Linux Device Drivers: mmap and DMA

Linux Device Drivers, 2nd Edition 2nd Edition June 2001 0-59600-008-1, Order Number: 0081 586 pages, $39.95

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Chapter 13
mmap and DMA

Contents:

This chapter delves into the area of Linux memory management, with an emphasis on techniques that are useful to the device driver writer. The material in this chapter is somewhat advanced, and not everybody will need a grasp of it. Nonetheless, many tasks can only be done through digging more deeply into the memory management subsystem; it also provides an interesting look into how an important part of the kernel works.

The material in this chapter is divided into three sections. The first covers the implementation of the mmapsystem call, which allows the mapping of device memory directly into a user process's address space. We then cover the kernel kiobuf mechanism, which provides direct access to user memory from kernel space. The kiobuf system may be used to implement "raw I/O'' for certain kinds of devices. The final section covers direct memory access (DMA) I/O operations, which essentially provide peripherals with direct access to system memory.

Of course, all of these techniques require an understanding of how Linux memory management works, so we start with an overview of that subsystem.

Linux is, of course, a virtual memory system, meaning that the addresses seen by user programs do not directly correspond to the physical addresses used by the hardware. Virtual memory introduces a layer of indirection, which allows a number of nice things. With virtual memory, programs running on the system can allocate far more memory than is physically available; indeed, even a single process can have a virtual address space larger than the system's physical memory. Virtual memory also allows playing a number of tricks with the process's address space, including mapping in device memory.

Recent developments have eliminated the limitations on memory, and 32-bit systems can now work with well over 4 GB of system memory (assuming, of course, that the processor itself can address that much memory). The limitation on how much memory can be directly mapped with logical addresses remains, however. Only the lowest portion of memory (up to 1 or 2 GB, depending on the hardware and the kernel configuration) has logical addresses; the rest (high memory) does not. High memory can require 64-bit physical addresses, and the kernel must set up explicit virtual address mappings to manipulate it. Thus, many kernel functions are limited to low memory only; high memory tends to be reserved for user-space process pages.

The kernel doesn't need to worry about doing page-table lookups during normal program execution, because they are done by the hardware. Nonetheless, the kernel must arrange things so that the hardware can do its work. It must build the page tables and look them up whenever the processor reports a page fault, that is, whenever the page associated with a virtual address needed by the processor is not present in memory. Device drivers, too, must be able to build page tables and handle faults when implementing mmap.

These inline functions[50] are used to retrieve the pgd, pmd, and pte entries associated with address. Page-table lookup begins with a pointer to struct mm_struct. The pointer associated with the memory map of the current process is current->mm, while the pointer to kernel space is described by &init_mm. Two-level processors define pmd_offset(dir,add) as (pmd_t *)dir, thus folding the pmd over the pgd. Functions that scan page tables are always declared as inline, and the compiler optimizes out any pmd lookup.

This macro returns a boolean value that indicates whether the data page is currently in memory. This is the most used of several functions that access the low bits in the pte -- the bits that are discarded by pte_page. Pages may be absent, of course, if the kernel has swapped them to disk (or if they have never been loaded). The page tables themselves, however, are always present in the current Linux implementation. Keeping page tables in memory simplifies the kernel code because pgd_offset and friends never fail; on the other hand, even a process with a "resident storage size'' of zero keeps its page tables in real RAM, wasting some memory that might be better used elsewhere.

Just seeing the list of these functions is not enough for you to be proficient in the Linux memory management algorithms; real memory management is much more complex and must deal with other complications, like cache coherence. The previous list should nonetheless be sufficient to give you a feel for how page management is implemented; it is also about all that you will need to know, as a device driver writer, to work occasionally with page tables. You can get more information from the include/asm and mm subtrees of the kernel source.

• An area for the program's executable code (often called text).

• One area each for data, including initialized data (that which has an explicitly assigned value at the beginning of execution), uninitialized data (BSS),[51] and the program stack.

  [51]The name BSS is a historical relic, from an old assembly operator meaning "Block started by symbol.'' The BSS segment of executable files isn't stored on disk, and the kernel maps the zero page to the BSS address range.

The memory areas of a process can be seen by looking in /proc/pid/maps(where pid, of course, is replaced by a process ID). /proc/self is a special case of /proc/pid, because it always refers to the current process. As an example, here are a couple of memory maps, to which we have added short comments after a sharp sign:

start-end perm offset major:minor inode image.

Each field in /proc/*/maps (except the image name) corresponds to a field in struct vm_area_struct, and is described in the following list.

A bit mask with the memory area's read, write, and execute permissions. This field describes what the process is allowed to do with pages belonging to the area. The last character in the field is either p for "private'' or s for "shared.''

The major and minor numbers of the device holding the file that has been mapped. Confusingly, for device mappings, the major and minor numbers refer to the disk partition holding the device special file that was opened by the user, and not the device itself.

image

The name of the file (usually an executable image) that has been mapped.

A driver that implements the mmap method needs to fill a VMA structure in the address space of the process mapping the device. The driver writer should therefore have at least a minimal understanding of VMAs in order to use them.

Let's look at the most important fields in struct vm_area_struct (defined in ). These fields may be used by device drivers in their mmap implementation. Note that the kernel maintains lists and trees of VMAs to optimize area lookup, and several fields of vm_area_struct are used to maintain this organization. VMAs thus can't be created at will by a driver, or the structures will break. The main fields of VMAs are as follows (note the similarity between these fields and the /proc output we just saw):

The offset of the area in the file, in pages. When a file or device is mapped, this is the file position of the first page mapped in this area.

A set of flags describing this area. The flags of the most interest to device driver writers are VM_IO and VM_RESERVED. VM_IO marks a VMA as being a memory-mapped I/O region. Among other things, the VM_IO flag will prevent the region from being included in process core dumps. VM_RESERVED tells the memory management system not to attempt to swap out this VMA; it should be set in most device mappings.

This method is intended to change the protection on a memory area, but is currently not used. Memory protection is handled by the page tables, and the kernel sets up the page-table entries separately.

When a process tries to access a page that belongs to a valid VMA, but that is currently not in memory, the nopagemethod is called (if it is defined) for the related area. The method returns the struct page pointer for the physical page, after, perhaps, having read it in from secondary storage. If the nopage method isn't defined for the area, an empty page is allocated by the kernel. The third argument, write_access, counts as "no-share'': a nonzero value means the page must be owned by the current process, whereas 0 means that sharing is possible.

This method handles write-protected page faults but is currently unused. The kernel handles attempts to write over a protected page without invoking the area-specific callback. Write-protect faults are used to implement copy-on-write. A private page can be shared across processes until one process writes to it. When that happens, the page is cloned, and the process writes on its own copy of the page. If the whole area is marked as read-only, a SIGSEGV is sent to the process, and the copy-on-write is not performed.

