ASOC声卡驱动框架
硬件平台mini2440开发板（S3C2440+UDA1341)
linux版本：linux-3.4.99
ASoC--ALSA System on Chip ，是建立在标准ALSA驱动层上，为了更好地支持嵌入式处理器和移动设备中的音频Codec的一套软件体系。在ASoc出现之前，内核对于SoC中的音频已经有部分的支持，不过会有一些局限性：
通常，就像软件领域里的抽象和重用一样，嵌入式设备的音频系统可以被划分为板载硬件（Machine）、Soc（Platform）、Codec三大部分，如下图所示：
下面就3大部分分别看下相关代码
1.platform:在该开发板中，platform包含了DMA和IIS的实现。
DMA：在/sound/soc/samsung/dma.c文件中，相关代码为：
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),
};
Static struct snd_soc_platform_driver samsung_asoc_platform = {
.ops = &dma_ops,
.pcm_new = dma_new,
.pcm_free = dma_free_dm_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,
};根据dev，drv驱动模型，查找“samsung-audio”在arch\arm\plat-samsung\devs.c文件中有相关dev的注册：
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),
}
};从而调用DMA中的probe函数，即上述的相关代码。
IIS：在\sound\soc\samsung\S3c24xx-i2s.c中实现了相关代码：
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,
},
};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,
};
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,
};该文件中配置了播放，录音，及数据格式等参数，并且注册了XX_dai_ops结构体，对于dev的注册，也是在 arch\arm\plat-samsung\devs.c函数中实现。
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),
}
};2.codec：在codec部分涉及到具体的声卡芯片了，在mini2440开发板中，用了UDA1341芯片，故关注sound\soc\codecs\uda134x.c文件。
static struct platform_driver uda134x_codec_driver = {
.driver = {
.name = "uda134x-codec",
.owner = THIS_MODULE,
},
.probe = uda134x_codec_probe,
.remove = __devexit_p(uda134x_codec_remove),
};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,
};
static struct snd_soc_dai_driver uda134x_dai = {
.name = "uda134x-hifi",
/* 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,
};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,
};
static int __devinit uda134x_codec_probe(struct platform_device *pdev)
{
return snd_soc_register_codec(&pdev->dev,
&soc_codec_dev_uda134x, &uda134x_dai, 1);
}该文件包含了UDA1341芯片的驱动函数，如果要调用该文件中的probe函数，需要注册在板子的初始化中注册该声卡的dev，在mach-smdk2440.c中添加：
static struct platform_device uda1340_codec = {
.name = "uda134x-codec",
.id = -1,
};machine：与自己板子相关的代码，因为在内核中能找到mini2440开发板类似的平台声卡，故可以直接拿来用。
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,
},
};要使上述probe函数呗调用，需在mach-smdk2440.c中添加：
static struct platform_device smdk2440_audio = {
.name = "s3c24xx_uda134x",
.id = 0,
.dev = {
.platform_data = &smdk2440_audio_pins,
},
};s3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);
if (!s3c24xx_uda134x_snd_device) {
printk(KERN_ERR "S3C24XX_UDA134X SoC Audio: "
"Unable to register\n");
return -ENOMEM;
}
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);
if (ret) {
printk(KERN_ERR "S3C24XX_UDA134X SoC Audio: Unable to add\n");
platform_device_put(s3c24xx_uda134x_snd_device);
}
static struct snd_soc_ops s3c24xx_uda134x_ops = {
.startup = s3c24xx_uda134x_startup,
.shutdown = s3c24xx_uda134x_shutdown,
.hw_params = s3c24xx_uda134x_hw_params,
};
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",
};
static struct snd_soc_card snd_soc_s3c24xx_uda134x = {
.name = "S3C24XX_UDA134X",
.owner = THIS_MODULE,
.dai_link = &s3c24xx_uda134x_dai_link,
.num_links = 1,
};之后看到代码将machine和之前说的platform，codec关联起来。大概就构成了ASOC的框架。