上文从下到上的介绍了spi子系统，现在反过来从上到下的来介绍spi子系统的使用：
int spi_register_driver(struct spi_driver *sdrv)
{
sdrv->driver.bus = &spi_bus_type;
if (sdrv->probe)
sdrv->driver.probe = spi_drv_probe;
if (sdrv->remove)
sdrv->driver.remove = spi_drv_remove;
if (sdrv->shutdown)
sdrv->driver.shutdown = spi_drv_shutdown;
return driver_register(&sdrv->driver);
}
2.6内核的典型做法，不直接使用原始设备驱动，而是使用包装后的抽象设备驱动spi_driver，
间接与原始设备驱动建立联系，并最终通过调用driver_register来注册原始设备驱动(要充分理解2.6内核的抽象化思想)。
注：
以后我们也不会直接与原始设备打交道了，而是通过spi_device来间接操作spi设备了^_^
/**
* spi_write_then_read - SPI synchronous write followed by read
* @spi: device with which data will be exchanged
* @txbuf: data to be written (need not be dma-safe)
* @n_tx: size of txbuf, in bytes
* @rxbuf: buffer into which data will be read
* @n_rx: size of rxbuf, in bytes (need not be dma-safe)
*
* This performs a half duplex MicroWire style transaction with the
* device, sending txbuf and then reading rxbuf. The return value
* is zero for success, else a negative errno status code.
* This call may only be used from a context that may sleep.
*
* Parameters to this routine are always copied using a small buffer;
* performance-sensitive or bulk transfer code should instead use
* spi_{async,sync}() calls with dma-safe buffers.
*/
/*
* spi_write_then_read比较简单，容易说明spi的使用，用它来作例子比较合适
*/
int spi_write_then_read(struct spi_device *spi,
const u8 *txbuf, unsigned n_tx,
u8 *rxbuf, unsigned n_rx)
{
static DECLARE_MUTEX(lock);
int status;
struct spi_message message;
struct spi_transfer x[2];
u8 *local_buf;
/* Use preallocated DMA-safe buffer. We can't avoid copying here,
* (as a pure convenience thing), but we can keep heap costs
* out of the hot path ...
*/
if ((n_tx + n_rx) > SPI_BUFSIZ)//SPI_BUFSIZ == 32
return -EINVAL;
/* 这里初始化message结构里面用于存放struct spi_transfer指针的链表头 */
spi_message_init(&message);//INIT_LIST_HEAD(&message->transfers);
memset(x, 0, sizeof x);
/* 留意到没有：tx和rx个占一个工作添加到message的struct spi_transfer链表里，稍后被bitbang_work从链表里提出来处理(后面会讲到) */
if (n_tx) {
x[0].len = n_tx;
spi_message_add_tail(&x[0], &message);//list_add_tail(&t->transfer_list, &m->transfers);
}
if (n_rx) {
x[1].len = n_rx;
spi_message_add_tail(&x[1], &message);
}
/* ... unless someone else is using the pre-allocated buffer */
/* 如果有人在用这个预分配的缓存，那没办法了，只能再分配一个临时的，用完再释放掉 */
if (down_trylock(&lock)) {
local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
if (!local_buf)
return -ENOMEM;
} else
local_buf = buf;//否则就采用预分配的缓存吧
/* local_buf的前部分用来存放要发送的数据，后部分用来存放接收到的数据 */
memcpy(local_buf, txbuf, n_tx);
x[0].tx_buf = local_buf;
x[1].rx_buf = local_buf + n_tx;
/* do the i/o */
status = spi_sync(spi, &message);//同步io，等待spi传输完成，然后返回用户所接收的数据和状态
if (status == 0) {
memcpy(rxbuf, x[1].rx_buf, n_rx);
status = message.status;
}
if (x[0].tx_buf == buf)//如果使用的是预分配的缓存，释放锁好让其它人使用
up(&lock);
else
kfree(local_buf);//如果使用的是临时申请的缓存，释放之
return status;
}
/*
* spi_sync - blocking/synchronous SPI data transfers
* @spi: device with which data will be exchanged
* @message: describes the data transfers
*
* This call may only be used from a context that may sleep. The sleep
* is non-interruptible, and has no timeout. Low-overhead controller
* drivers may DMA directly into and out of the message buffers.
