基于i.MX6X的音频驱动分析（三）音频驱动ASoC的机器层
前面一节的内容我们提到，ASoC被分为Machine、Platform和Codec三大部分。
其中的Machine驱动负责Platform和Codec之间的耦合以及部分和设备或板子特定的代码，再次引用上一节的内容：
Machine驱动负责处理机器特有的一些控件和音频事件（例如，当播放音频时，需要先行打开一个放大器）；单独的Platform和Codec驱动是不能工作的，它必须由Machine驱动把它们结合在一起才能完成整个设备的音频处理工作。
ASoC的一切都从Machine驱动开始，包括声卡的注册，绑定Platform和Codec驱动等等，下面就让我们从Machine驱动开始讨论吧
注册Platform Device
ASoC把声卡注册为Platform Device，以WM8960为例 ，涉及文件：sound/soc/imx/imx-wm8960.c
其模块初始化函数为：module_init(imx_asoc_init);
static int __init imx_asoc_init(void)
{
int ret;
ret = platform_driver_register(&imx_wm8960_driver);
if (ret < 0)
goto exit;
if (machine_is_mx6q_sabresd())
imx_dai[0].codec_name = "wm8960-codec.0-001a";
else if (machine_is_mx6sl_arm2() machine_is_mx6sl_evk())
imx_dai[0].codec_name = "wm8960.1-001a";
imx_snd_device = platform_device_alloc("soc-audio", 6);
if (!imx_snd_device)
goto err_device_alloc;
platform_set_drvdata(imx_snd_device, &snd_soc_card_imx);
ret = platform_device_add(imx_snd_device);
if (0 == ret)
goto exit;
platform_device_put(imx_snd_device);
err_device_alloc:
platform_driver_unregister(&imx_wm8960_driver);
exit:
return ret;
}由此可见，模块初始化时，注册了一个名为soc-audio的Platform设备，同时把snd_soc_card_imx设到platform_device结构的dev.drvdata字段中。
这里引出了第一个数据结构snd_soc_card的实例snd_soc_card_imx，它的定义如下（数据结构snd_soc_card的定义在include/sound/soc.h）：
static struct snd_soc_dai_link imx_dai[] = {
{
.name = "HiFi",
.stream_name = "HiFi",
.codec_dai_name = "wm8960-hifi",//指定codec dai的名字
.codec_name = "wm8960-codec.0-001a",//指定codec的名字
.cpu_dai_name = "imx-ssi.1", //指定cpu dai的名字
.platform_name = "imx-pcm-audio.1",//指定platform的名字
.init = imx_wm8960_init,
.ops = &imx_hifi_ops,
},
};
static struct snd_soc_card snd_soc_card_imx = {
.name = "wm8960-audio",
.dai_link = imx_dai,
.num_links = ARRAY_SIZE(imx_dai),
};通过snd_soc_card结构，又引出了Machine驱动的另外两个数据结构：
- snd_soc_dai_link（实例：imx_dai[] ）
- snd_soc_ops（实例：imx_hifi_ops ）
snd_soc_dai_link中，指定了Platform、Codec、codec_dai、cpu_dai的名字，稍后Machine驱动将会利用这些名字去匹配已经在系统中注册的platform，codec，dai，这些注册的部件都是在另外相应的Platform驱动和Codec驱动的代码文件中定义的，这样看来，Machine驱动的设备初始化代码无非就是选择合适Platform和Codec以及dai，用他们填充以上几个数据结构，然后注册Platform设备即可。当然还要实现连接Platform和Codec的dai_link对应的ops实现，本例中就是imx_hifi_ops：
static struct snd_soc_ops imx_hifi_ops = {
.startup = imx_hifi_startup,
.shutdown = imx_hifi_shutdown,
.hw_params = imx_hifi_hw_params,
.hw_free = imx_hifi_hw_free,
.trigger = imx_hifi_trigger,
};注册Platform Driver
按照Linux的设备模型，有platform_device，就一定会有platform_driver。ASoC的platform_driver在以下文件中定义：sound/soc/soc-core.c。
还是先从模块的入口看起：
static int __init snd_soc_init(void)
{
...
