本文将对Linux内核中的SPI驱动源码进行分析，包括SPI驱动框架的基本结构、各文件的作用、重要的数据结构和函数等。
SPI(Serial Peripheral Interface)是一种串行通信接口，常常用于与数字外设进行通信，如传感器、存储器、网卡等。Linux内核提供了SPI驱动框架，用于向上层应用程序提供SPI接口。本文将对该框架进行深入分析。
一、SPI驱动框架的基本结构
在Linux内核中，SPI驱动框架的代码位于/drivers/spi目录下。该目录下的源文件主要包括以下几个：
- spi.c：SPI总线设备驱动程序。
- spi-bitbang.c：位压缩SPI驱动程序。
- spi-dw-dma.c：SPI DMA驱动程序。
- spi-dw-mmio.c：SPI MMIO驱动程序。
- spi-fsl-dspi.c：FSL DSPI驱动程序。
- spi-imx.c：i.MX SPI驱动程序。
- spi-pl022.c：ARM PrimeCell PL022 SPI驱动程序。
- spi-s3c24xx.c：Samsung S3C24xx SPI驱动程序。
- spi-tegra20-sflash.c：Nvidia SPI Flash驱动程序。
- spi-ti-qspi.c：TI Quad SPI驱动程序。
这些驱动程序分别对应不同的SPI控制器。其中，spi.c是SPI驱动的核心文件，提供了SPI驱动框架的基本结构和主要函数。
二、spi.c的结构和作用
1、SPI驱动框架的初始化
SPI驱动框架的初始化主要在spi_init()函数中完成。该函数首先调用spi_bus_type_init()函数，注册SPI设备总线，然后向/sys/class下的spi_master目录中创建spi设备目录，最后调用probe_master()函数，搜索当前系统中的SPI设备并添加到bus层中。该函数的代码如下：
static int __init spi_init(void)
{
int status;
status = spi_bus_type_init();
if (status)
goto out;
status = class_register(&spi_master_class);
if (status)
goto bus_unregister;
status = spi_proc_init();
if (status)
goto class_unregister;
status = spi_gpio_register_board_info(NULL, 0);
if (status)
goto proc_cleanup;
status = spi_read_configfile();
if (status)
goto board_cleanup;
status = spi_master_probe_devices();
if (status)
goto board_cleanup;
printk(KERN_INFO "%s\n", spi_revision);
return 0;
board_cleanup:
spi_board_cleanup();
proc_cleanup:
spi_proc_cleanup();
class_unregister:
class_unregister(&spi_master_class);
bus_unregister:
spi_bus_type_exit();
out:
return status;
}2、SPI总线设备的添加和删除
当SPI总线设备（spi_master）被发现并添加到bus层时，会自动调用spi_master_add()函数，该函数会为SPI总线设备创建一个spi_master结构体，并将其添加到bus层中。
static int spi_master_add(struct spi_master *master)
{
struct device *dev = master->dev.parent;
struct spi_controller *ctlr = master->controller;
mutex_lock(&spi_mutex);
/*
• Implementation restriction: each SPI MASTER talks with other
• devices at constant signal levels, which don't change once
• operation starts. We don't provide any synchronization
• primitives that would be necessary for anything else.
*/
if (master->num_chipselect)
dev_warn(dev, "num_chipselect should == 1 when !is_slave\n");
if (!ctlr) {
ctlr = kzalloc(sizeof(struct spi_controller), GFP_KERNEL);
if (!ctlr) {
mutex_unlock(&spi_mutex);
return -ENOMEM;
}
ctlr->master = master;
master->controller = ctlr;
master->bits_per_word_mask = 0xFFFF;
if (!spi_controller_is_slave(master)) {
ctlr->max_speed_hz = spi_max_speed_hz(&ctlr->dev, master);
ctlr->setup = spi_master_setup;
ctlr->transfer_one = spi_transfer_one;
} else {
ctlr->max_speed_hz = master->max_speed_hz;
ctlr->setup = spi_slave_setup;
ctlr->transfer_one = spi_transfer_one_slave;
}
ctlr->bits_per_word_mask = master->bits_per_word_mask;
ctlr->flags = 0;
ctlr->mode_bits = master->mode_bits;
if (spi_controller_is_slave(master)) {
ctlr->mode_bits = 0;
ctlr->flags = SPI_CONTROLLER_SLAVE;
ctlr->bus_num = spi_slave_controller_id++;
idr_init(&ctlr->idr);
} else {
ctlr->mode_bits &= ctlr->controller_ops->get_mode_bits;
ctlr->flags |= SPI_CONTROLLER_MASTER;
ctlr->bus_num = spi_master_controller_id++;
}
dev_set_drvdata(dev, master);
dev_info(dev, "registered, %s%s%s%s%s\n",
ctlr->flags & SPI_CONTROLLER_MASTER ? "master" : "",
ctlr->flags & SPI_CONTROLLER_SLAVE ? "slave" : "",
ctlr->flags & SPI_CONTROLLER_CS_WORD ? "cs-high" : "",
ctlr->flags & SPI_CONTROLLER_NEEDS_POLL ? ", polling" : "",
ctlr->mode_bits ? ", mode " : "");
list_add_tail(&ctlr->list, &ctlr_list);
}
mutex_unlock(&spi_mutex);
return 0;
}当SPI总线设备从bus层中删除时，会自动调用spi_master_del()函数，该函数会删除spi_master结构体并释放相关资源。