This method is called when a page is selected to be swapped out. A return value of 0 signals success; any other value signals an error. In case of error, the process owning the page is sent a SIGBUS. It is highly unlikely that a driver will ever need to implement swapout; device mappings are not something that the kernel can just write to disk.

Memory mapping is one of the most interesting features of modern Unix systems. As far as drivers are concerned, memory mapping can be used to provide user programs with direct access to device memory.

cat /proc/731/maps
08048000-08327000 r-xp 00000000 08:01 55505    /usr/X11R6/bin/XF86_SVGA
08327000-08369000 rw-p 002de000 08:01 55505    /usr/X11R6/bin/XF86_SVGA
40015000-40019000 rw-s fe2fc000 08:01 10778    /dev/mem
40131000-40141000 rw-s 000a0000 08:01 10778    /dev/mem
40141000-40941000 rw-s f4000000 08:01 10778    /dev/mem
     ...

The full list of the X server's VMAs is lengthy, but most of the entries are not of interest here. We do see, however, three separate mappings of /dev/mem, which give some insight into how the X server works with the video card. The first mapping shows a 16 KB region mapped at fe2fc000. This address is far above the highest RAM address on the system; it is, instead, a region of memory on a PCI peripheral (the video card). It will be a control region for that card. The middle mapping is at a0000, which is the standard location for video RAM in the 640 KB ISA hole. The last /dev/memmapping is a rather larger one at f4000000 and is the video memory itself. These regions can also be seen in /proc/iomem:

Mapping a device means associating a range of user-space addresses to device memory. Whenever the program reads or writes in the assigned address range, it is actually accessing the device. In the X server example, using mmap allows quick and easy access to the video card's memory. For a performance-critical application like this, direct access makes a large difference.

As you might suspect, not every device lends itself to the mmap abstraction; it makes no sense, for instance, for serial ports and other stream-oriented devices. Another limitation of mmap is that mapping is PAGE_SIZE grained. The kernel can dispose of virtual addresses only at the level of page tables; therefore, the mapped area must be a multiple of PAGE_SIZE and must live in physical memory starting at an address that is a multiple of PAGE_SIZE. The kernel accommodates for size granularity by making a region slightly bigger if its size isn't a multiple of the page size.

These limits are not a big constraint for drivers, because the program accessing the device is device dependent anyway. It needs to know how to make sense of the memory region being mapped, so the PAGE_SIZE alignment is not a problem. A bigger constraint exists when ISA devices are used on some non-x86 platforms, because their hardware view of ISA may not be contiguous. For example, some Alpha computers see ISA memory as a scattered set of 8-bit, 16-bit, or 32-bit items, with no direct mapping. In such cases, you can't use mmap at all. The inability to perform direct mapping of ISA addresses to Alpha addresses is due to the incompatible data transfer specifications of the two systems. Whereas early Alpha processors could issue only 32-bit and 64-bit memory accesses, ISA can do only 8-bit and 16-bit transfers, and there's no way to transparently map one protocol onto the other.

There are sound advantages to using mmap when it's feasible to do so. For instance, we have already looked at the X server, which transfers a lot of data to and from video memory; mapping the graphic display to user space dramatically improves the throughput, as opposed to an lseek/writeimplementation. Another typical example is a program controlling a PCI device. Most PCI peripherals map their control registers to a memory address, and a demanding application might prefer to have direct access to the registers instead of repeatedly having to call ioctl to get its work done.

The filp argument in the method is the same as that introduced in Chapter 3, "Char Drivers", while vma contains the information about the virtual address range that is used to access the device. Much of the work has thus been done by the kernel; to implement mmap, the driver only has to build suitable page tables for the address range and, if necessary, replace vma->vm_ops with a new set of operations.

The arguments to remap_page_range are fairly straightforward, and most of them are already provided to you in the VMA when your mmap method is called. The one complication has to do with caching: usually, references to device memory should not be cached by the processor. Often the system BIOS will set things up properly, but it is also possible to disable caching of specific VMAs via the protection field. Unfortunately, disabling caching at this level is highly processor dependent. The curious reader may wish to look at the function pgprot_noncached from drivers/char/mem.c to see what's involved. We won't discuss the topic further here.

If your driver needs to do a simple, linear mapping of device memory into a user address space, remap_page_range is almost all you really need to do the job. The following code comes from drivers/char/mem.c and shows how this task is performed in a typical module called simple (Simple Implementation Mapping Pages with Little Enthusiasm):

The /dev/mem code checks to see if the requested offset (stored in vma->vm_pgoff) is beyond physical memory; if so, the VM_IO VMA flag is set to mark the area as being I/O memory. The VM_RESERVED flag is always set to keep the system from trying to swap this area out. Then it is just a matter of calling remap_page_range to create the necessary page tables.

Here, we will provide open and close operations for our VMA. These operations will be called anytime a process opens or closes the VMA; in particular, the open method will be invoked anytime a process forks and creates a new reference to the VMA. The open and close VMA methods are called in addition to the processing performed by the kernel, so they need not reimplement any of the work done there. They exist as a way for drivers to do any additional processing that they may require.

So, we will override the default vma->vm_ops with operations that keep track of the usage count. The code is quite simple -- a complete mmap implementation for a modularized /dev/mem looks like the following:

This code relies on the fact that the kernel initializes to NULL the vm_ops field in the newly created area before calling f_op->mmap. The code just shown checks the current value of the pointer as a safety measure, should something change in future kernels.

The nopage method, therefore, must be implemented if you want to support the mremap system call. But once you have nopage, you can choose to use it extensively, with some limitations (described later). This method is shown in the next code fragment. In this implementation of mmap, the device method only replaces vma->vm_ops. The nopagemethod takes care of "remapping'' one page at a time and returning the address of its struct page structure. Because we are just implementing a window onto physical memory here, the remapping step is simple -- we need only locate and return a pointer to the struct page for the desired address.

An implementation of /dev/mem using nopage looks like the following:

Since, once again, we are simply mapping main memory here, the nopage function need only find the correct struct page for the faulting address and increment its reference count. The required sequence of events is thus to calculate the desired physical address, turn it into a logical address with __va, and then finally to turn it into a struct page with virt_to_page. It would be possible, in general, to go directly from the physical address to the struct page, but such code would be difficult to make portable across architectures. Such code might be necessary, however, if one were trying to map high memory, which, remember, has no logical addresses. simple, being simple, does not worry about that (rare) case.

The nopage method normally returns a pointer to a struct page. If, for some reason, a normal page cannot be returned (e.g., the requested address is beyond the device's memory region), NOPAGE_SIGBUS can be returned to signal the error. nopage can also return NOPAGE_OOM to indicate failures caused by resource limitations.

