分类： 嵌入式
2013-07-09 17:33:36
Linux的SPI子系统采用主机驱动和外设驱动分离的思想，首先主机SPI控制器是一种平台设备，因此它以platform的方式注册进内核，外设的信息是以boardinfo形式静态定义的，在创建spi_master时，会根据外设的bus_num和主机的bus_num是否相等，来选择是否将该外设挂接在该SPI主控制器下。先看SPI子系统中几个关键的数据结构：
struct spi_master用来描述一个SPI主控制器
struct spi_master {
struct device dev;
s16 bus_num; /*总线编号*/
u16 num_chipselect;/*支持的外设数量*/
u16 dma_alignment;
int (*transfer)(struct spi_device *spi, struct spi_message *mesg);/*用于将消息添加到队列*/
void (*cleanup)(struct spi_device *spi);
};
struct spi_device用来描述一个SPI从设备
struct spi_device {
struct device dev;
struct spi_master *master; /*从设备所属的SPI主控器*/
u32 max_speed_hz; /*最大传输频率*/
u8 chip_select; /*片选号，用于区别其他从设备*/
u8 mode; /*传输模式*/
/*各个mode的定义*/
#define SPI_CPHA 0x01 /* clock phase */
#define SPI_CPOL 0x02 /* clock polarity */
#define SPI_MODE_0 (0|0) /* (original MicroWire) */
#define SPI_MODE_1 (0|SPI_CPHA)
#define SPI_MODE_2 (SPI_CPOL|0)
#define SPI_MODE_3 (SPI_CPOL|SPI_CPHA)
#define SPI_CS_HIGH 0x04 /* chipselect active high? */
#define SPI_LSB_FIRST 0x08 /* per-word bits-on-wire */
#define SPI_3WIRE 0x10 /* SI/SO signals shared */
#define SPI_LOOP 0x20 /* loopback mode */
u8 bits_per_word; /*每个字的比特数*/
int irq; /*所使用的中断*/
void *controller_state;
void *controller_data;
char modalias[32]; /*设备名，在和从设备驱动匹配时会用到*/
};
struct spi_driver用来描述一个SPI从设备的驱动，它的形式和struct platform_driver是一致的
struct spi_driver {
int (*probe)(struct spi_device *spi);
int (*remove)(struct spi_device *spi);
void (*shutdown)(struct spi_device *spi);
int (*suspend)(struct spi_device *spi, pm_message_t mesg);
int (*resume)(struct spi_device *spi);
struct device_driver driver;
};
SPI子系统初始化的第一步就是将SPI总线注册进内核，并且在/sys下创建一个spi_master的类，以后注册的从设备都将挂接在该总线下
static int __init spi_init(void)
{
int status;
buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
if (!buf) {
status = -ENOMEM;
goto err0;
}
status = bus_register(&spi_bus_type);//注册SPI总线
if (status < 0)
goto err1;
status = class_register(&spi_master_class);//注册spi_master类
if (status < 0)
goto err2;
return 0;
err2:
bus_unregister(&spi_bus_type);
err1:
kfree(buf);
buf = NULL;
err0:
return status;
}
我们来看spi_bus_type的定义
struct bus_type spi_bus_type = {
.name = "spi",
.dev_attrs = spi_dev_attrs,
.match = spi_match_device,
.uevent = spi_uevent,
.suspend = spi_suspend,
.resume = spi_resume,
};
来看挂接在SPI总线下的从设备和从设备驱动是如何匹配的，也就是spi_match_device函数
static const struct spi_device_id *spi_match_id(const struct spi_device_id *id,
const struct spi_device *sdev)
{
while (id->name[0]) {
if (!strcmp(sdev->modalias, id->name))
return id;
id++;
}
return NULL;
}