return platform_driver_register(&soc_driver);
}
module_init(snd_soc_init);soc_driver的定义如下：
/* 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,
};我们看到platform_driver的name字段为soc-audio，正好与platform_device中的名字相同，按照Linux的设备模型，platform总线会匹配这两个名字相同的device和driver，同时会触发soc_probe的调用，它正是整个ASoC驱动初始化的入口。
初始化入口soc_probe()
soc_probe函数本身很简单，它先从platform_device参数中取出snd_soc_card，然后调用snd_soc_register_card，通过snd_soc_register_card，为snd_soc_pcm_runtime数组申请内存，每一个dai_link对应snd_soc_pcm_runtime数组的一个单元，然后把snd_soc_card中的dai_link配置复制到相应的snd_soc_pcm_runtime中。
/* probes a new socdev */
static int soc_probe(struct platform_device *pdev)
{
struct snd_soc_card *card = platform_get_drvdata(pdev);//从platform_device参数中取出snd_soc_card
int ret = 0;
/*
* no card, so machine driver should be registering card
* we should not be here in that case so ret error
*/
if (!card)
return -EINVAL;
/* Bodge while we unpick instantiation */
card->dev = &pdev->dev;
ret = snd_soc_register_card(card);//为snd_soc_pcm_runtime数组申请内存
if (ret != 0) {
dev_err(&pdev->dev, "Failed to register card\n");
return ret;
}
return 0;
}/**
* snd_soc_register_card - Register a card with the ASoC core
*
* @card: Card to register
*
*/
int snd_soc_register_card(struct snd_soc_card *card)
{
int i;
if (!card->name !card->dev)
return -EINVAL;
dev_set_drvdata(card->dev, card);
snd_soc_initialize_card_lists(card);
soc_init_card_debugfs(card);
card->rtd = kzalloc(sizeof(struct snd_soc_pcm_runtime) *
(card->num_links + card->num_aux_devs),
GFP_KERNEL);
if (card->rtd == NULL)
return -ENOMEM;
card->rtd_aux = &card->rtd[card->num_links];
for (i = 0; i < card->num_links; i++)
card->rtd[i].dai_link = &card->dai_link[i];
INIT_LIST_HEAD(&card->list);
card->instantiated = 0;
mutex_init(&card->mutex);
mutex_lock(&client_mutex);
list_add(&card->list, &card_list);
snd_soc_instantiate_cards();
mutex_unlock(&client_mutex);
dev_dbg(card->dev, "Registered card '%s'\n", card->name);
return 0;
}
EXPORT_SYMBOL_GPL(snd_soc_register_card);/*
* Attempt to initialise any uninitialised cards. Must be called with
* client_mutex.