static int spi_master_del(struct spi_master *master)
{
int my_bus_num = master->controller->bus_num;
mutex_lock(&spi_mutex);
if (my_bus_num < 0) { /* not yet attached */
mutex_unlock(&spi_mutex);
return -EINVAL;
}
if (!spi_controller_is_slave(master)) {
if (spi_master_get(master)) {
mutex_unlock(&spi_mutex);
return -EINVAL;
}
}
dev_info(&master->dev, "removed\n");
spi_controller_cleanup(master->controller);
kfree(master->controller);
return 0;
}三、重要的数据结构和函数
1、spi_device
spi_device结构体表示一个SPI设备，包含了设备的名称、片选信号、总线速率、数据位数、SPI传输设置等信息。该结构体被定义在include/linux/spi/spi.h头文件中，其定义如下：
struct spi_device {
struct device dev;
spinlock_t regs_lock;
const struct spi_device *next;
u32 max_speed_hz;
u8 chip_select;
u8 mode;
u8 bit_order;
u16 flags;
u32 irq;
struct mutex io_mutex;
/* RT signal stuff */
struct rt_mutex rt;
struct spi_controller *controller;
};spi_transfer结构体表示一次SPI传输，包含了传输的缓冲区、字节长度、传输设置等信息以及一个回调函数，用于在传输完成时通知上层应用程序。该结构体被定义在include/linux/spi/spi.h头文件中，其定义如下：
struct spi_transfer {
const void *tx_buf;
void *rx_buf;
unsigned len;
u32 speed_hz;
u16 delay_usecs;
u8 bits_per_word;
/* Used internally, by spi_sync() and the SPI core code */
u8 cs_change:1;
u8 do_read:1;
u8 tx_nbits:6; /* internal, for packing only */
u8 rx_nbits:6; /* internal, for packing only */
u16 rdy_for_tx:1;
u16 rdy_for_rx:1;
u16 cs_change_delay:14;
u16 large_buf:1;
u8 *tx_buf_wr;
u8 *rx_buf_wr;
void *private_data;
void (*complete)(void *private_data);
};3、spi_sync()
spi_sync()函数用于同步传输数据，该函数会等待传输完成并返回传输结果。该函数的代码如下：
int spi_sync(struct spi_device *spi, struct spi_transfer *t)
{
DECLARE_COMPLETION_ONSTACK(done);
int status;
t->complete = spi_complete;
t->private_data = &done;
t->rdy_for_tx = t->rdy_for_rx = 0;
t->cs_change = spi->controller->cs_gpiod ? 1 : 0;
status = spi_async(spi, t);
if (status == 0) {
wait_for_completion(&done);
status = t->status;
if (status == -ETIMEDOUT)
status = -EIO;
}
return status;
}4、spi_async()
spi_async()函数用于异步传输数据，该函数会启动SPI传输，并立即返回，不等待传输完成。该函数的代码如下：
int spi_async(struct spi_device *spi, struct spi_transfer *t)
{
struct spi_message msg;
int status;
memset(&msg, 0, sizeof(msg));
msg.spi = spi;
msg.complete = spi_complete;
msg.context = t;
msg.state = NULL;
msg.is_dma_mapped = false;
spi_prepare_message(&msg, t);
status = spi_async_locked(spi_get_parent_master(spi), &msg);
if (status == -EBUSY)
return -EAGAIN;
t->status = status;
if (msg.is_dma_mapped)
dma_unmap_sg(&spi->dev, msg.sgbuf, msg.nents, msg.direction);
if (msg.is_dma_mapped && msg.context && spi_need_dma_clean_up_on_error()) {
struct spi_controller *ctlr = spi->controller;
struct spi_transfer *xfer = msg.contexte
if (xfer->tx_buf && ctlr->dma_tx && ctlr->dma_tx->device->dev) {
dma_sync_sg_for_device(ctlr->dma_tx->device->dev,
msg.sgbuf,
msg.nents,
(ctlr->dma_tx_dir == DMA_MEM_TO_DEV) ?
DMA_TO_DEVICE : DMA_FROM_DEVICE);
dma_unmap_sg(ctlr->dma_tx->device->dev,
msg.sgbuf,
msg.nents,
ctlr->dma_tx_dir);
}
if (xfer->rx_buf && ctlr->dma_rx && ctlr->dma_rx->device->dev) {
dma_sync_sg_for_device(ctlr->dma_rx->device->dev,
msg.sgbuf,
msg.nents,
(ctlr->dma_rx_dir == DMA_MEM_TO_DEV) ?
DMA_TO_DEVICE : DMA_FROM_DEVICE);
dma_unmap_sg(ctlr->dma_rx->device->dev,
msg.sgbuf,
msg.nents,
ctlr->dma_rx_dir);
}
}
if (status == -EINPROGRESS || status == -EBUSY) {
status = 0;
} else if (unlikely(status)) {
dev_err(spi->dev.parent, "%s: spi_sync failed with status %d\n",
func, status);
}
return status;
}四、总结
本文分析了Linux内核中的SPI驱动源码，介绍了SPI驱动框架的基本结构、spi.c的结构和作用以及SPI驱动中的重要数据结构和函数。通读本文后，读者应该了解了SPI设备的工作原理和Linux内核中提供的SPI驱动框架的实现方式，理解了相关代码的运行过程和涉及的系统调用，有助于读者熟练掌握SPI驱动的编写技巧。