static int spi_match_device(struct device *dev, struct device_driver *drv)
{
const struct spi_device *spi = to_spi_device(dev);
const struct spi_driver *sdrv = to_spi_driver(drv);
/* Attempt an OF style match */
if (of_driver_match_device(dev, drv))
return 1;
//如果驱动中定义id_table,查看driver结构体id_table->name与 平台设备结构体dev->modalias 是否相同
if (sdrv->id_table)
return !!spi_match_id(sdrv->id_table, spi);
//如果驱动中没有定义id_table,则对比drv->name 与 dev->modalias是否相同
return strcmp(spi->modalias, drv->name) == 0;
}
这里可以看到是将struct device_driver中的name字段与struct spi_device中的modalias字段进行匹配
这里已经完成了SPI子系统初始化的第一步，也就是注册SPI总线，这一步是和平台无关的，第二步是和平台相关的初始化，下一节再做介绍。
本节以spidev设备驱动为例，来阐述SPI数据传输的过程。spidev是内核中一个通用的设备驱动，我们注册的从设备都可以使用该驱动，只需在注册 时将从设备的modalias字段设置为"spidev",这样才能和spidev驱动匹配成功。我们要传输的数据有时需要分为一段一段的(比如先发送， 后读取，就需要两个字段)，每个字段都被封装成一个transfer,N个transfer可以被添加到message中，作为一个消息包进行传输。当用 户发出传输数据的请求时，message并不会立刻传输到从设备，而是由之前定义的transfer()函数将message放入一个等待队列中，这些 message会以FIFO的方式有workqueue调度进行传输，这样能够避免SPI从设备同一时间对主SPI控制器的竞争。和之前一样，还是习惯先 画一张图来描述数据传输的主要过程。
在使用spidev设备驱动时，需要先初始化spidev. spidev是以字符设备的形式注册进内核的。
s3c24xx平台下的transfer函数是在bitbang_start()函数中定义的，为bitbang_transfer()
这里可以看到transfer函数不负责实际的数据传输，而是将message添加到 等待队列中。同样在spi_bitbang_start()中，有这样一个定义INIT_WORK(&bitbang->work, bitbang_work);因此bitbang_work()函数会被调度运行，类似于底半部机制
static int __init spidev_init(void)
{
int status;
/* Claim our 256 reserved device numbers. Then register a class
* that will key udev/mdev to add/remove /dev nodes. Last, register
* the driver which manages those device numbers.
*/
BUILD_BUG_ON(N_SPI_MINORS > 256);
/*将spidev作为字符设备注册*/
status = register_chrdev(SPIDEV_MAJOR, "spi", &spidev_fops);
if (status < 0)
return status;
/*创建spidev类*/
spidev_class = class_create(THIS_MODULE, "spidev");
if (IS_ERR(spidev_class)) {
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
return PTR_ERR(spidev_class);
}
/*注册spidev的driver,可与modalias字段为"spidev"的spi_device匹配*/
status = spi_register_driver(&spidev_spi);
if (status < 0) {
class_destroy(spidev_class);
unregister_chrdev(SPIDEV_MAJOR, spidev_spi.driver.name);
}
return status;
}
与相应的从设备匹配成功后，则调用spidev中的probe函数
static int spidev_probe(struct spi_device *spi)
{
struct spidev_data *spidev;
int status;
unsigned long minor;
/* Allocate driver data */
spidev = kzalloc(sizeof(*spidev), GFP_KERNEL);
if (!spidev)
return -ENOMEM;
/* Initialize the driver data */
spidev->spi = spi;//设定spi
spin_lock_init(&spidev->spi_lock);
mutex_init(&spidev->buf_lock);
INIT_LIST_HEAD(&spidev->device_entry);
/* If we can allocate a minor number, hook up this device.
* Reusing minors is fine so long as udev or mdev is working.