*/
static void snd_soc_instantiate_cards(void)
{
struct snd_soc_card *card;
list_for_each_entry(card, &card_list, list)
snd_soc_instantiate_card(card);
}最后，大部分的工作都在snd_soc_instantiate_card中实现，下面就看看snd_soc_instantiate_card做了些什么：
static void snd_soc_instantiate_card(struct snd_soc_card *card)
{
struct snd_soc_codec *codec;
struct snd_soc_codec_conf *codec_conf;
enum snd_soc_compress_type compress_type;
int ret, i;
mutex_lock(&card->mutex);
if (card->instantiated) {
mutex_unlock(&card->mutex);
return;
}
/* bind DAIs */
for (i = 0; i < card->num_links; i++)
soc_bind_dai_link(card, i);
/* bind completed ? */
if (card->num_rtd != card->num_links) {
mutex_unlock(&card->mutex);
return;
}
/* initialize the register cache for each available codec */
list_for_each_entry(codec, &codec_list, list) {
if (codec->cache_init)
continue;
/* by default we don't override the compress_type */
compress_type = 0;
/* check to see if we need to override the compress_type */
for (i = 0; i < card->num_configs; ++i) {
codec_conf = &card->codec_conf[i];
if (!strcmp(codec->name, codec_conf->dev_name)) {
compress_type = codec_conf->compress_type;
if (compress_type && compress_type
!= codec->compress_type)
break;
}
}
ret = snd_soc_init_codec_cache(codec, compress_type);
if (ret < 0) {
mutex_unlock(&card->mutex);
return;
}
}
/* card bind complete so register a sound card */
ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
card->owner, 0, &card->snd_card);
if (ret < 0) {
printk(KERN_ERR "asoc: can't create sound card for card %s\n",
card->name);
mutex_unlock(&card->mutex);
return;
}
card->snd_card->dev = card->dev;
card->dapm.bias_level = SND_SOC_BIAS_OFF;
card->dapm.dev = card->dev;
card->dapm.card = card;
list_add(&card->dapm.list, &card->dapm_list);
#ifdef CONFIG_DEBUG_FS
snd_soc_dapm_debugfs_init(&card->dapm, card->debugfs_card_root);
#endif
#ifdef CONFIG_PM_SLEEP
/* deferred resume work */
INIT_WORK(&card->deferred_resume_work, soc_resume_deferred);
#endif
if (card->dapm_widgets)
snd_soc_dapm_new_controls(&card->dapm, card->dapm_widgets,
card->num_dapm_widgets);
/* initialise the sound card only once */
if (card->probe) {
ret = card->probe(card);
if (ret < 0)
goto card_probe_error;
}
for (i = 0; i < card->num_links; i++) {
ret = soc_probe_dai_link(card, i);
if (ret < 0) {
pr_err("asoc: failed to instantiate card %s: %d\n",
card->name, ret);
goto probe_dai_err;
}
}
for (i = 0; i < card->num_aux_devs; i++) {
ret = soc_probe_aux_dev(card, i);
if (ret < 0) {
pr_err("asoc: failed to add auxiliary devices %s: %d\n",
card->name, ret);
goto probe_aux_dev_err;
}
}
/* We should have a non-codec control add function but we don't */
if (card->controls)
snd_soc_add_controls(list_first_entry(&card->codec_dev_list,
struct snd_soc_codec,
card_list),
card->controls,
card->num_controls);
if (card->dapm_routes)
snd_soc_dapm_add_routes(&card->dapm, card->dapm_routes,
card->num_dapm_routes);
snprintf(card->snd_card->shortname, sizeof(card->snd_card->shortname),