*
* Note that the SPI device's chip select is active during the message,
* and then is normally disabled between messages. Drivers for some
* frequently-used devices may want to minimize costs of selecting a chip,
* by leaving it selected in anticipation that the next message will go
* to the same chip. (That may increase power usage.)
*
* Also, the caller is guaranteeing that the memory associated with the
* message will not be freed before this call returns.
*
* The return value is a negative error code if the message could not be
* submitted, else zero. When the value is zero, then message->status is
* also defined: it's the completion code for the transfer, either zero
* or a negative error code from the controller driver.
*/
int spi_sync(struct spi_device *spi, struct spi_message *message)
{
DECLARE_COMPLETION_ONSTACK(done);//声明一个完成变量
int status;
message->complete = spi_complete;//spi传输完成后的回调函数
message->context = &done;
status = spi_async(spi, message);
if (status == 0)
wait_for_completion(&done);//等待spi传输，调用spi_complete后返回
message->context = NULL;
return status;
}
/*
* spi_async -- asynchronous SPI transfer
* @spi: device with which data will be exchanged
* @message: describes the data transfers, including completion callback
*
* This call may be used in_irq and other contexts which can't sleep,
* as well as from task contexts which can sleep.
*
* The completion callback is invoked in a context which can't sleep.
* Before that invocation, the value of message->status is undefined.
* When the callback is issued, message->status holds either zero (to
* indicate complete success) or a negative error code. After that
* callback returns, the driver which issued the transfer request may
* deallocate the associated memory; it's no longer in use by any SPI
* core or controller driver code.
*
* Note that although all messages to a spi_device are handled in
* FIFO order, messages may go to different devices in other orders.
* Some device might be higher priority, or have various "hard" access
* time requirements, for example.
*
* On detection of any fault during the transfer, processing of
* the entire message is aborted, and the device is deselected.
* Until returning from the associated message completion callback,
* no other spi_message queued to that device will be processed.
* (This rule applies equally to all the synchronous transfer calls,
* which are wrappers around this core asynchronous primitive.)
*/
static inline int
spi_async(struct spi_device *spi, struct spi_message *message)
{
printk("spi_async\n");
message->spi = spi;
return spi->master->transfer(spi, message);//调用spi_bitbang_transfer传输数据
}
/*
* spi_bitbang_transfer - default submit to transfer queue
*/
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);
/*
* 还记得spi_alloc_master函数中调用spi_master_set_devdata把struct s3c24xx_spi结构存放起来吧?
* 而struct spi_bitbang结构正是struct s3c24xx_spi结构所包含的第一个结构
*/
if (bitbang->shutdown)
return -ESHUTDOWN;
spin_lock_irqsave(&bitbang->lock, flags);
if (!spi->max_speed_hz)
status = -ENETDOWN;
else {
list_add_tail(&m->queue, &bitbang->queue);//把message加入到bitang的等待队列中
queue_work(bitbang->workqueue, &bitbang->work);//把bitbang-work加入bitbang->workqueue中，调度运行
}
spin_unlock_irqrestore(&bitbang->lock, flags);
return status;
}
好了，稍微总结一下：
spi的读写请求通过：spi_transfer->spi_message->spi_bitbang添加都bitbang->queue中，被bitbang->work反方向提取出来执行(后面会提到)。
通 过queue_work(bitbang->workqueue, &bitbang->work)把bitbang-work加入bitbang->workqueue后，在某个合适的时间， bitbang->work将被调度运行，bitbang_work函数将被调用：
/*
* SECOND PART ... simple transfer queue runner.
*
* This costs a task context per controller, running the queue by
* performing each transfer in sequence. Smarter hardware can queue
* several DMA transfers at once, and process several controller queues
* in parallel; this driver doesn't match such hardware very well.
*
* Drivers can provide word-at-a-time i/o primitives, or provide
* transfer-at-a-time ones to leverage dma or fifo hardware.