All the examples we've seen so far are reimplementations of /dev/mem; they remap physical addresses into user space. The typical driver, however, wants to map only the small address range that applies to its peripheral device, not all of memory. In order to map to user space only a subset of the whole memory range, the driver needs only to play with the offsets. The following lines will do the trick for a driver mapping a region of simple_region_size bytes, beginning at physical address simple_region_start (which should be page aligned).

In addition to calculating the offsets, this code introduces a check that reports an error when the program tries to map more memory than is available in the I/O region of the target device. In this code, psize is the physical I/O size that is left after the offset has been specified, and vsize is the requested size of virtual memory; the function refuses to map addresses that extend beyond the allowed memory range.

Note that the user process can always use mremapto extend its mapping, possibly past the end of the physical device area. If your driver has no nopage method, it will never be notified of this extension, and the additional area will map to the zero page. As a driver writer, you may well want to prevent this sort of behavior; mapping the zero page onto the end of your region is not an explicitly bad thing to do, but it is highly unlikely that the programmer wanted that to happen.

Of course, a more thorough implementation could check to see if the faulting address is within the device area, and perform the remapping if that is the case. Once again, however, nopagewill not work with PCI memory areas, so extension of PCI mappings is not possible. In Linux, a page of physical addresses is marked as "reserved'' in the memory map to indicate that it is not available for memory management. On the PC, for example, the range between 640 KB and 1 MB is marked as reserved, as are the pages that host the kernel code itself.

The limitations of remap_page_range can be seen by running mapper, one of the sample programs in misc-progs in the files provided on the O'Reilly FTP site. mapper is a simple tool that can be used to quickly test the mmapsystem call; it maps read-only parts of a file based on the command-line options and dumps the mapped region to standard output. The following session, for instance, shows that /dev/mem doesn't map the physical page located at address 64 KB -- instead we see a page full of zeros (the host computer in this examples is a PC, but the result would be the same on other platforms):

morgana.root# ./mapper /dev/mem 0x10000 0x1000 | od -Ax -t x1
mapped "/dev/mem" from 65536 to 69632
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
001000

The inability of remap_page_range to deal with RAM suggests that a device like scullpcan't easily implement mmap, because its device memory is conventional RAM, not I/O memory. Fortunately, a relatively easy workaround is available to any driver that needs to map RAM into user space; it uses the nopage method that we have seen earlier.

The way to map real RAM to user space is to use vm_ops->nopage to deal with page faults one at a time. A sample implementation is part of the scullp module, introduced in Chapter 7, "Getting Hold of Memory".

scullp is the page oriented char device. Because it is page oriented, it can implement mmap on its memory. The code implementing memory mapping uses some of the concepts introduced earlier in "Memory Management in Linux".

Before examining the code, let's look at the design choices that affect the mmap implementation in scullp.

• scullp doesn't release device memory as long as the device is mapped. This is a matter of policy rather than a requirement, and it is different from the behavior of scull and similar devices, which are truncated to a length of zero when opened for writing. Refusing to free a mapped scullp device allows a process to overwrite regions actively mapped by another process, so you can test and see how processes and device memory interact. To avoid releasing a mapped device, the driver must keep a count of active mappings; the vmas field in the device structure is used for this purpose.

• Memory mapping is performed only when the scullp order parameter is 0. The parameter controls how get_free_pagesis invoked (see Chapter 7, "Getting Hold of Memory", "get_free_page and Friends"). This choice is dictated by the internals of get_free_pages, the allocation engine exploited by scullp. To maximize allocation performance, the Linux kernel maintains a list of free pages for each allocation order, and only the page count of the first page in a cluster is incremented by get_free_pages and decremented by free_pages. The mmap method is disabled for a scullp device if the allocation order is greater than zero, because nopage deals with single pages rather than clusters of pages. (Return to "A scull Using Whole Pages: scullp" in Chapter 7, "Getting Hold of Memory" if you need a refresher on scullp and the memory allocation order value.)

This implementation of scullp_mmap is very short, because it relies on the nopage function to do all the interesting work:

 
int scullp_mmap(struct file *filp, struct vm_area_struct *vma)
{
    struct inode *inode = INODE_FROM_F(filp);

    /* refuse to map if order is not 0 */
    if (scullp_devices[MINOR(inode->i_rdev)].order)
        return -ENODEV;

    /* don't do anything here: "nopage" will fill the holes */
    vma->vm_ops = &scullp_vm_ops;
    vma->vm_flags |= VM_RESERVED;
    vma->vm_private_data = scullp_devices + MINOR(inode->i_rdev);
    scullp_vma_open(vma);
    return 0;
}

The purpose of the leading conditional is to avoid mapping devices whose allocation order is not 0. scullp's operations are stored in the vm_ops field, and a pointer to the device structure is stashed in the vm_private_data field. At the end, vm_ops->open is called to update the usage count for the module and the count of active mappings for the device.

 
void scullp_vma_open(struct vm_area_struct *vma)
{
    ScullP_Dev *dev = scullp_vma_to_dev(vma);

    dev->vmas++;
    MOD_INC_USE_COUNT;
}

void scullp_vma_close(struct vm_area_struct *vma)
{
    ScullP_Dev *dev = scullp_vma_to_dev(vma);

    dev->vmas--;
    MOD_DEC_USE_COUNT;
}

The function sculls_vma_to_dev simply returns the contents of the vm_private_data field. It exists as a separate function because kernel versions prior to 2.4 lacked that field, requiring that other means be used to get that pointer. See "Backward Compatibility" at the end of this chapter for details.

Most of the work is then performed by nopage. In the scullp implementation, the address parameter to nopage is used to calculate an offset into the device; the offset is then used to look up the correct page in the scullpmemory tree.

 
struct page *scullp_vma_nopage(struct vm_area_struct *vma,
                                unsigned long address, int write)
{
    unsigned long offset;
    ScullP_Dev *ptr, *dev = scullp_vma_to_dev(vma);
    struct page *page = NOPAGE_SIGBUS;
    void *pageptr = NULL; /* default to "missing" */

    down(&dev->sem);
    offset = (address - vma->vm_start) + VMA_OFFSET(vma);
    if (offset >= dev->size) goto out; /* out of range */

    /*
     * Now retrieve the scullp device from the list, then the page.
     * If the device has holes, the process receives a SIGBUS when
     * accessing the hole.
     */
    offset >>= PAGE_SHIFT; /* offset is a number of pages */
    for (ptr = dev; ptr && offset >= dev->qset;) {
        ptr = ptr->next;
        offset -= dev->qset;
    }
    if (ptr && ptr->data) pageptr = ptr->data[offset];
    if (!pageptr) goto out; /* hole or end-of-file */
    page = virt_to_page(pageptr);
    
    /* got it, now increment the count */
    get_page(page);
out:
    up(&dev->sem);
    return page;
}

scullp uses memory obtained with get_free_pages. That memory is addressed using logical addresses, so all scullp_nopage has to do to get a struct page pointer is to call virt_to_page.