"%s", card->name);
snprintf(card->snd_card->longname, sizeof(card->snd_card->longname),
"%s", card->long_name ? card->long_name : card->name);
snprintf(card->snd_card->driver, sizeof(card->snd_card->driver),
"%s", card->driver_name ? card->driver_name : card->name);
for (i = 0; i < ARRAY_SIZE(card->snd_card->driver); i++) {
switch (card->snd_card->driver[i]) {
case '_':
case '-':
case '\0':
break;
default:
if (!isalnum(card->snd_card->driver[i]))
card->snd_card->driver[i] = '_';
break;
}
}
if (card->late_probe) {
ret = card->late_probe(card);
if (ret < 0) {
dev_err(card->dev, "%s late_probe() failed: %d\n",
card->name, ret);
goto probe_aux_dev_err;
}
}
ret = snd_card_register(card->snd_card);
if (ret < 0) {
printk(KERN_ERR "asoc: failed to register soundcard for %s\n", card->name);
goto probe_aux_dev_err;
}
#ifdef CONFIG_SND_SOC_AC97_BUS
/* register any AC97 codecs */
for (i = 0; i < card->num_rtd; i++) {
ret = soc_register_ac97_dai_link(&card->rtd[i]);
if (ret < 0) {
printk(KERN_ERR "asoc: failed to register AC97 %s\n", card->name);
while (--i >= 0)
soc_unregister_ac97_dai_link(card->rtd[i].codec);
goto probe_aux_dev_err;
}
}
#endif
card->instantiated = 1;
mutex_unlock(&card->mutex);
return;
probe_aux_dev_err:
for (i = 0; i < card->num_aux_devs; i++)
soc_remove_aux_dev(card, i);
probe_dai_err:
soc_remove_dai_links(card);
card_probe_error:
if (card->remove)
card->remove(card);
snd_card_free(card->snd_card);
mutex_unlock(&card->mutex);
}该函数首先利用card->instantiated来判断该卡是否已经实例化，如果已经实例化则直接返回，否则遍历每一对dai_link，进行codec、platform、dai的绑定工作，下只是代码的部分选节，详细的代码请直接参考完整的代码树。
/* bind DAIs */
for (i = 0; i < card->num_links; i++)
soc_bind_dai_link(card, i);ASoC定义了三个全局的链表头变量：codec_list、dai_list、platform_list，系统中所有的Codec、DAI、Platform都在注册时连接到这三个全局链表上。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_instantiate_card接着初始化Codec的寄存器缓存，然后调用标准的alsa函数创建声卡实例：
/* card bind complete so register a sound card */
ret = snd_card_create(SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
card->owner, 0, &card->snd_card);
if (ret < 0) {
printk(KERN_ERR "asoc: can't create sound card for card %s\n",
card->name);
mutex_unlock(&card->mutex);
return;
}
card->snd_card->dev = card->dev;
card->dapm.bias_level = SND_SOC_BIAS_OFF;
card->dapm.dev = card->dev;
card->dapm.card = card;
list_add(&card->dapm.list, &card->dapm_list);然后，依次调用各个子结构的probe函数：
/* initialise the sound card only once */
if (card->probe) {
ret = card->probe(card);
if (ret < 0)
goto card_probe_error;
}
for (i = 0; i < card->num_links; i++) {
ret = soc_probe_dai_link(card, i);
if (ret < 0) {
pr_err("asoc: failed to instantiate card %s: %d\n",
card->name, ret);
goto probe_dai_err;
}
}
for (i = 0; i < card->num_aux_devs; i++) {
ret = soc_probe_aux_dev(card, i);
if (ret < 0) {
pr_err("asoc: failed to add auxiliary devices %s: %d\n",
card->name, ret);
goto probe_aux_dev_err;
}
}在上面的soc_probe_dai_link()函数中做了比较多的事情，把他展开继续讨论：
static int soc_probe_dai_link(struct snd_soc_card *card, int num)
{
...
