linux 驱动 i2c

http://www.cnblogs.com/swnuwangyun/p/4233460.html

在分析snd_soc_codec_driver的结构体时，发现有些芯片的驱动中定义了字段reg_word_size, reg_cache_size, reg_cache_default，但没有定义read/write，如wm8993:

static struct snd_soc_codec_driver soc_codec_dev_wm8993 = {
.probe =     wm8993_probe,
.remove =     wm8993_remove,
.suspend =    wm8993_suspend,
.resume =    wm8993_resume,
.set_bias_level = wm8993_set_bias_level,
.reg_cache_size = ARRAY_SIZE(wm8993_reg_defaults),
.reg_word_size = sizeof(u16),
.reg_cache_default = wm8993_reg_defaults,
.volatile_register = wm8993_volatile,
};

而另外的一些芯片驱动中，则定义了字段read, write，如wm8400和cx20442:

static struct snd_soc_codec_driver soc_codec_dev_wm8400 = {
.probe =    wm8400_codec_probe,
.remove =    wm8400_codec_remove,
.suspend =    wm8400_suspend,
.resume =    wm8400_resume,
.read = wm8400_read,
.write = wm8400_write,
.set_bias_level = wm8400_set_bias_level,
};

static struct snd_soc_codec_driver cx20442_codec_dev = {
.probe =     cx20442_codec_probe,
.remove =     cx20442_codec_remove,
.reg_cache_default = &cx20442_reg,
.reg_cache_size = 1,
.reg_word_size = sizeof(u8),
.read = cx20442_read_reg_cache,
.write = cx20442_write,
.dapm_widgets = cx20442_dapm_widgets,
.num_dapm_widgets = ARRAY_SIZE(cx20442_dapm_widgets),
.dapm_routes = cx20442_audio_map,
.num_dapm_routes = ARRAY_SIZE(cx20442_audio_map),
};

猜测read/write应该和snd_soc_read/write有关，在soc_core.c中注意到snd_soc_read的源码：

unsigned int snd_soc_read(struct snd_soc_codec *codec, unsigned int reg)
{
unsigned int ret;

ret = codec->read(codec, reg);
dev_dbg(codec->dev, "read %x => %x\n", reg, ret);
trace_snd_soc_reg_read(codec, reg, ret);

return ret;
}
EXPORT_SYMBOL_GPL(snd_soc_read);

因此，要想使用snd_soc_read，必须要设置codec->read回调函数，当我们提供了read/write函数时，在snd_soc_register_codec函数中会设置codec->read

int snd_soc_register_codec(struct device *dev,
const struct snd_soc_codec_driver *codec_drv,
struct snd_soc_dai_driver *dai_drv,
int num_dai)
{
...

codec->write = codec_drv->write;
codec->read = codec_drv->read;
codec->volatile_register = codec_drv->volatile_register;

OK，这里和我们soc_codec_dev_wm8400以及cx20442_codec_dev都对应的上，snd_soc_read最终会调用我们提供的回调函数。

问题来了，soc_codec_dev_wm8993中并没有提供回调函数，snd_soc_read是如何工作的呢？刚开始百思不得其解，肯定会有什么地方设置了codec->read！继续找代码，终于在soc_cache.c中找到了一个神奇的函数：snd_soc_codec_set_cache_io，看看代码片段：

int snd_soc_codec_set_cache_io(struct snd_soc_codec *codec,
int addr_bits, int data_bits,
enum snd_soc_control_type control)
{
...

codec->write = io_types[i].write;
codec->read = io_types[i].read;
codec->bulk_write_raw = snd_soc_hw_bulk_write_raw;

果然，它设置了codec->read！而在wm8993的probe函数中，有如下的调用：

ret = snd_soc_codec_set_cache_io(codec, 8, 16, SND_SOC_I2C);

它设置了I2C的地址宽度为8位，寄存器宽度为16位，I2C通信方式，如果根据这些参数继续追踪io_types[i].read，则会发现它最终调用了I2C的标准读函数：

 

static unsigned int do_i2c_read(struct snd_soc_codec *codec,
void *reg, int reglen,
void *data, int datalen)
{
struct i2c_msg xfer[2];
int ret;
struct i2c_client *client = codec->control_data;

/* Write register */
xfer[0].addr = client->addr;
xfer[0].flags = 0;
xfer[0].len = reglen;
xfer[0].buf = reg;
xfer[0].scl_rate = 100 * 1000;

/* Read data */
xfer[1].addr = client->addr;
xfer[1].flags = I2C_M_RD;
xfer[1].len = datalen;
xfer[1].buf = data;

ret = i2c_transfer(client->adapter, xfer, 2);
if (ret == 2)
return 0;
else if (ret < 0)
return ret;
else
return -EIO;
}