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。
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：设置接口
b. DMA：传送数据
3）codec
a. DAI
b. 控制接口
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
/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\n");
......
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.
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,
};
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。
原文：https://www.cnblogs.com/-glb/p/14359899.html