一、什么是ASOC
在嵌入式系统里面的声卡驱动为ASOC（ALSA System on Chip） ，它是在ALSA 驱动程序上封装的一层，分为3大部分，Machine，Platform和Codec ,三部分的关系如下图所示：其中Machine是指我们的开发板，Platform是指Soc,而Codec是指编解码器（如uda1341）
二、嵌入式系统声卡注册过程
这里我们以mini2440开发板为例
安装新驱动insmod alsa/driver/myalsa/platform/s3c2440_iis.ko
insmod alsa/driver/myalsa/platform/s3c2440_dma.ko
insmod alsa/driver/myalsa/codec/uda1341.ko
insmod alsa/driver/myalsa/machine/s3c2440_uda1341.ko 首先我们往内核安装我们写好的声卡驱动（Machine部分的ko文件一定要放在最后面，Platform和Codec 则无特殊要求，下面会讲为什么）
1.platform部分
1.1 cpu_dai
安装s3c2440_iis.ko的时候会注册平台驱动s3c24xx_iis_driver，因为内核中有名字同为"s3c24xx-iis"的平台设备，所以调用了对应的probe函数s3c24xx_iis_dev_probe，该函数最终会把s3c24xx_i2s_dai（cpu_dai硬件操作相关）放入链表dai_list，并命名为"s3c24xx-iis"（后面Machine驱动根据这个名字来实例化声卡）
1. platform:
1.1 s3c24xx-i2s.c : 把s3c24xx_i2s_dai放入链表dai_list, .name = "s3c24xx-iis",
s3c24xx_iis_dev_probesnd_soc_register_dai(&pdev->dev, &s3c24xx_i2s_dai);list_add(&dai->list, &dai_list);平台驱动
static struct platform_driver s3c24xx_iis_driver = {.probe = s3c24xx_iis_dev_probe,.remove = s3c24xx_iis_dev_remove,.driver = {.name = "s3c24xx-iis",.owner = THIS_MODULE,},
};平台设备
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 = &s3c_device_iis_dmamask,.coherent_dma_mask = 0xffffffffUL}
};static 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,
};//cpu_dai相关的硬件操作
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,
};1.2 platform_dma
安装s3c2440_dma.ko时，把samsung_asoc_platform放入了链表platform_list，并命名为"samsung-audio"（后面Machineko驱动根据这个名字来实例化声卡）
1.2 sound/soc/samsung/dma.c : 把samsung_asoc_platform放入了链表platform_list, .name = "samsung-audio",
samsung_asoc_platform_probesnd_soc_register_platform(&pdev->dev, &samsung_asoc_platform);list_add(&platform->list, &platform_list);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,
};
//cpu_dam相关的硬件操作
static struct snd_soc_platform_driver samsung_asoc_platform = {.ops = &dma_ops,.pcm_new = dma_new,.pcm_free = dma_free_dma_buffers,
};2. codec注册部分
安装uda1341.ko 时，把soc_codec_dev_uda134x和uda134x_dai分别放入dai_list和codec_list链表（用于后面的Machineko驱动实例化声卡）
2. codec: uda134x.c
uda134x_codec_probesnd_soc_register_codec(&pdev->dev,&soc_codec_dev_uda134x, &uda134x_dai, 1);struct snd_soc_codec *codec;codec->driver = codec_drv; = &soc_codec_dev_uda134xsnd_soc_register_dais(dev, dai_drv, num_dai); // uda134x_dailist_add(&dai->list, &dai_list); : 把uda134x_dai放入了链表dai_listlist_add(&codec->list, &codec_list); 把soc_codec_dev_uda134x放入了链表codec_list//设置codec芯片的硬件操作
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 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,
};//codec_dai相关的硬件操作
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,
};3. machine部分
machine部分内容很多，总的调用过程如下：
