前言
针对嵌入式设备，linux在ALSA的基础上又封装了一层，名为ASOC（ALSA system on chip）。使用ASOC就不需要自己调用snd_card_create、snd_ctrl_create等函数自己来创建声卡。ASOC将驱动程序分为3部分：
1）machine：单板相关的内容。抽象出一个snd_soc_card结构体 ，重点关注snd_soc_dai_link结构体 ，此结构体表明以下两点：
a. 表明platform是哪一个， CPU DAI是哪一个，DMA是哪一个，即哪个DMA负责数据传输。
b. 表明codec是哪一个，codec DAI是哪一个
2）platform
a. DAI：抽象出snd_soc_dai_driver结构体
b. DMA：抽象出snd_soc_platform_driver结构体
3）codec
a. DAI：抽象出一个snd_soc_dai_driver结构体
b. 控制接口：抽象出一个snd_soc_codec_driver结构体。
在Linux驱动中，肯定有多个platform和codec。对于某个单板，使用哪款主芯片(platform)，哪款codec芯片在machine中指定。
内核中带有UDA1341的驱动程序，但是没有WM8976的驱动程序。首先分析UDA1341的驱动程序
1. machine
/sound/soc/samsung/S3c24xx_uda134x.c
static struct platform_driver s3c24xx_uda134x_driver = {
.probe = s3c24xx_uda134x_probe,
.remove = s3c24xx_uda134x_remove,
.driver = {
.name = "s3c24xx_uda134x",
.owner = THIS_MODULE,
},
};
有平台driver，肯定有同名的平台device（这是一种老的方式，现在都使用设备树了），搜索s3c24xx_uda134x
arch/arm/mach-s3c24xx/Mach-mini2440.c
static struct platform_device mini2440_audio = {
.name = "s3c24xx_uda134x",
.id = 0,
.dev = {
.platform_data = &mini2440_audio_pins,
},
};
当内核中有同名的平台driver和平台device时，platform_driver 中的probe函数将会调用。
static int s3c24xx_uda134x_probe(struct platform_device *pdev)
{
int ret;
printk(KERN_INFO "S3C24XX_UDA134X SoC Audio driver
");
......
s3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);
platform_set_drvdata(s3c24xx_uda134x_snd_device,
&snd_soc_s3c24xx_uda134x);
platform_device_add_data(s3c24xx_uda134x_snd_device, &s3c24xx_uda134x, sizeof(s3c24xx_uda134x));
ret = platform_device_add(s3c24xx_uda134x_snd_device);
return ret;
}
又出现了平台device soc-audio，在内核中搜索soc-audio
/sound/soc/Soc-core.c
/* ASoC platform driver */
static struct platform_driver soc_driver = {
.driver = {
.name = "soc-audio",
.owner = THIS_MODULE,
.pm = &snd_soc_pm_ops,
},
.probe = soc_probe,
.remove = soc_remove,
};
soc_probe函数将被调用
/* probes a new socdev */
static int soc_probe(struct platform_device *pdev)
{
struct snd_soc_card *card = platform_get_drvdata(pdev);
int ret = 0;
........
card->dev = &pdev->dev;
ret = snd_soc_register_card(card);
return 0;
}
注册一个snd_soc_card的结构体。该结构体具体指哪一个？就是snd_soc_s3c24xx_uda134x
/sound/soc/samsung/S3c24xx_uda134x.c
static struct snd_soc_card snd_soc_s3c24xx_uda134x = {
.name = "S3C24XX_UDA134X",
.owner = THIS_MODULE,
.dai_link = &s3c24xx_uda134x_dai_link,
.num_links = 1,
};
static struct snd_soc_dai_link s3c24xx_uda134x_dai_link = {
.name = "UDA134X",
.stream_name = "UDA134X",
.codec_name = "uda134x-codec",//用哪一个codec
.codec_dai_name = "uda134x-hifi",//codec芯片中的哪一个DAI，因为有些codec中有多个数字接口
.cpu_dai_name = "s3c24xx-iis",//2440的DAI接口
.ops = &s3c24xx_uda134x_ops,
.platform_name = "samsung-audio",//DMA
};
static struct snd_soc_ops s3c24xx_uda134x_ops = {
.startup = s3c24xx_uda134x_startup,
.shutdown = s3c24xx_uda134x_shutdown,
.hw_params = s3c24xx_uda134x_hw_params,
};
machine部分到此结束，它主要就是构造了一个snd_soc_card结构体 。在该结构体中有一个dai_link，在dai_link中指定了codec_name，code_dai_name，cpu_dai_name以及platform_name.