*/
mutex_lock(&device_list_lock);
minor = find_first_zero_bit(minors, N_SPI_MINORS);//寻找没被占用的次设备号
if (minor < N_SPI_MINORS) {
struct device *dev;
/*计算设备号*/
spidev->devt = MKDEV(SPIDEV_MAJOR, minor);
/*在spidev_class下创建设备*/
dev = device_create(spidev_class, &spi->dev, spidev->devt,
spidev, "spidev%d.%d",
spi->master->bus_num, spi->chip_select);
status = IS_ERR(dev) ? PTR_ERR(dev) : 0;
} else {
dev_dbg(&spi->dev, "no minor number available!\n");
status = -ENODEV;
}
if (status == 0) {
set_bit(minor, minors);//将minors的相应位置位，表示该位对应的次设备号已被占用
list_add(&spidev->device_entry, &device_list);//将创建的spidev添加到device_list
}
mutex_unlock(&device_list_lock);
if (status == 0)
spi_set_drvdata(spi, spidev);
else
kfree(spidev);
return status;
}
然后就可以利用spidev模块提供的接口来实现主从设备之间的数据传输了。我们以spidev_write()函数为例来分析数据传输的过程，实际上spidev_read()和其是差不多的，只是前面的一些步骤不一样，可以参照上图。
static ssize_t
spidev_write(struct file *filp, const char __user *buf,
size_t count, loff_t *f_pos)
{
struct spidev_data *spidev;
ssize_t status = 0;
unsigned long missing;
/* chipselect only toggles at start or end of operation */
if (count > bufsiz)
return -EMSGSIZE;
spidev = filp->private_data;
mutex_lock(&spidev->buf_lock);
//将用户要发送的数据拷贝到spidev->buffer
missing = copy_from_user(spidev->buffer, buf, count);
if (missing == 0) {//全部拷贝成功，则调用spidev_sysn_write()
status = spidev_sync_write(spidev, count);
} else
status = -EFAULT;
mutex_unlock(&spidev->buf_lock);
return status;
}
static inline ssize_t
spidev_sync_write(struct spidev_data *spidev, size_t len)
{
struct spi_transfer t = {//设置传输字段
.tx_buf = spidev->buffer,
.len = len,
};
struct spi_message m;//创建message
spi_message_init(&m);
spi_message_add_tail(&t, &m);//将transfer添加到message中
return spidev_sync(spidev, &m);
}
我们来看看struct spi_transfer和struct spi_message是如何定义的
struct spi_transfer {
/* it's ok if tx_buf == rx_buf (right?)
* for MicroWire, one buffer must be null
* buffers must work with dma_*map_single() calls, unless
* spi_message.is_dma_mapped reports a pre-existing mapping
*/
const void *tx_buf;//发送缓冲区
void *rx_buf;//接收缓冲区
unsigned len; //传输数据的长度
dma_addr_t tx_dma;
dma_addr_t rx_dma;
unsigned cs_change:1; //该位如果为1，则表示当该transfer传输完后，改变片选信号
u8 bits_per_word;//字比特数
u16 delay_usecs; //传输后的延时
u32 speed_hz; //指定的时钟
struct list_head transfer_list;//用于将该transfer链入message
};
struct spi_message {
struct list_head transfers;//用于链接spi_transfer
struct spi_device *spi; //指向目的从设备
unsigned is_dma_mapped:1;
/* REVISIT: we might want a flag affecting the behavior of the
* last transfer ... allowing things like "read 16 bit length L"
* immediately followed by "read L bytes". Basically imposing
* a specific message scheduling algorithm.
*
* Some controller drivers (message-at-a-time queue processing)
* could provide that as their default scheduling algorithm. But
* others (with multi-message pipelines) could need a flag to
* tell them about such special cases.
*/
/* completion is reported through a callback */
void (*complete)(void *context);//用于异步传输完成时调用的回调函数
void *context; //回调函数的参数
unsigned actual_length; //实际传输的长度
int status;
/* for optional use by whatever driver currently owns the
* spi_message ... between calls to spi_async and then later
* complete(), that's the spi_master controller driver.