s3c24xx_uda134x_probes3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);platform_set_drvdata(s3c24xx_uda134x_snd_device, &snd_soc_s3c24xx_uda134x); platform_device_add(s3c24xx_uda134x_snd_device);.....soc_probesnd_soc_register_card(card); // card = &snd_soc_s3c24xx_uda134xcard->rtd = devm_kzalloc(card->dev,...card->rtd[i].dai_link = &card->dai_link[i]; // &s3c24xx_uda134x_dai_linklist_add(&card->list, &card_list);snd_soc_instantiate_cards(); // 实例化声卡snd_soc_instantiate_card(card);3.1 /* bind DAIs */for (i = 0; i < card->num_links; i++)soc_bind_dai_link(card, i);3.1.1 /* find CPU DAI */rtd->cpu_dai = cpu_dai; = //&s3c24xx_i2s_dai3.1.2 /* find_codec */rtd->codec = codec; = // codec, codec->driver=&soc_codec_dev_uda134x3.1.3 /* find CODEC DAI */ rtd->codec_dai = codec_dai; // = &uda134x_dai3.1.4 /* find_platform */rtd->platform = platform; // = &samsung_asoc_platform3.2 /* initialize the register cache for each available codec */ret = snd_soc_init_codec_cache(codec, compress_type);3.3 snd_card_createerr = snd_ctl_create(card);static struct snd_device_ops ops = {.dev_free = snd_ctl_dev_free,.dev_register = snd_ctl_dev_register,.dev_disconnect = snd_ctl_dev_disconnect,};snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);dev->ops = ops;3.4 /* early DAI link probe */soc_probe_dai_link /* probe the cpu_dai *//* probe the CODEC *//* probe the platform *//* probe the CODEC DAI *//* create the pcm */ret = soc_new_pcm(rtd, num);struct snd_pcm_ops *soc_pcm_ops = &rtd->ops;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_newstatic struct snd_device_ops ops = {.dev_free = snd_pcm_dev_free,.dev_register = snd_pcm_dev_register,.dev_disconnect = snd_pcm_dev_disconnect,};err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops)dev->ops = ops;pcm->private_data = rtd;3.5 snd_card_registersnd_device_register_allerr = dev->ops->dev_register(dev)调用前面snd_pcm_new的snd_pcm_dev_registersnd_pcm_dev_registererr = snd_register_device_for_dev(devtype, pcm->card,pcm->device,&snd_pcm_f_ops[cidx],pcm, str, dev);安装s3c2440_uda1341.ko时，注册平台驱动s3c24xx_uda134x_driver，因为内核中有同名的平台设备mini2440_audio，所以调用了s3c24xx_uda134x_probe函数
//平台驱动
static struct platform_driver s3c24xx_uda134x_driver = {.probe = s3c24xx_uda134x_probe,.remove = s3c24xx_uda134x_remove,.driver = {.name = "s3c24xx_uda134x",.owner = THIS_MODULE,},
};//平台设备
static struct platform_device mini2440_audio = {.name = "s3c24xx_uda134x",.id = 0,.dev = {.platform_data = &mini2440_audio_pins,},
};①s3c24xx_uda134x_probe函数调用platform_set_drvdata把snd_soc_card结构体snd_soc_s3c24xx_uda134x（含有dai_link：用来负责连接Platform
和Codec，实例化的时候会用到）保存在pdev（platform_device）->dev->p->driver_data中，然后platform_device_add(s3c24xx_uda134x_snd_device)，因为内核中有同名的平台驱动soc_driver，所以调用该驱动的probe函数(soc_probe函数)
s3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);platform_set_drvdata(s3c24xx_uda134x_snd_device, &snd_soc_s3c24xx_uda134x); dev_set_drvdata(&pdev->dev, data);dev->p->driver_data = data; platform_device_add(s3c24xx_uda134x_snd_device);//指定了platform和codec
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",.dai_link = &s3c24xx_uda134x_dai_link,.num_links = 1, //只有1个dai_link
};/* 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,
};②soc_probe函数调用platform_get_drvdata函数把第①步的snd_soc_card结构体snd_soc_s3c24xx_uda134x（含有dai_link：用来负责连接Platform和Codec，实例化的时候会用到）取出来，然后调用snd_soc_register_card函数注册它。