The scullp device now works as expected, as you can see in this sample output from the mapper utility. Here we send a directory listing of /dev (which is long) to the scullp device, and then use the mapper utility to look at pieces of that listing with mmap.

morgana% ls -l /dev > /dev/scullp
morgana% ./mapper /dev/scullp 0 140
mapped "/dev/scullp" from 0 to 140
total 77
-rwxr-xr-x    1 root     root        26689 Mar  2  2000 MAKEDEV
crw-rw-rw-    1 root     root      14,  14 Aug 10 20:55 admmidi0
morgana% ./mapper /dev/scullp 8192 200
mapped "/dev/scullp" from 8192 to 8392
0
crw --  --  -- -    1 root     root     113,   1 Mar 26  1999 cum1
crw --  --  -- -    1 root     root     113,   2 Mar 26  1999 cum2
crw --  --  -- -    1 root     root     113,   3 Mar 26  1999 cum3

Although it's rarely necessary, it's interesting to see how a driver can map a virtual address to user space using mmap. A true virtual address, remember, is an address returned by a function like vmalloc or kmap -- that is, a virtual address mapped in the kernel page tables. The code in this section is taken from scullv, which is the module that works like scullp but allocates its storage through vmalloc.

Most of the scullv implementation is like the one we've just seen for scullp, except that there is no need to check the order parameter that controls memory allocation. The reason for this is that vmalloc allocates its pages one at a time, because single-page allocations are far more likely to succeed than multipage allocations. Therefore, the allocation order problem doesn't apply to vmalloced space.

Most of the work of vmalloc is building page tables to access allocated pages as a continuous address range. The nopage method, instead, must pull the page tables back apart in order to return a struct page pointer to the caller. Therefore, the nopageimplementation for scullv must scan the page tables to retrieve the page map entry associated with the page.

The function is similar to the one we saw for scullp, except at the end. This code excerpt only includes the part of nopage that differs from scullp:

 
pgd_t *pgd; pmd_t *pmd; pte_t *pte;
unsigned long lpage;

/*
 * After scullv lookup, "page" is now the address of the page
 * needed by the current process. Since it's a vmalloc address,
 * first retrieve the unsigned long value to be looked up
 * in page tables.
 */
lpage = VMALLOC_VMADDR(pageptr);
spin_lock(&init_mm.page_table_lock);
pgd = pgd_offset(&init_mm, lpage);
pmd = pmd_offset(pgd, lpage);
pte = pte_offset(pmd, lpage);
page = pte_page(*pte);
spin_unlock(&init_mm.page_table_lock);
    
/* got it, now increment the count */
get_page(page);
out:
up(&dev->sem);
return page;

The page tables are looked up using the functions introduced at the beginning of this chapter. The page directory used for this purpose is stored in the memory structure for kernel space, init_mm. Note that scullv obtains the page_table_lock prior to traversing the page tables. If that lock were not held, another processor could make a change to the page table while scullv was halfway through the lookup process, leading to erroneous results.

Based on this discussion, you might also want to map addresses returned by ioremap to user space. This mapping is easily accomplished because you can use remap_page_range directly, without implementing methods for virtual memory areas. In other words, remap_page_range is already usable for building new page tables that map I/O memory to user space; there's no need to look in the kernel page tables built by vremap as we did in scullv.

As of version 2.3.12, the Linux kernel supports an I/O abstraction called the kernel I/O buffer, or kiobuf. The kiobuf interface is intended to hide much of the complexity of the virtual memory system from device drivers (and other parts of the system that do I/O). Many features are planned for kiobufs, but their primary use in the 2.4 kernel is to facilitate the mapping of user-space buffers into the kernel.

Locking a kiovec in this manner is unnecessary, however, for most applications of kiobufs seen in device drivers.

Unix systems have long provided a "raw'' interface to some devices -- block devices in particular -- which performs I/O directly from a user-space buffer and avoids copying data through the kernel. In some cases much improved performance can be had in this manner, especially if the data being transferred will not be used again in the near future. For example, disk backups typically read a great deal of data from the disk exactly once, then forget about it. Running the backup via a raw interface will avoid filling the system buffer cache with useless data.

The Linux kernel has traditionally not provided a raw interface, for a number of reasons. As the system gains in popularity, however, more applications that expect to be able to do raw I/O (such as large database management systems) are being ported. So the 2.3 development series finally added raw I/O; the driving force behind the kiobuf interface was the need to provide this capability.

Raw I/O is not always the great performance boost that some people think it should be, and driver writers should not rush out to add the capability just because they can. The overhead of setting up a raw transfer can be significant, and the advantages of buffering data in the kernel are lost. For example, note that raw I/O operations almost always must be synchronous -- the write system call cannot return until the operation is complete. Linux currently lacks the mechanisms that user programs need to be able to safely perform asynchronous raw I/O on a user buffer.

In this section, we add a raw I/O capability to the sbull sample block driver. When kiobufs are available, sbull actually registers two devices. The block sbull device was examined in detail in Chapter 12, "Loading Block Drivers". What we didn't see in that chapter was a second, char device (called sbullr), which provides raw access to the RAM-disk device. Thus, /dev/sbull0 and /dev/sbullr0 access the same memory; the former using the traditional, buffered mode and the second providing raw access via the kiobuf mechanism.

It is worth noting that in Linux systems, there is no need for block drivers to provide this sort of interface. The raw device, in drivers/char/raw.c, provides this capability in an elegant, general way for all block devices. The block drivers need not even know they are doing raw I/O. The raw I/O code in sbull is essentially a simplification of the raw device code for demonstration purposes.

Raw I/O to a block device must always be sector aligned, and its length must be a multiple of the sector size. Other kinds of devices, such as tape drives, may not have the same constraints. sbullr behaves like a block device and enforces the alignment and length requirements. To that end, it defines a few symbols:

The sbullr raw device will be registered only if the hard-sector size is equal to SBULLR_SECTOR. There is no real reason why a larger hard-sector size could not be supported, but it would complicate the sample code unnecessarily.