2. platform
2.1 cpu dai
搜索s3c24xx-iis
/sound/soc/samsung/S3c24xx-i2s.c
static struct platform_driver s3c24xx_iis_driver = {
.probe = s3c24xx_iis_dev_probe,
.remove = __devexit_p(s3c24xx_iis_dev_remove),
.driver = {
.name = "s3c24xx-iis",
.owner = THIS_MODULE,
},
};
有平台driver，必然有同名的平台device
/arch/arm/plat-samsung/Devs.c
struct platform_device s3c_device_iis = {
.name = "s3c24xx-iis",
.id = -1,
.num_resources = ARRAY_SIZE(s3c_iis_resource),
.resource = s3c_iis_resource,
.dev = {
.dma_mask = &samsung_device_dma_mask,
.coherent_dma_mask = DMA_BIT_MASK(32),
}
};
因此函数s3c24xx_iis_dev_probe将被调用，
static __devinit int s3c24xx_iis_dev_probe(struct platform_device *pdev)
{
return snd_soc_register_dai(&pdev->dev, &s3c24xx_i2s_dai);
}
static struct snd_soc_dai_driver s3c24xx_i2s_dai = {
.probe = s3c24xx_i2s_probe,
.suspend = s3c24xx_i2s_suspend,
.resume = s3c24xx_i2s_resume,
.playback = {
.channels_min = 2,
.channels_max = 2,
.rates = S3C24XX_I2S_RATES,
.formats = SNDRV_PCM_FMTBIT_S8 | SNDRV_PCM_FMTBIT_S16_LE,},
.capture = {
.channels_min = 2,
.channels_max = 2,
.rates = S3C24XX_I2S_RATES,
.formats = SNDRV_PCM_FMTBIT_S8 | SNDRV_PCM_FMTBIT_S16_LE,},
.ops = &s3c24xx_i2s_dai_ops,
};
至此，platform中最重要的cpu DAI接口——> snd_soc_dai_driver 就出现了
static const struct snd_soc_dai_ops s3c24xx_i2s_dai_ops = {
.trigger = s3c24xx_i2s_trigger,
.hw_params = s3c24xx_i2s_hw_params,
.set_fmt = s3c24xx_i2s_set_fmt,
.set_clkdiv = s3c24xx_i2s_set_clkdiv,
.set_sysclk = s3c24xx_i2s_set_sysclk,
};
2.2 DMA
根据结构体snd_soc_dai_link中的 platform_name = "samsung-audio"，在代码中搜索samsung-audio
/sound/soc/samsung/Dma.c
static struct platform_driver asoc_dma_driver = {
.driver = {
.name = "samsung-audio",
.owner = THIS_MODULE,
},
.probe = samsung_asoc_platform_probe,
.remove = __devexit_p(samsung_asoc_platform_remove),
};
又出现了一个平台driver，要想这个平台driver起作用，必定会有一个平台device.