soc_probestruct snd_soc_card *card = platform_get_drvdata(pdev);return dev->p->driver_data;snd_soc_register_card(card); // card = &snd_soc_s3c24xx_uda134x③snd_soc_register_card函数取出snd_soc_card结构体的dai_link（snd_soc_s3c24xx_uda134x结构体只有一个dai_link，这里的for只执行一次），然后调用snd_soc_instantiate_cards函数实例化声卡，其实就是根据snd_soc_s3c24xx_uda134x结构体里面的dai_link指定的名字，在dai_list、platform_list、codec_list链表找到我们之前platform和codec注册部分放入队列的那四个结构体。（soc_bind_dai_link函数逐个扫描这三个链表，根据card->dai_link[]中的名称进行匹配，匹配后把相应的codec，dai和platform实例赋值到（snd_soc_card）card->rtd[]中（snd_soc_pcm_runtime）。经过这个过程后，snd_soc_pcm_runtime：（card->rtd）中保存了本Machine中使用的Codec，DAI和Platform驱动的信息）
snd_soc_register_cardcard->rtd = kzalloc(sizeof(struct snd_soc_pcm_runtime) *(card->num_links + card->num_aux_devs),GFP_KERNEL);for (i = 0; i < card->num_links; i++)card->rtd[i].dai_link = &card->dai_link[i];list_add(&card->list, &card_list);snd_soc_instantiate_cards();snd_soc_instantiate_card(card);soc_bind_dai_link(card, i);/* find CPU DAI */rtd->cpu_dai = cpu_dai; = //&s3c24xx_i2s_dai/* find_codec */rtd->codec = codec; = // codec, codec->driver=&soc_codec_dev_uda134x/* find CODEC DAI */ rtd->codec_dai = codec_dai; // = &uda134x_dai/* find_platform */rtd->platform = platform; // = &samsung_asoc_platform④调用传统ALSA声卡驱动的API，snd_card_create函数，该函数再调用snd_ctl_create，snd_ctl_create函数调用snd_device_new把snd_device_ops结构体ops填充到snd_device的ops成员（dev->ops = ops;），然后把snd_device挂到（snd_card）card->devices链表中。后面第⑧步snd_card_register的时候会用到snd_device_ops结构体的dev_register成员。目前我们只是先把它挂在一个链表中，如下图：
3.3 snd_card_createerr = snd_ctl_create(card);static struct snd_device_ops ops = {.dev_free = snd_ctl_dev_free,.dev_register = snd_ctl_dev_register,.dev_disconnect = snd_ctl_dev_disconnect,};snd_device_new(card, SNDRV_DEV_CONTROL, card, &ops);dev->ops = ops;
⑤soc_probe_dai_link函数先是调用前面第三步找出来的cpu_dai等的probe函数（例如：s3c24xx_i2s_dai结构体的s3c24xx_iis_dev_probe），说白了就是硬件初始化。然后调用soc_new_pcm。
soc_probe_dai_link /* probe the cpu_dai */ret = cpu_dai->driver->probe(cpu_dai);/* probe the CODEC */ret = soc_probe_codec(card, codec);/* probe the platform */ret = platform->driver->probe(platform);/* probe the CODEC DAI */ret = codec_dai->driver->probe(codec_dai);/* create the pcm */ret = soc_new_pcm(rtd, num);⑥soc_new_pcm函数函数先是给card->rtd->ops结构体里的函数指针赋值（APP调用的程序的过程中会通过这些函数再去调用到硬件相关的函数（如前面platform的s3c24xx_i2s_dai）），然后把card->rtd->ops的地址赋给substream->ops。最后调用snd_pcm_new。
ret = soc_new_pcm(rtd, num);struct snd_pcm_ops *soc_pcm_ops = &rtd->ops;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_newsnd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, soc_pcm_ops);substream->ops = ops;
或snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, soc_pcm_ops);substream->ops = ops;⑦snd_pcm_new是传统的ALSA声卡驱动API，这一步和第④步是相似的，区别在于第④步对应的是snd_control，这里是snd_pcm的。snd_pcm_new执行完又回到soc_new_pcm函数的，这里有一个(snd_pcm)pcm->private_data = rtd;后面我们APP调用程序的时候会从pcm->private_data取出rtd（rtd里有对应我们开发板的cpu_dai、codec_dai...）
