Linux驱动分析之SPI设备

Posted 2021-12-08 嵌入式软件开发交流

前言

前面我们对SPI控制器驱动进行了分析，接下来来分析SPI设备驱动。我们以DS1302驱动作为分析对象。DS1302是一款RTC芯片，估计很多人在学单片机时用到过。RTC芯片算是比较简单的，也方便分析理解。

SPI设备驱动分析

内核：4.20

芯片：DS1302 RTC

下面的代码分析主要都在注释中，会按照驱动中函数的执行顺序分析。我们不需要去关心RTC的具体内容，因为它主要是一些读写寄存器的过程。应主要关注SPI的通信。

(1) 装载和卸载函数

//dts匹配表
static const struct of_device_id ds1302_dt_ids[] = {
  { .compatible = "maxim,ds1302", },
  { /* sentinel */ }
};
MODULE_DEVICE_TABLE(of, ds1302_dt_ids);


static struct spi_driver ds1302_driver = {
  .driver.name  = "rtc-ds1302",
  .driver.of_match_table = of_match_ptr(ds1302_dt_ids),
  .probe    = ds1302_probe,
  .remove    = ds1302_remove,
};
//封装了spi_register_driver和spi_unregister_driver
module_spi_driver(ds1302_driver);

module_spi_driver宏定义在 include/linux/spi/spi.h， 具体看一下源码

#define module_spi_driver(__spi_driver) \\
  module_driver(__spi_driver, spi_register_driver, \\
      spi_unregister_driver)

#define module_driver(__driver, __register, __unregister, ...) \\
static int __init __driver##_init(void) \\
{ \\
  return __register(&(__driver) , ##__VA_ARGS__); \\
} \\
module_init(__driver##_init); \\
static void __exit __driver##_exit(void) \\
{ \\
  __unregister(&(__driver) , ##__VA_ARGS__); \\
} \\
module_exit(__driver##_exit);

所以只是对 spi_register_driver 和 spi_unregister_driver 做了封装。

(2) probe()函数

static int ds1302_probe(struct spi_device *spi)
{
  struct rtc_device  *rtc;
  u8    addr;
  u8    buf[4];
  u8    *bp;
  int    status;


    //检查是不是8bit传输
  if (spi->bits_per_word && (spi->bits_per_word != 8)) {
    dev_err(&spi->dev, "bad word length\\n");
    return -EINVAL;
  } else if (spi->max_speed_hz > 2000000) {//检查最大速率
    dev_err(&spi->dev, "speed is too high\\n");
    return -EINVAL;
  } else if (spi->mode & SPI_CPHA) { 
    dev_err(&spi->dev, "bad mode\\n");
    return -EINVAL;
  }
    //使用spi读写一下寄存器，检查是否可以写(DS1302有个寄存器是设置写保护的)
  addr = RTC_ADDR_CTRL << 1 | RTC_CMD_READ;
  status = spi_write_then_read(spi, &addr, sizeof(addr), buf, 1);
    ......


  spi_set_drvdata(spi, spi);
    //注册rtc
  rtc = devm_rtc_device_register(&spi->dev, "ds1302",
      &ds1302_rtc_ops, THIS_MODULE);


  return 0;
}

(3) RTC设置和读取函数

//读取时间
static int ds1302_rtc_get_time(struct device *dev, struct rtc_time *time)
{
  struct spi_device  *spi = dev_get_drvdata(dev);
  u8    addr = RTC_CLCK_BURST << 1 | RTC_CMD_READ;
  u8    buf[RTC_CLCK_LEN - 1];
  int    status;


   //spi读取时间
  status = spi_write_then_read(spi, &addr, sizeof(addr),
      buf, sizeof(buf));
  if (status < 0)
    return status;