The sbullr implementation adds little to the existing sbull code. In particular, the open and close methods from sbull are used without modification. Since sbullr is a char device, however, it needs read and writemethods. Both are defined to use a single transfer function as follows:

 
ssize_t sbullr_read(struct file *filp, char *buf, size_t size, 
                    loff_t *off)
{
    Sbull_Dev *dev = sbull_devices + 
                    MINOR(filp->f_dentry->d_inode->i_rdev);
    return sbullr_transfer(dev, buf, size, off, READ);
}

ssize_t sbullr_write(struct file *filp, const char *buf, size_t size,
                loff_t *off)
{
    Sbull_Dev *dev = sbull_devices + 
                    MINOR(filp->f_dentry->d_inode->i_rdev);
    return sbullr_transfer(dev, (char *) buf, size, off, WRITE);
}

 
static int sbullr_transfer (Sbull_Dev *dev, char *buf, size_t count,
                loff_t *offset, int rw)
{
    struct kiobuf *iobuf;       
    int result;
    
    /* Only block alignment and size allowed */
    if ((*offset & SBULLR_SECTOR_MASK) || (count & SBULLR_SECTOR_MASK))
        return -EINVAL;
    if ((unsigned long) buf & SBULLR_SECTOR_MASK)
        return -EINVAL;

    /* Allocate an I/O vector */
    result = alloc_kiovec(1, &iobuf);
    if (result)
        return result;

    /* Map the user I/O buffer and do the I/O. */
    result = map_user_kiobuf(rw, iobuf, (unsigned long) buf, count);
    if (result) {
        free_kiovec(1, &iobuf);
        return result;
    }
    spin_lock(&dev->lock);
    result = sbullr_rw_iovec(dev, iobuf, rw, 
                    *offset >> SBULLR_SECTOR_SHIFT,
                    count >> SBULLR_SECTOR_SHIFT);
    spin_unlock(&dev->lock);

    /* Clean up and return. */
    unmap_kiobuf(iobuf);
    free_kiovec(1, &iobuf);
    if (result > 0)
        *offset += result << SBULLR_SECTOR_SHIFT;
    return result << SBULLR_SECTOR_SHIFT;
}

 
static int sbullr_rw_iovec(Sbull_Dev *dev, struct kiobuf *iobuf, int rw,
                int sector, int nsectors)
{
    struct request fakereq;
    struct page *page;
    int offset = iobuf->offset, ndone = 0, pageno, result;

    /* Perform I/O on each sector */
    fakereq.sector = sector;
    fakereq.current_nr_sectors = 1;
    fakereq.cmd = rw;
    
    for (pageno = 0; pageno < iobuf->nr_pages; pageno++) {
        page = iobuf->maplist[pageno];
        while (ndone < nsectors) {
            /* Fake up a request structure for the operation */
            fakereq.buffer = (void *) (kmap(page) + offset);
            result = sbull_transfer(dev, &fakereq);
	    kunmap(page);
            if (result == 0)
                return ndone;
            /* Move on to the next one */
            ndone++;
            fakereq.sector++;
            offset += SBULLR_SECTOR;
            if (offset >= PAGE_SIZE) {
                offset = 0;
                break;
            }
        }
    }
    return ndone;
}

Some quick tests copying data show that a copy to or from an sbullr device takes roughly two-thirds the system time as the same copy to the block sbull device. The savings is gained by avoiding the extra copy through the buffer cache. Note that if the same data is read several times over, that savings will evaporate -- especially for a real hardware device. Raw device access is often not the best approach, but for some applications it can be a major improvement.

Although kiobufs remain controversial in the kernel development community, there is interest in using them in a wider range of contexts. There is, for example, a patch that implements Unix pipes with kiobufs -- data is copied directly from one process's address space to the other with no buffering in the kernel at all. A patch also exists that makes it easy to use a kiobuf to map kernel virtual memory into a process's address space, thus eliminating the need for a nopage implementation as shown earlier.

Direct memory access, or DMA, is the advanced topic that completes our overview of memory issues. DMA is the hardware mechanism that allows peripheral components to transfer their I/O data directly to and from main memory without the need for the system processor to be involved in the transfer. Use of this mechanism can greatly increase throughput to and from a device, because a great deal of computational overhead is eliminated.

To exploit the DMA capabilities of its hardware, the device driver needs to be able to correctly set up the DMA transfer and synchronize with the hardware. Unfortunately, because of its hardware nature, DMA is very system dependent. Each architecture has its own techniques to manage DMA transfers, and the programming interface is different for each. The kernel can't offer a unified interface, either, because a driver can't abstract too much from the underlying hardware mechanisms. Some steps have been made in that direction, however, in recent kernels.

This chapter concentrates mainly on the PCI bus, since it is currently the most popular peripheral bus available. Many of the concepts are more widely applicable, though. We also touch on how some other buses, such as ISA and SBus, handle DMA.

Before introducing the programming details, let's review how a DMA transfer takes place, considering only input transfers to simplify the discussion.

The second case comes about when DMA is used asynchronously. This happens, for example, with data acquisition devices that go on pushing data even if nobody is reading them. In this case, the driver should maintain a buffer so that a subsequent read call will return all the accumulated data to user space. The steps involved in this kind of transfer are slightly different:

3. The peripheral device writes the data to the buffer and raises another interrupt when it's done.

A variant of the asynchronous approach is often seen with network cards. These cards often expect to see a circular buffer (often called a DMA ring buffer) established in memory shared with the processor; each incoming packet is placed in the next available buffer in the ring, and an interrupt is signaled. The driver then passes the network packets to the rest of the kernel, and places a new DMA buffer in the ring.

Another relevant item introduced here is the DMA buffer. To exploit direct memory access, the device driver must be able to allocate one or more special buffers, suited to DMA. Note that many drivers allocate their buffers at initialization time and use them until shutdown -- the word allocate in the previous lists therefore means "get hold of a previously allocated buffer.''

The main problem with the DMA buffer is that when it is bigger than one page, it must occupy contiguous pages in physical memory because the device transfers data using the ISA or PCI system bus, both of which carry physical addresses. It's interesting to note that this constraint doesn't apply to the SBus (see "SBus" in Chapter 15, "Overview of Peripheral Buses"), which uses virtual addresses on the peripheral bus. Some architectures can also use virtual addresses on the PCI bus, but a portable driver cannot count on that capability.

Although DMA buffers can be allocated either at system boot or at runtime, modules can only allocate their buffers at runtime. Chapter 7, "Getting Hold of Memory" introduced these techniques: "Boot-Time Allocation" talked about allocation at system boot, while "The Real Story of kmalloc" and "get_free_page and Friends" described allocation at runtime. Driver writers must take care to allocate the right kind of memory when it will be used for DMA operations -- not all memory zones are suitable. In particular, high memory will not work for DMA on most systems -- the peripherals simply cannot work with addresses that high.

Most devices on modern buses can handle 32-bit addresses, meaning that normal memory allocations will work just fine for them. Some PCI devices, however, fail to implement the full PCI standard and cannot work with 32-bit addresses. And ISA devices, of course, are limited to 16-bit addresses only.