/arch/arm/plat-samsung/Devs.c
/* ASOC DMA */
struct platform_device samsung_asoc_dma = {
.name = "samsung-audio",
.id = -1,
.dev = {
.dma_mask = &samsung_device_dma_mask,
.coherent_dma_mask = DMA_BIT_MASK(32),
}
};
/sound/soc/samsung/Dma.c
static int __devinit samsung_asoc_platform_probe(struct platform_device *pdev)
{
return snd_soc_register_platform(&pdev->dev, &samsung_asoc_platform);
}
static struct snd_soc_platform_driver samsung_asoc_platform = {
.ops = &dma_ops,
.pcm_new = dma_new,
.pcm_free = dma_free_dma_buffers,
};
static struct snd_pcm_ops dma_ops = {
.open = dma_open,
.close = dma_close,
.ioctl = snd_pcm_lib_ioctl,
.hw_params = dma_hw_params,
.hw_free = dma_hw_free,
.prepare = dma_prepare,
.trigger = dma_trigger,
.pointer = dma_pointer,
.mmap = dma_mmap,
};
至此，platform的DMA这部分分析完毕，这部分主要就是注册了一个snd_soc_platform_driver的结构体。
3. codec
搜索 uda134x-codec
/sound/soc/codecs/Uda134x.c
static struct platform_driver uda134x_codec_driver = {
.driver = {
.name = "uda134x-codec",
.owner = THIS_MODULE,
},
.probe = uda134x_codec_probe,
.remove = __devexit_p(uda134x_codec_remove),
};
内核中肯定存在名为uda134x-codec的platform device
/arch/arm/mach-s3c24xx/Mach-mini2440.c
static struct platform_device uda1340_codec = {
.name = "uda134x-codec",
.id = -1,
};
函数uda134x_codec_probe将被调用
static int __devinit uda134x_codec_probe(struct platform_device *pdev)
{
return snd_soc_register_codec(&pdev->dev,
&soc_codec_dev_uda134x, &uda134x_dai, 1);
}
3.1 soc_codec_dev_uda134x
这个结构体其实就是codec中的控制接口。
static struct snd_soc_codec_driver soc_codec_dev_uda134x = {
.probe = uda134x_soc_probe,
.remove = uda134x_soc_remove,
.suspend = uda134x_soc_suspend,
.resume = uda134x_soc_resume,
.reg_cache_size = sizeof(uda134x_reg),
.reg_word_size = sizeof(u8),
.reg_cache_default = uda134x_reg,
.reg_cache_step = 1,
.read = uda134x_read_reg_cache,
.write = uda134x_write,
.set_bias_level = uda134x_set_bias_level,
};
3.2 uda134x_dai
static struct snd_soc_dai_driver uda134x_dai = { .name = "uda134x-hifi",//这个地方正好对应s3c24xx_uda134x_dai_link结构体中的codec_dai_name
/* playback capabilities */ .playback = { .stream_name = "Playback", .channels_min = 1, .channels_max = 2, .rates = UDA134X_RATES, .formats = UDA134X_FORMATS, }, /* capture capabilities */ .capture = { .stream_name = "Capture", .channels_min = 1, .channels_max = 2, .rates = UDA134X_RATES, .formats = UDA134X_FORMATS, }, /* pcm operations */ .ops = &uda134x_dai_ops, };
3.2.1 uda134x_dai_ops
static const struct snd_soc_dai_ops uda134x_dai_ops = {
.startup = uda134x_startup,
.shutdown = uda134x_shutdown,
.hw_params = uda134x_hw_params,
.digital_mute = uda134x_mute,
.set_sysclk = uda134x_set_dai_sysclk,
.set_fmt = uda134x_set_dai_fmt,
};
4. ASOC如何与ALSA建立联系
4.1 platform
4.1.1 s3c244-i2s.c
把s3c24xx_i2s_dai放入了链表dai_list中。.name = "s3c24xx-iis"
s3c24xx_iis_dev_probe
snd_soc_register_dai(&pdev->dev, &s3c24xx_i2s_dai)
list_add(&dai->list, &dai_list);
4.1.2 /sound/soc/samsung/dma.c
把结构体samsung_asoc_platform放入了链表platform_list中。name = "samsung-audio",
samsung_asoc_platform_probe
snd_soc_register_platform(&pdev->dev, &samsung_asoc_platform);
list_add(&platform->list, &platform_list);
4.2 uda134x.c
.name = "uda134x-codec",
uda134x_codec_probe
snd_soc_register_codec(&pdev->dev,&soc_codec_dev_uda134x, &uda134x_dai, 1);
struct snd_soc_codec *codec;
//分配一个snd_soc_codec结构体
codec = kzalloc(sizeof(struct snd_soc_codec), GFP_KERNEL);
codec->driver = codec_drv; //codec_drv即soc_codec_dev_uda134x
snd_soc_register_dais(dev, dai_drv, num_dai); //dai_drv即uda134x_dai
//把结构体uda134x_dai放入了链表dai_list中。在链表dai_list中既有s3c24xx_i2s_dai，也有uda134x_dai
list_add(&dai->list, &dai_list);
list_add(&codec->list, &codec_list); //把上面分配的codec放入链表codec_list中。
它们之间是如何联系起来的，就是通过machine中的snd_soc_dai_link中的名字把它们联系起来。下面具体来分析
4.3 s3c24xx_uda134x_probe
s3c24xx_uda134x_probe
//分配一个名为soc-audio的平台设备
s3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);
//设置该设备的私有数据
platform_set_drvdata(s3c24xx_uda134x_snd_device, &snd_soc_s3c24xx_uda134x);
//添加该设备，此时将导致soc-core.c中的soc_driver中的soc_probe函数将被调用。因为在该结构体中有同名soc-audio platform driver.