snd_pcm_newstatic struct snd_device_ops ops = {.dev_free = snd_pcm_dev_free,.dev_register = snd_pcm_dev_register,.dev_disconnect = snd_pcm_dev_disconnect,};err = snd_device_new(card, SNDRV_DEV_PCM, pcm, &ops)dev->ops = ops;pcm->private_data = rtd;⑧snd_card_register，该函数会调用前面第④、⑦步的snd_ctl_dev_register和snd_pcm_dev_register（通过snd_device_register_all()注册所有挂在该声卡下的逻辑设备，snd_device_register_all()实际上是通过snd_card的devices链表，遍历所有的snd_device，并且调用snd_device的ops->dev_register()来实现各自设备的注册的），这两个函数也是传统的ALSA声卡的API，它们最后都会调用到snd_register_device_for_dev函数，将snd_ctl_f_ops、snd_pcm_f_ops作为snd_register_device_for_dev的参数被传入，并被记录在snd_minors[minor]中的字段f_ops中，并创建设备节点（device_create（class_create则是在sound_core.c中））。用户程序需要打开control、pcm设备时，驱动程序通过snd_minors[]全局数组和此设备号，可以获得snd_ctl_f_ops、snd_pcm_f_ops结构中的各个回调函数（在另外一篇文章里介绍）
3.5 snd_card_registersnd_device_register_allerr = dev->ops->dev_register(dev)调用前面snd_ctl_create的snd_ctl_dev_register和snd_pcm_new的snd_pcm_dev_registersnd_ctl_dev_registersnd_register_device(SNDRV_DEVICE_TYPE_CONTROL, card, -1,&snd_ctl_f_ops, card, name)snd_register_device_for_devpreg->type = type;preg->card = card ? card->number : -1;preg->device = dev;preg->f_ops = f_ops;preg->private_data = private_data;snd_minors[minor] = preg;preg->dev = device_create(sound_class, device, MKDEV(major, minor),private_data, "%s", name);和snd_pcm_dev_registererr = snd_register_device_for_dev(devtype, pcm->card,pcm->device,&snd_pcm_f_ops[cidx],pcm, str, dev);static const struct file_operations snd_ctl_f_ops =
{.owner = THIS_MODULE,.read = snd_ctl_read,.open = snd_ctl_open,.release = snd_ctl_release,.llseek = no_llseek,.poll = snd_ctl_poll,.unlocked_ioctl = snd_ctl_ioctl,.compat_ioctl = snd_ctl_ioctl_compat,.fasync = snd_ctl_fasync,
};const struct file_operations snd_pcm_f_ops[2] = {{.owner = THIS_MODULE,.write = snd_pcm_write,.aio_write = snd_pcm_aio_write,.open = snd_pcm_playback_open,.release = snd_pcm_release,.llseek = no_llseek,.poll = snd_pcm_playback_poll,.unlocked_ioctl = snd_pcm_playback_ioctl,.compat_ioctl = snd_pcm_ioctl_compat,.mmap = snd_pcm_mmap,.fasync = snd_pcm_fasync,.get_unmapped_area = snd_pcm_get_unmapped_area,},{.owner = THIS_MODULE,.read = snd_pcm_read,.aio_read = snd_pcm_aio_read,.open = snd_pcm_capture_open,.release = snd_pcm_release,.llseek = no_llseek,.poll = snd_pcm_capture_poll,.unlocked_ioctl = snd_pcm_capture_ioctl,.compat_ioctl = snd_pcm_ioctl_compat,.mmap = snd_pcm_mmap,.fasync = snd_pcm_fasync,.get_unmapped_area = snd_pcm_get_unmapped_area,}
};
至此，machine部分的任务已经完成。
4. 字符设备注册
在sound/core/sound.c中有alsa_sound_init()函数，register_chrdev中的参数major与之前创建pcm设备是device_create时的major是同一个，这样的结果是，当应用程序open设备文件/dev/snd/pcmCxDxp时，会进入snd_fops的open回调函数，该open函数以次设备号为索引，从snd_minors全局数组中取出当初注册conrol、pcm设备时填充的snd_minor结构，然后从snd_minor结构中取出control、pcm设备的f_ops，并且把file->f_op替换为pcm设备的f_ops，紧接着直接调用pcm设备的f_ops->open()...
#define CONFIG_SND_MAJOR 116 /* standard configuration */
static int major = CONFIG_SND_MAJOR;static const struct file_operations snd_fops =
{.owner = THIS_MODULE,.open = snd_open,.llseek = noop_llseek,
};alsa_sound_initregister_chrdev(major, "alsa", &snd_fops)调用过程我们在下一篇文章再来详细介绍。