For devices with this kind of limitation, memory should be allocated from the DMA zone by adding the GFP_DMA flag to the kmalloc or get_free_pagescall. When this flag is present, only memory that can be addressed with 16 bits will be allocated.

We have seen how get_free_pages (and therefore kmalloc) can't return more than 128 KB (or, more generally, 32 pages) of consecutive memory space. But the request is prone to fail even when the allocated buffer is less than 128 KB, because system memory becomes fragmented over time.[52]

[52]The word fragmentation is usually applied to disks, to express the idea that files are not stored consecutively on the magnetic medium. The same concept applies to memory, where each virtual address space gets scattered throughout physical RAM, and it becomes difficult to retrieve consecutive free pages when a DMA buffer is requested.

Actually, there is another way to allocate DMA space: perform aggressive allocation until you are able to get enough consecutive pages to make a buffer. We strongly discourage this allocation technique if there's any other way to achieve your goal. Aggressive allocation results in high machine load, and possibly in a system lockup if your aggressiveness isn't correctly tuned. On the other hand, sometimes there is no other way available.

In practice, the code invokes kmalloc(GFP_ATOMIC) until the call fails; it then waits until the kernel frees some pages, and then allocates everything once again. If you keep an eye on the pool of allocated pages, sooner or later you'll find that your DMA buffer of consecutive pages has appeared; at this point you can release every page but the selected buffer. This kind of behavior is rather risky, though, because it may lead to a deadlock. We suggest using a kernel timer to release every page in case allocation doesn't succeed before a timeout expires.

We're not going to show the code here, but you'll find it in misc-modules/allocator.c; the code is thoroughly commented and designed to be called by other modules. Unlike every other source accompanying this book, the allocator is covered by the GPL. The reason we decided to put the source under the GPL is that it is neither particularly beautiful nor particularly clever, and if someone is going to use it, we want to be sure that the source is released with the module.

A device driver using DMA has to talk to hardware connected to the interface bus, which uses physical addresses, whereas program code uses virtual addresses.

As a matter of fact, the situation is slightly more complicated than that. DMA-based hardware uses bus, rather than physical, addresses. Although ISA and PCI addresses are simply physical addresses on the PC, this is not true for every platform. Sometimes the interface bus is connected through bridge circuitry that maps I/O addresses to different physical addresses. Some systems even have a page-mapping scheme that can make arbitrary pages appear contiguous to the peripheral bus.

The virt_to_bus conversion must be used when the driver needs to send address information to an I/O device (such as an expansion board or the DMA controller), while bus_to_virt must be used when address information is received from hardware connected to the bus.

The functions in this section require a struct pci_dev structure for your device. The details of setting up a PCI device are covered in Chapter 15, "Overview of Peripheral Buses". Note, however, that the routines described here can also be used with ISA devices; in that case, the struct pci_dev pointer should simply be passed in as NULL.

Dealing with difficult hardware

The first question that must be answered before performing DMA is whether the given device is capable of such operation on the current host. Many PCI devices fail to implement the full 32-bit bus address space, often because they are modified versions of old ISA hardware. The Linux kernel will attempt to work with such devices, but it is not always possible.

The function pci_dma_supported should be called for any device that has addressing limitations:

int pci_dma_supported(struct pci_dev *pdev, dma_addr_t mask);

Here, mask is a simple bit mask describing which address bits the device can successfully use. If the return value is nonzero, DMA is possible, and your driver should set the dma_mask field in the PCI device structure to the mask value. For a device that can only handle 16-bit addresses, you might use a call like this:

if (pci_dma_supported (pdev, 0xffff))
    pdev->dma_mask = 0xffff;
else {
    card->use_dma = 0;   /* We'll have to live without DMA */
    printk (KERN_WARN, "mydev: DMA not supported\n");
}

int pci_set_dma_mask(struct pci_dev *pdev, dma_addr_t mask);

For devices that can handle 32-bit addresses, there is no need to call pci_dma_supported.

A DMA mapping is a combination of allocating a DMA buffer and generating an address for that buffer that is accessible by the device. In many cases, getting that address involves a simple call to virt_to_bus; some hardware, however, requires that mapping registers be set up in the bus hardware as well. Mapping registers are an equivalent of virtual memory for peripherals. On systems where these registers are used, peripherals have a relatively small, dedicated range of addresses to which they may perform DMA. Those addresses are remapped, via the mapping registers, into system RAM. Mapping registers have some nice features, including the ability to make several distributed pages appear contiguous in the device's address space. Not all architectures have mapping registers, however; in particular, the popular PC platform has no mapping registers.

Setting up a useful address for the device may also, in some cases, require the establishment of a bounce buffer. Bounce buffers are created when a driver attempts to perform DMA on an address that is not reachable by the peripheral device -- a high-memory address, for example. Data is then copied to and from the bounce buffer as needed. Making code work properly with bounce buffers requires adherence to some rules, as we will see shortly.

The DMA mapping sets up a new type, dma_addr_t, to represent bus addresses. Variables of type dma_addr_t should be treated as opaque by the driver; the only allowable operations are to pass them to the DMA support routines and to the device itself.

These are set up for a single operation. Some architectures allow for significant optimizations when streaming mappings are used, as we will see, but these mappings also are subject to a stricter set of rules in how they may be accessed. The kernel developers recommend the use of streaming mappings over consistent mappings whenever possible. There are two reasons for this recommendation. The first is that, on systems that support them, each DMA mapping uses one or more mapping registers on the bus. Consistent mappings, which have a long lifetime, can monopolize these registers for a long time, even when they are not being used. The other reason is that, on some hardware, streaming mappings can be optimized in ways that are not available to consistent mappings.

void *pci_alloc_consistent(struct pci_dev *pdev, size_t size,
                           dma_addr_t *bus_addr);

This function handles both the allocation and the mapping of the buffer. The first two arguments are our PCI device structure and the size of the needed buffer. The function returns the result of the DMA mapping in two places. The return value is a kernel virtual address for the buffer, which may be used by the driver; the associated bus address, instead, is returned in bus_addr. Allocation is handled in this function so that the buffer will be placed in a location that works with DMA; usually the memory is just allocated with get_free_pages (but note that the size is in bytes, rather than an order value).

void pci_free_consistent(struct pci_dev *pdev, size_t size,
                         void *cpu_addr, dma_handle_t bus_addr);

These two symbols should be reasonably self-explanatory. If data is being sent to the device (in response, perhaps, to a write system call), PCI_DMA_TODEVICE should be used; data going to the CPU, instead, will be marked with PCI_DMA_FROMDEVICE.

dma_addr_t pci_map_single(struct pci_dev *pdev, void *buffer,
                          size_t size, int direction);

The return value is the bus address that you can pass to the device, or NULL if something goes wrong.

void pci_unmap_single(struct pci_dev *pdev, dma_addr_t bus_addr,
                      size_t size, int direction);

• Once a buffer has been mapped, it belongs to the device, not the processor. Until the buffer has been unmapped, the driver should not touch its contents in any way. Only after pci_unmap_single has been called is it safe for the driver to access the contents of the buffer (with one exception that we'll see shortly). Among other things, this rule implies that a buffer being written to a device cannot be mapped until it contains all the data to write.