  /* Decode the registers */
  time->tm_sec = bcd2bin(buf[RTC_ADDR_SEC]);
  time->tm_min = bcd2bin(buf[RTC_ADDR_MIN]);
  time->tm_hour = bcd2bin(buf[RTC_ADDR_HOUR]);
  time->tm_wday = buf[RTC_ADDR_DAY] - 1;
  time->tm_mday = bcd2bin(buf[RTC_ADDR_DATE]);
  time->tm_mon = bcd2bin(buf[RTC_ADDR_MON]) - 1;
  time->tm_year = bcd2bin(buf[RTC_ADDR_YEAR]) + 100;


  return 0;
}
//设置时间
static int ds1302_rtc_set_time(struct device *dev, struct rtc_time *time)
{
  struct spi_device  *spi = dev_get_drvdata(dev);
  u8    buf[1 + RTC_CLCK_LEN];
  u8    *bp;
  int    status;


  /* Enable writing */
  bp = buf;
  *bp++ = RTC_ADDR_CTRL << 1 | RTC_CMD_WRITE;
  *bp++ = RTC_CMD_WRITE_ENABLE;
    //关闭写保护
  status = spi_write_then_read(spi, buf, 2,
      NULL, 0);
  if (status)
    return status;


  /* Write registers starting at the first time/date address. */
  bp = buf;
  *bp++ = RTC_CLCK_BURST << 1 | RTC_CMD_WRITE;


  *bp++ = bin2bcd(time->tm_sec);
  *bp++ = bin2bcd(time->tm_min);
  *bp++ = bin2bcd(time->tm_hour);
  *bp++ = bin2bcd(time->tm_mday);
  *bp++ = bin2bcd(time->tm_mon + 1);
  *bp++ = time->tm_wday + 1;
  *bp++ = bin2bcd(time->tm_year % 100);
  *bp++ = RTC_CMD_WRITE_DISABLE;


  //只有写，没有读
  return spi_write_then_read(spi, buf, sizeof(buf),
      NULL, 0);
}


static const struct rtc_class_ops ds1302_rtc_ops = {
  .read_time  = ds1302_rtc_get_time,
  .set_time  = ds1302_rtc_set_time,
};

上面读取和设置都是调用spi_write_then_read来进行Spi通信，这个是Linux帮我们封装好的接口函数。看一下具体实现：

int spi_write_then_read(struct spi_device *spi,
    const void *txbuf, unsigned n_tx,
    void *rxbuf, unsigned n_rx)
{
  static DEFINE_MUTEX(lock);


  int      status;
  struct spi_message  message;
  struct spi_transfer  x[2];
  u8      *local_buf;


  if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
    local_buf = kmalloc(max((unsigned)SPI_BUFSIZ, n_tx + n_rx),
            GFP_KERNEL | GFP_DMA);
    if (!local_buf)
      return -ENOMEM;
  } else {
    local_buf = buf;
  }
    //初始化spi_message
  spi_message_init(&message);
    //将要传的数据放到spi_transfer，然后追加到spi_message
  memset(x, 0, sizeof(x));
  if (n_tx) {
    x[0].len = n_tx;
    spi_message_add_tail(&x[0], &message);
  }
  if (n_rx) {
    x[1].len = n_rx;
    spi_message_add_tail(&x[1], &message);
  }


  memcpy(local_buf, txbuf, n_tx);
  x[0].tx_buf = local_buf;
  x[1].rx_buf = local_buf + n_tx;


  //进行SPI发送
  status = spi_sync(spi, &message);
  if (status == 0)
    memcpy(rxbuf, x[1].rx_buf, n_rx);


  if (x[0].tx_buf == buf)
    mutex_unlock(&lock);
  else
    kfree(local_buf);


  return status;
}

spi_sync最终会调用spi_master->transfer()；传递给spi_sync函数的参数中有spi_device, 而spi_device中又包含spi_master的指针。所以就能找到了对应的spi控制器进行数据发送。

总结

大部分的SPI设备驱动框架都差不多，大家可以配合下面两篇文章一起看。这样更能理解。我们会发现，SPI设备驱动内容其实就是使用SPI控制器(spi_master)去对具体芯片设备进行读写。