*/
struct list_head queue; //用于将该message链入bitbang等待队列
void *state;
};
继续跟踪源码，进入spidev_sync(),从这一步开始，read和write就完全一样了
static ssize_t
spidev_sync(struct spidev_data *spidev, struct spi_message *message)
{
DECLARE_COMPLETION_ONSTACK(done);
int status;
message->complete = spidev_complete;//设置回调函数
message->context = &done;
spin_lock_irq(&spidev->spi_lock);
if (spidev->spi == NULL)
status = -ESHUTDOWN;
else
status = spi_async(spidev->spi, message);//调用spi核心层的函数spi_async()
spin_unlock_irq(&spidev->spi_lock);
if (status == 0) {
wait_for_completion(&done);
status = message->status;
if (status == 0)
status = message->actual_length;
}
return status;
}
static inline int
spi_async(struct spi_device *spi, struct spi_message *message)
{
message->spi = spi;
/*调用master的transfer函数将message放入等待队列*/
return spi->master->transfer(spi, message);
}
int spi_bitbang_transfer(struct spi_device *spi, struct spi_message *m)
{
struct spi_bitbang *bitbang;
unsigned long flags;
int status = 0;
m->actual_length = 0;
m->status = -EINPROGRESS;
bitbang = spi_master_get_devdata(spi->master);
spin_lock_irqsave(&bitbang->lock, flags);
if (!spi->max_speed_hz)
status = -ENETDOWN;
else {
list_add_tail(&m->queue, &bitbang->queue);//将message添加到bitbang的等待队列
queue_work(bitbang->workqueue, &bitbang->work);//调度运行work
}
spin_unlock_irqrestore(&bitbang->lock, flags);
return status;
}
static void bitbang_work(struct work_struct *work)
{
struct spi_bitbang *bitbang =
container_of(work, struct spi_bitbang, work);//获取bitbang
unsigned long flags;
spin_lock_irqsave(&bitbang->lock, flags);
bitbang->busy = 1;
while (!list_empty(&bitbang->queue)) {//等待队列不为空
struct spi_message *m;
struct spi_device *spi;
unsigned nsecs;
struct spi_transfer *t = NULL;
unsigned tmp;
unsigned cs_change;
int status;
int (*setup_transfer)(struct spi_device *,
struct spi_transfer *);
/*取出等待队列中的的第一个message*/
m = container_of(bitbang->queue.next, struct spi_message,
queue);
list_del_init(&m->queue);//将message从队列中删除
spin_unlock_irqrestore(&bitbang->lock, flags);
/* FIXME this is made-up ... the correct value is known to
* word-at-a-time bitbang code, and presumably chipselect()
* should enforce these requirements too?
*/
nsecs = 100;
spi = m->spi;
tmp = 0;
cs_change = 1;
status = 0;
setup_transfer = NULL;
/*遍历message中的所有传输字段,逐一进行传输*/
list_for_each_entry (t, &m->transfers, transfer_list) {
/* override or restore speed and wordsize */
if (t->speed_hz || t->bits_per_word) {
setup_transfer = bitbang->setup_transfer;
if (!setup_transfer) {
status = -ENOPROTOOPT;
break;
}
}
/*调用setup_transfer根据transfer中的信息进行时钟、字比特数的设定*/
if (setup_transfer) {
status = setup_transfer(spi, t);
if (status < 0)
break;
}
/* set up default clock polarity, and activate chip;
* this implicitly updates clock and spi modes as
* previously recorded for this device via setup().
* (and also deselects any other chip that might be
* selected ...)
*/
if (cs_change) {//使能外设的片选
bitbang->chipselect(spi, BITBANG_CS_ACTIVE);
ndelay(nsecs);
}
cs_change = t->cs_change;//这里确定进行了这个字段的传输后是否要改变片选状态
if (!t->tx_buf && !t->rx_buf && t->len) {
status = -EINVAL;
break;
}
/* transfer data. the lower level code handles any
* new dma mappings it needs. our caller always gave
* us dma-safe buffers.
*/
if (t->len) {
/* REVISIT dma API still needs a designated
* DMA_ADDR_INVALID; ~0 might be better.