/* set default power off timeout */
rtd->pmdown_time = pmdown_time;
/* probe the cpu_dai */
if (!cpu_dai->probed) {
if (cpu_dai->driver->probe) {
ret = cpu_dai->driver->probe(cpu_dai);
}
cpu_dai->probed = 1;
/* mark cpu_dai as probed and add to card cpu_dai list */
list_add(&cpu_dai->card_list, &card->dai_dev_list);
}
/* probe the CODEC */
if (!codec->probed) {
ret = soc_probe_codec(card, codec);
}
/* probe the platform */
if (!platform->probed) {
if (platform->driver->probe) {
ret = platform->driver->probe(platform);
}
/* mark platform as probed and add to card platform list */
platform->probed = 1;
list_add(&platform->card_list, &card->platform_dev_list);
}
/* probe the CODEC DAI */
if (!codec_dai->probed) {
if (codec_dai->driver->probe) {
ret = codec_dai->driver->probe(codec_dai);
}
/* mark cpu_dai as probed and add to card cpu_dai list */
codec_dai->probed = 1;
list_add(&codec_dai->card_list, &card->dai_dev_list);
}
/* DAPM dai link stream work */
INIT_DELAYED_WORK(&rtd->delayed_work, close_delayed_work);
ret = soc_post_component_init(card, codec, num, 0);
if (ret)
return ret;
ret = device_create_file(&rtd->dev, &dev_attr_pmdown_time);
/* create the pcm */
ret = soc_new_pcm(rtd, num);
/* add platform data for AC97 devices */
if (rtd->codec_dai->driver->ac97_control)
snd_ac97_dev_add_pdata(codec->ac97, rtd->cpu_dai->ac97_pdata);
return 0;
}该函数出了挨个调用了codec，dai和platform驱动的probe函数外，在最后还调用了soc_new_pcm()函数用于创建标准alsa驱动的pcm逻辑设备。现在把该函数的部分代码也贴出来：
/* create a new pcm */
static int soc_new_pcm(struct snd_soc_pcm_runtime *rtd, int num)
{
struct snd_pcm_ops *soc_pcm_ops;
rtd->pcm = pcm;
pcm->private_data = rtd;
soc_pcm_ops->open = soc_pcm_open;
soc_pcm_ops->close = soc_codec_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->pointer = soc_pcm_pointer;
soc_pcm_ops->trigger = soc_pcm_trigger;
soc_pcm_ops->ioctl = soc_pcm_ioctl;
if (platform->driver->ops) {
soc_pcm_ops->mmap = platform->driver->ops->mmap;
soc_pcm_ops->copy = platform->driver->ops->copy;
soc_pcm_ops->silence = platform->driver->ops->silence;
soc_pcm_ops->ack = platform->driver->ops->ack;
soc_pcm_ops->page = platform->driver->ops->page;
}
rtd->ops = soc_pcm_ops;
if (playback)
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, soc_pcm_ops);
if (capture)
snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, soc_pcm_ops);
if (platform->driver->pcm_new) {
ret = platform->driver->pcm_new(rtd->card->snd_card,
codec_dai, pcm);
if (ret < 0) {
pr_err("asoc: platform pcm constructor failed\n");
kfree(rtd->ops);
return ret;
}
}
pcm->private_free = platform->driver->pcm_free;
printk(KERN_INFO "asoc: %s <-> %s mapping ok\n", codec_dai->name,
cpu_dai->name);
return ret;
}该函数首先初始化snd_soc_runtime中的snd_pcm_ops字段，也就是rtd->ops中的部分成员，例如open，close，hw_params等，紧接着调用标准alsa驱动中的创建pcm的函数snd_pcm_new()创建声卡的pcm实例，pcm的private_data字段设置为该runtime变量rtd，然后用platform驱动中的snd_pcm_ops替换部分pcm中的snd_pcm_ops字段，最后，调用platform驱动的pcm_new回调，该回调实现该platform下的dma内存申请和dma初始化等相关工作。到这里，声卡和他的pcm实例创建完成。
回到snd_soc_instantiate_card函数，完成snd_card和snd_pcm的创建后，接着对dapm和dai支持的格式做出一些初始化合设置工作后，调用了 card->late_probe(card)进行一些最后的初始化合设置工作，最后则是调用标准alsa驱动的声卡注册函数对声卡进行注册：
if (card->late_probe) {
ret = card->late_probe(card);
if (ret < 0) {
dev_err(card->dev, "%s late_probe() failed: %d\n",
card->name, ret);
goto probe_aux_dev_err;
}
}
ret = snd_card_register(card->snd_card);
if (ret < 0) {
printk(KERN_ERR "asoc: failed to register soundcard for %s\n", card->name);
goto probe_aux_dev_err;
}至此，整个Machine驱动的初始化已经完成，通过各个子结构的probe调用，实际上，也完成了部分Platfrom驱动和Codec驱动的初始化工作，整个过程可以用以下的序列图表示：
基于3.0内核的soc probe序列图