You may be wondering why the driver can no longer work with a buffer once it has been mapped. There are actually two reasons why this rule makes sense. First, when a buffer is mapped for DMA, the kernel must ensure that all of the data in that buffer has actually been written to memory. It is likely that some data will remain in the processor's cache, and must be explicitly flushed. Data written to the buffer by the processor after the flush may not be visible to the device.

Second, consider what happens if the buffer to be mapped is in a region of memory that is not accessible to the device. Some architectures will simply fail in this case, but others will create a bounce buffer. The bounce buffer is just a separate region of memory that is accessible to the device. If a buffer is mapped with a direction of PCI_DMA_TODEVICE, and a bounce buffer is required, the contents of the original buffer will be copied as part of the mapping operation. Clearly, changes to the original buffer after the copy will not be seen by the device. Similarly, PCI_DMA_FROMDEVICE bounce buffers are copied back to the original buffer by pci_unmap_single; the data from the device is not present until that copy has been done.

void pci_sync_single(struct pci_dev *pdev, dma_handle_t bus_addr,
                     size_t size, int direction);

Scatter-gather mappings are a special case of streaming DMA mappings. Suppose you have several buffers, all of which need to be transferred to or from the device. This situation can come about in several ways, including from a readv or writev system call, a clustered disk I/O request, or a list of pages in a mapped kernel I/O buffer. You could simply map each buffer in turn and perform the required operation, but there are advantages to mapping the whole list at once.

One reason is that some smart devices can accept a scatterlist of array pointers and lengths and transfer them all in one DMA operation; for example, "zero-copy'' networking is easier if packets can be built in multiple pieces. Linux is likely to take much better advantage of such devices in the future. Another reason to map scatterlists as a whole is to take advantage of systems that have mapping registers in the bus hardware. On such systems, physically discontiguous pages can be assembled into a single, contiguous array from the device's point of view. This technique works only when the entries in the scatterlist are equal to the page size in length (except the first and last), but when it does work it can turn multiple operations into a single DMA and speed things up accordingly.

int pci_map_sg(struct pci_dev *pdev, struct scatterlist *list,
               int nents, int direction);

void pci_unmap_sg(struct pci_dev *pdev, struct scatterlist *list,
                  int nents, int direction);

void pci_dma_sync_sg(struct pci_dev *pdev, struct scatterlist *sg,
                     int nents, int direction);

The actual form of DMA operations on the PCI bus is very dependent on the device being driven. Thus, this example does not apply to any real device; instead, it is part of a hypothetical driver called dad (DMA Acquisition Device). A driver for this device might define a transfer function like this:

int dad_transfer(struct dad_dev *dev, int write, void *buffer, 
                 size_t count)
{
    dma_addr_t bus_addr;
    unsigned long flags;

    /* Map the buffer for DMA */
    dev->dma_dir = (write ? PCI_DMA_TODEVICE : PCI_DMA_FROMDEVICE);
    dev->dma_size = count;
    bus_addr = pci_map_single(dev->pci_dev, buffer, count, 
                              dev->dma_dir);
    dev->dma_addr = bus_addr;

    /* Set up the device */
    writeb(dev->registers.command, DAD_CMD_DISABLEDMA);
    writeb(dev->registers.command, write ? DAD_CMD_WR : DAD_CMD_RD);
    writel(dev->registers.addr, cpu_to_le32(bus_addr));
    writel(dev->registers.len, cpu_to_le32(count));

    /* Start the operation */
    writeb(dev->registers.command, DAD_CMD_ENABLEDMA);
    return 0;
}

This function maps the buffer to be transferred and starts the device operation. The other half of the job must be done in the interrupt service routine, which would look something like this:

void dad_interrupt(int irq, void *dev_id, struct pt_regs *regs)
{
    struct dad_dev *dev = (struct dad_dev *) dev_id;

    /* Make sure it's really our device interrupting */

    /* Unmap the DMA buffer */
    pci_unmap_single(dev->pci_dev, dev->dma_addr, dev->dma_size,
		         dev->dma_dir);

    /* Only now is it safe to access the buffer, copy to user, etc. */
    ...
}

SPARC-based systems have traditionally included a Sun-designed bus called the SBus. This bus is beyond the scope of this chapter, but a quick mention is worthwhile. There is a set of functions (declared in ) for performing DMA mappings on the SBus; they have names like sbus_alloc_consistent and sbus_map_sg. In other words, the SBus DMA API looks almost exactly like the PCI interface. A detailed look at the function definitions will be required before working with DMA on the SBus, but the concepts will match those discussed earlier for the PCI bus.

The ISA bus allows for two kinds of DMA transfers: native DMA and ISA bus master DMA. Native DMA uses standard DMA-controller circuitry on the motherboard to drive the signal lines on the ISA bus. ISA bus master DMA, on the other hand, is handled entirely by the peripheral device. The latter type of DMA is rarely used and doesn't require discussion here because it is similar to DMA for PCI devices, at least from the driver's point of view. An example of an ISA bus master is the 1542 SCSI controller, whose driver is drivers/scsi/aha1542.c in the kernel sources.

The controller holds information about the DMA transfer, such as the direction, the memory address, and the size of the transfer. It also contains a counter that tracks the status of ongoing transfers. When the controller receives a DMA request signal, it gains control of the bus and drives the signal lines so that the device can read or write its data.

The peripheral device

The device must activate the DMA request signal when it's ready to transfer data. The actual transfer is managed by the DMAC; the hardware device sequentially reads or writes data onto the bus when the controller strobes the device. The device usually raises an interrupt when the transfer is over.

The device driver

The original DMA controller used in the PC could manage four "channels," each associated with one set of DMA registers. Four devices could store their DMA information in the controller at the same time. Newer PCs contain the equivalent of two DMAC devices:[53] the second controller (master) is connected to the system processor, and the first (slave) is connected to channel 0 of the second controller.[54]

[53]These circuits are now part of the motherboard's chipset, but a few years ago they were two separate 8237 chips.

The channel argument is a number between 0 and 7 or, more precisely, a positive number less than MAX_DMA_CHANNELS. On the PC, MAX_DMA_CHANNELS is defined as 8, to match the hardware. The name argument is a string identifying the device. The specified name appears in the file /proc/dma, which can be read by user programs.