platform_device_add(s3c24xx_uda134x_snd_device)
.....
soc_probe
//card其实就是snd_soc_s3c24xx_uda134x
struct snd_soc_card *card = platform_get_drvdata(pdev);
snd_soc_register_card(card);
snd_soc_instantiate_cards();
struct snd_soc_card *card;
list_for_each_entry(card, &card_list, list)//对card_list中的每一个card，执行snd_soc_instantiate_card(card)
snd_soc_instantiate_card(card);
4.3.1/* bind DAIs */
for (i = 0; i < card->num_links; i++)
soc_bind_dai_link(card, i);
/*find CPU DAI from registered DAIs*/
list_for_each_entry(cpu_dai, &dai_list, list) {
if (strcmp(cpu_dai->name, dai_link->cpu_dai_name))
rtd->cpu_dai = cpu_dai //即s3c24xx_i2s_dai
....
/*find CODEC from registered CODECs*/
list_for_each_entry(codec, &codec_list, list) {
if (dai_link->codec_of_node) {
if (strcmp(codec->name, dai_link->codec_name))
rtd->codec = codec //即code->codec_drv = soc_codec_dev_uda134x
.....
/*
* CODEC found, so find CODEC DAI from registered DAIs from
* this CODEC
*/
list_for_each_entry(codec_dai, &dai_list, list) {
if (codec->dev == codec_dai->dev &&
!strcmp(codec_dai->name,
dai_link->codec_dai_name)) {
rtd->codec_dai = codec_dai;//即s3c24xx_i2s_dai
.......
/* no, then find one from the set of registered platforms */
list_for_each_entry(platform, &platform_list, list) {
if (strcmp(platform->name, platform_name))
rtd->platform = platform;//即samsung_asoc_platform
........
4.3.2 /* initialize the register cache for each available codec */
list_for_each_entry(codec, &codec_list, list) {
//初始化codec_list链表中的每个codec中的寄存器，这不是我们所关心的点
ret = snd_soc_init_codec_cache(codec, compress_type);
}
4.3.3/* card bind complete so register a sound card */
ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
card->owner, 0, &card->snd_card);
.....
4.3.4/* early DAI link probe */
soc_probe_dai_link(card, i, order)
/* probe the cpu_dai，调用cpu_dai中的probe函数 */
/* probe the CODEC，调用codec中的probe函数*/
/* probe the platform，调用platform中的probe函数 */
/* probe the CODEC DAI */
* create the pcm */
ret = soc_new_pcm(rtd, num);
soc_pcm_ops->open = soc_pcm_open;
soc_pcm_ops->close = soc_pcm_close;
soc_pcm_ops->hw_params = soc_pcm_hw_params;
soc_pcm_ops->hw_free = soc_pcm_hw_free;
soc_pcm_ops->prepare = soc_pcm_prepare;
soc_pcm_ops->trigger = soc_pcm_trigger;
soc_pcm_ops->pointer = soc_pcm_pointer;
......
snd_pcm_new
4.3.5 snd_card_register(card->snd_card);
在文章的开始，曾经说过当使用ASOC驱动时，它将自动调用snd_card_create、snd_pcm_new、以及snd_card_register。从这个地方也可以看出ASOC是在ALSA的基础上，进行了封装。
附：
static struct snd_soc_card snd_soc_s3c24xx_uda134x = { .name = "S3C24XX_UDA134X", .owner = THIS_MODULE, .dai_link = &s3c24xx_uda134x_dai_link, .num_links = 1, }; static struct snd_soc_dai_link s3c24xx_uda134x_dai_link = { .name = "UDA134X", .stream_name = "UDA134X", .codec_name = "uda134x-codec", .codec_dai_name = "uda134x-hifi", .cpu_dai_name = "s3c24xx-iis", .ops = &s3c24xx_uda134x_ops, .platform_name = "samsung-audio", };