*/
static void bitbang_work(void *_bitbang)
{
struct spi_bitbang *bitbang = _bitbang;
unsigned long flags;
spin_lock_irqsave(&bitbang->lock, flags);
bitbang->busy = 1;//置忙标志
while (!list_empty(&bitbang->queue)) { //遍历bitbang->queue链表
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 *);
m = container_of(bitbang->queue.next, struct spi_message, queue);//获取spi_message结构
list_del_init(&m->queue);//把spi_messae从queue里删除
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;
list_for_each_entry (t, &m->transfers, transfer_list) {//从spi_message结构的transfers链表中获取spi_transfer结构
if (bitbang->shutdown) {
status = -ESHUTDOWN;
break;
}
/* override or restore speed and wordsize */
/* 本messae传输中，需要重设条件，调用setup_transfer函数 */
if (t->speed_hz || t->bits_per_word) {
setup_transfer = bitbang->setup_transfer;
if (!setup_transfer) {
status = -ENOPROTOOPT;
break;
}
}
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) { //片选激活spi
bitbang->chipselect(spi, BITBANG_CS_ACTIVE);
ndelay(nsecs);
}
cs_change = t->cs_change;
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;
status = bitbang->txrx_bufs(spi, t);//调用s3c24xx_spi_txrx开始传输数据
}
if (status != t->len) {
if (status > 0)
status = -EMSGSIZE;
break;
}
m->actual_length += status;
status = 0;
/* protocol tweaks before next transfer */
if (t->delay_usecs)
udelay(t->delay_usecs);
if (!cs_change)
continue;//不用重新片选，继续下一个message的传输
if (t->transfer_list.next == &m->transfers)//链表遍历完毕，退出循环
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);//需要重新片选的话...
ndelay(nsecs);
}
m->status = status;//所用spi_message传输完毕
m->complete(m->context);//应答返回变量，通知等待spi传输完毕的进程(具体来说就是spi_sync函数了)
/* restore speed and wordsize */
/* 前面重设过条件的，在这恢复之 */
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);//重新获取自旋锁，遍历工作者队列的下一个工作
}
bitbang->busy = 0;//处理完毕，清除忙标志
spin_unlock_irqrestore(&bitbang->lock, flags);
}
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;//发送指针
hw->rx = t->rx_buf;//接收指针
hw->len = t->len;//需要发送/接收的数据数目
hw->count = 0;//存放实际spi传输的数据数目
/* send the first byte */
writeb(hw_txbyte(hw, 0), hw->regs + S3C2410_SPTDAT);
wait_for_completion(&hw->done);
/*
* 非常有意思，这里虽然只发送第一字节，可是中断里会帮你发送完其它的字节(并接收数据)，
* 直到所有的数据发送完毕且所要接收的数据接收完毕(首要)才返回
*/
return hw->count;
}
static irqreturn_t s3c24xx_spi_irq(int irq, void *dev, struct pt_regs *regs)
{
struct s3c24xx_spi *hw = dev;
unsigned int spsta = readb(hw->regs + S3C2410_SPSTA);
unsigned int count = hw->count;
if (hw->len){
if (spsta & S3C2410_SPSTA_DCOL) {
dev_dbg(hw->dev, "data-collision\n");//检测冲突
complete(&hw->done);
goto irq_done;
}
if (!(spsta & S3C2410_SPSTA_READY)) {
dev_dbg(hw->dev, "spi not ready for tx?\n");//设备忙
complete(&hw->done);
goto irq_done;
}
hw->count++;
if (hw->rx)
hw->rx[count] = readb(hw->regs + S3C2410_SPRDAT);//接收数据
count++;
if (count < hw->len)
writeb(hw_txbyte(hw, count), hw->regs + S3C2410_SPTDAT);//发送其它数据(或空数据0xFF)
else
complete(&hw->done);//发送接收完毕，通知s3c24xx_spi_txrx函数
}
irq_done:
return IRQ_HANDLED;
}
static inline unsigned int hw_txbyte(struct s3c24xx_spi *hw, int count)
{
return hw->tx ? hw->tx[count] : 0xff;
//如果还有数据没接收完且要发送的数据经已发送完毕，发送空数据0xFF
}
注：
这里要注意的是：在spi提供的write_then_read函数中，写和读数据是分开两个阶段来进行的(写数据的时候不读数据;读数据的时候发送空数据0xff)。
总结：
简单的spi子系统大致就是这样，相对比较简单易懂，具体的应用可以参考一下代spi接口的触摸屏控制芯片驱动：
driver/input/touchscreen/ads7846.c
不过看明白它需要多花些时间了，因为毕竟这个驱动不仅和spi子系统打交道而且还和input子系统打交道，可不是那么容易应付的哦^_^