The return value from request_dma is 0 for success and -EINVAL or -EBUSY if there was an error. The former means that the requested channel is out of range, and the latter means that another device is holding the channel.

We also suggest that you request the DMA channel after you've requested the interrupt line and that you release it before the interrupt. This is the conventional order for requesting the two resources; following the convention avoids possible deadlocks. Note that every device using DMA needs an IRQ line as well; otherwise, it couldn't signal the completion of data transfer.

In a typical case, the code for open looks like the following, which refers to our hypothetical dad module. The dad device as shown uses a fast interrupt handler without support for shared IRQ lines.

int dad_open (struct inode *inode, struct file *filp)
{
    struct dad_device *my_device; 

    /* ... */
    if ( (error = request_irq(my_device.irq, dad_interrupt,
                              SA_INTERRUPT, "dad", NULL)) )
        return error; /* or implement blocking open */

    if ( (error = request_dma(my_device.dma, "dad")) ) {
        free_irq(my_device.irq, NULL);
        return error; /* or implement blocking open */
    }
    /* ... */
    return 0;
}

void dad_close (struct inode *inode, struct file *filp)
{
    struct dad_device *my_device;

    /* ... */
    free_dma(my_device.dma);
    free_irq(my_device.irq, NULL);
    /* ... */
}

The driver needs to configure the DMA controller either when read or write is called, or when preparing for asynchronous transfers. This latter task is performed either at open time or in response to an ioctl command, depending on the driver and the policy it implements. The code shown here is the code that is typically called by the read or write device methods.

This subsection provides a quick overview of the internals of the DMA controller so you will understand the code introduced here. If you want to learn more, we'd urge you to read and some hardware manuals describing the PC architecture. In particular, we don't deal with the issue of 8-bit versus 16-bit data transfers. If you are writing device drivers for ISA device boards, you should find the relevant information in the hardware manuals for the devices.

Indicates whether the channel must read from the device (DMA_MODE_READ) or write to it (DMA_MODE_WRITE). A third mode exists, DMA_MODE_CASCADE, which is used to release control of the bus. Cascading is the way the first controller is connected to the top of the second, but it can also be used by true ISA bus-master devices. We won't discuss bus mastering here.

In addition to these functions, there are a number of housekeeping facilities that must be used when dealing with DMA devices:

A DMA channel can be disabled within the controller. The channel should be disabled before the controller is configured, to prevent improper operation (the controller is programmed via eight-bit data transfers, and thus none of the previous functions is executed atomically).

This function clears the DMA flip-flop. The flip-flop is used to control access to 16-bit registers. The registers are accessed by two consecutive 8-bit operations, and the flip-flop is used to select the least significant byte (when it is clear) or the most significant byte (when it is set). The flip-flop automatically toggles when 8 bits have been transferred; the programmer must clear the flip-flop (to set it to a known state) before accessing the DMA registers.

The only thing that remains to be done is to configure the device board. This device-specific task usually consists of reading or writing a few I/O ports. Devices differ in significant ways. For example, some devices expect the programmer to tell the hardware how big the DMA buffer is, and sometimes the driver has to read a value that is hardwired into the device. For configuring the board, the hardware manual is your only friend.

As with other parts of the kernel, both memory mapping and DMA have seen a number of changes over the years. This section describes the things a driver writer must take into account in order to write portable code.

Changes to Memory Management

The 2.3 development series saw major changes in the way memory management worked. The 2.2 kernel was quite limited in the amount of memory it could use, especially on 32-bit processors. With 2.4, those limits have been lifted; Linux is now able to manage all the memory that the processor is able to address. Some things have had to change to make all this possible; overall, however, the scale of the changes at the API level is surprisingly small.

Thus, for example, pte_page returned an unsigned long value instead of struct page *. The virt_to_page macro did not exist at all; if you needed to find a struct page entry you had to go directly to the memory map to get it. The macro MAP_NR would turn a logical address into an index in mem_map; thus, the current virt_to_page macro could be defined (and, in sysdep.h in the sample code, is defined) as follows:

struct page has also changed with time; in particular, the virtual field is present in Linux 2.4 only.

The vm_area_struct structure saw a number of changes in the 2.3 development series, and more in 2.1. These included the following:

• The vm_private_data field did not exist in Linux 2.2, so drivers had no way of storing their own information in the VMA. A number of them did so anyway, using the vm_pte field, but it would be safer to obtain the minor device number from vm_file and use it to retrieve the needed information.

In the 2.0 kernel, the init_mm structure was not exported to modules. Thus, a module that wished to access init_mm had to dig through the task table to find it (as part of the init process). When running on a 2.0 kernel, scullp finds init_mm with this bit of code:

 
static struct mm_struct *init_mm_ptr;
#define init_mm (*init_mm_ptr) /* to avoid ifdefs later */

static void retrieve_init_mm_ptr(void)
{
    struct task_struct *p;

    for (p = current ; (p = p->next_task) != current ; )
        if (p->pid == 0)
            break;

    init_mm_ptr = p->mm;
}

This chapter introduced the following symbols related to memory handling. The list doesn't include the symbols introduced in the first section, as that section is a huge list in itself and those symbols are rarely useful to device drivers.

#include

These functions convert between kernel virtual and bus addresses. Bus addresses must be used to talk to peripheral devices.

int pci_dma_supported(struct pci_dev *pdev, dma_addr_t mask);

void *pci_alloc_consistent(struct pci_dev *pdev, size_t size, dma_addr_t *bus_addr)

void pci_free_consistent(struct pci_dev *pdev, size_t size, void *cpuaddr, dma_handle_t bus_addr);

dma_addr_t pci_map_single(struct pci_dev *pdev, void *buffer, size_t size, int direction);

void pci_unmap_single(struct pci_dev *pdev, dma_addr_t bus_addr, size_t size, int direction);

void pci_sync_single(struct pci_dev *pdev, dma_handle_t bus_addr, size_t size, int direction)

Synchronizes a buffer that has a streaming mapping. This function must be used if the processor must access a buffer while the streaming mapping is in place (i.e., while the device owns the buffer).

The scatterlist structure describes an I/O operation that involves more than one buffer. The macros sg_dma_address and sg_dma_len may be used to extract bus addresses and buffer lengths to pass to the device when implementing scatter-gather operations.

pci_map_sg(struct pci_dev *pdev, struct scatterlist *list, int nents, int direction);

pci_unmap_sg(struct pci_dev *pdev, struct scatterlist *list, int nents, int direction);

pci_dma_sync_sg(struct pci_dev *pdev, struct scatterlist *sg, int nents, int direction)

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