*/
if (!m->is_dma_mapped)
t->rx_dma = t->tx_dma = 0;
/*调用针对于平台的传输函数txrx_bufs*/
status = bitbang->txrx_bufs(spi, t);
}
if (status > 0)
m->actual_length += status;
if (status != t->len) {
/* always report some kind of error */
if (status >= 0)
status = -EREMOTEIO;
break;
}
status = 0;
/* protocol tweaks before next transfer */
/*如果要求在传输完一个字段后进行delay,则进行delay*/
if (t->delay_usecs)
udelay(t->delay_usecs);
if (!cs_change)
continue;
/*最后一个字段传输完毕了，则跳出循环*/
if (t->transfer_list.next == &m->transfers)
break;
/* sometimes a short mid-message deselect of the chip
* may be needed to terminate a mode or command
*/
ndelay(nsecs);
bitbang->chipselect(spi, BITBANG_CS_INACTIVE);
ndelay(nsecs);
}
m->status = status;
m->complete(m->context);
/* restore speed and wordsize */
if (setup_transfer)
setup_transfer(spi, NULL);
/* normally deactivate chipselect ... unless no error and
* cs_change has hinted that the next message will probably
* be for this chip too.
*/
if (!(status == 0 && cs_change)) {
ndelay(nsecs);
bitbang->chipselect(spi, BITBANG_CS_INACTIVE);
ndelay(nsecs);
}
spin_lock_irqsave(&bitbang->lock, flags);
}
bitbang->busy = 0;
spin_unlock_irqrestore(&bitbang->lock, flags);
}
只要bitbang->queue等待队列不为空，就表示相应的SPI主控制器上还有 传输任务没有完成，因此bitbang_work()会被不断地调度执行。 bitbang_work()中的工作主要是两个循环，外循环遍历等待队列中的message,内循环遍历message中的transfer,在 bitbang_work()中，传输总是以transfer为单位的。当选定了一个transfer后，便会调用transfer_txrx()函数， 进行实际的数据传输，显然这个函数是针对于平台的SPI控制器而实现的，在s3c24xx平台中，该函数为s3c24xx_spi_txrx();
static int s3c24xx_spi_txrx(struct spi_device *spi, struct spi_transfer *t)
{
struct s3c24xx_spi *hw = to_hw(spi);
dev_dbg(&spi->dev, "txrx: tx %p, rx %p, len %d\n",
t->tx_buf, t->rx_buf, t->len);
hw->tx = t->tx_buf;//获取发送缓冲区
hw->rx = t->rx_buf;//获取读取缓存区
hw->len = t->len; //获取数据长度
hw->count = 0;
init_completion(&hw->done);//初始化完成量
/* send the first byte */
/*只发送第一个字节，其他的在中断中发送(读取)*/
writeb(hw_txbyte(hw, 0), hw->regs + S3C2410_SPTDAT);
wait_for_completion(&hw->done);
return hw->count;
}
static inline unsigned int hw_txbyte(struct s3c24xx_spi *hw, int count)
{
/*如果tx不为空，也就是说当前是从主机向从机发送数据，则直接将tx[count]发送过去，
如果tx为空，也就是说当前是从从机向主机发送数据，则向从机写入0*/
return hw->tx ? hw->tx[count] : 0;
}
负责SPI数据传输的中断函数：
static irqreturn_t s3c24xx_spi_irq(int irq, void *dev)
{
struct s3c24xx_spi *hw = dev;
unsigned int spsta = readb(hw->regs + S3C2410_SPSTA);
unsigned int count = hw->count;
/*冲突检测*/
if (spsta & S3C2410_SPSTA_DCOL) {
dev_dbg(hw->dev, "data-collision\n");
complete(&hw->done);
goto irq_done;
}
/*设备忙检测*/
if (!(spsta & S3C2410_SPSTA_READY)) {
dev_dbg(hw->dev, "spi not ready for tx?\n");
complete(&hw->done);
goto irq_done;
}
hw->count++;
if (hw->rx)//读取数据到缓冲区
hw->rx[count] = readb(hw->regs + S3C2410_SPRDAT);
count++;
if (count < hw->len)//向从机写入数据
writeb(hw_txbyte(hw, count), hw->regs + S3C2410_SPTDAT);
else//count == len，一个字段发送完成，唤醒完成量
complete(&hw->done);
irq_done:
return IRQ_HANDLED;
}
这里可以看到一点，即使tx为空，也就是说用户申请的是从从设备读取数据，也要不断地向从设备写入数据，只不过写入从设备的是无效数据(0)，这样做得目的是为了维持SPI总线上的时钟。至此，SPI框架已分析完毕。