概述
在ASOC小节中描述了整个ASOC的架构，其中Machine是ASOC架构中的关键部件，没有Machine部件，单独的Codec和Platform是无法工作的。因此本节则先从Machine部分开始，那应该如何开始呢？ 答案当然是从代码入手，先进入ASOC在kernel中的位置: kernel/sound/soc下
代码语言：javascript复制root@test:~/test/kernel/sound/soc$ ls
adi au1x blackfin codecs dwc generic jz4740 kirkwood mxs omap rockchip sh soc-cache.c soc-core.c soc-devres.c soc-io.c soc-pcm.c spear txx9
atmel bcm cirrus davinci fsl intel Kconfig Makefile nuc900 pxa samsung sirf soc-compress.c soc-dapm.c soc-generic-dmaengine-pcm.c soc-jack.c soc-utils.c tegra ux500此目录下就是当前支持ASOC架构的平台，在这里以samsung架构做为参考。kernel版本: 3.18
Machine代码分析
samsung平台的machine代码选择为: s3c24xx_uda134x.c
此代码先注册平台驱动s3c24xx_uda134x_driver， 当平台驱动和平台设备(以前在arch下，目前在dt中配置)的名字想匹配的时候，就会调用平台驱动中的probe函数s3c24xx_uda134x_probe。
代码语言：javascript复制 s3c24xx_uda134x_snd_device = platform_device_alloc("soc-audio", -1);
if (!s3c24xx_uda134x_snd_device) {
printk(KERN_ERR "S3C24XX_UDA134X SoC Audio: "
"Unable to registern");
return -ENOMEM;
}
platform_set_drvdata(s3c24xx_uda134x_snd_device,&snd_soc_s3c24xx_uda134x);此出分配名字为"soc-audio"的平台设备，然后将snd_soc_s3c24xx_uda134x设置到平台设备的dev->driver_data中。关于snd_soc_s3c24xx_uda134x结构在后面说明。
既然此处注册"soc-audio"的设备，就会存在名字为"soc-audio"的驱动，搜索"soc-audio"，就会发现在soc-core.c中存在。
代码语言：javascript复制/* 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,
};
static int __init snd_soc_init(void)
{
snd_soc_util_init();
return platform_driver_register(&soc_driver);
}当platform_device和platform_driver相匹配的话，就会调用soc_probe函数。
代码语言：javascript复制static int soc_probe(struct platform_device *pdev)
{
struct snd_soc_card *card = platform_get_drvdata(pdev);
/*
* no card, so machine driver should be registering card
* we should not be here in that case so ret error
*/
if (!card)
return -EINVAL;
dev_warn(&pdev->dev,
"ASoC: machine %s should use snd_soc_register_card()n",
card->name);
/* Bodge while we unpick instantiation */
card->dev = &pdev->dev;
return snd_soc_register_card(card);
}此处会调用snd_soc_register_card，会在ASOC core中注册一个card。 此处的card就是snd_soc_s3c24xx_uda134x结构。接下来谈论此结构的作用。
代码语言：javascript复制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 = "s3c24xx-iis",
};
static struct snd_soc_card snd_soc_s3c24xx_uda134x = {
.name = "S3C24XX_UDA134X",
.owner = THIS_MODULE,
.dai_link = &s3c24xx_uda134x_dai_link,
.num_links = 1,
};其中dai_link结构就是用作连接platform和codec的，指明到底用那个codec，那个platfrom。那是通过什么指定的？ 如果有兴趣可以详细看snd_soc_dai_link的注释，此注释写的非常清楚。
代码语言：javascript复制struct snd_soc_dai_link {
/* config - must be set by machine driver */
const char *name; /* Codec name */
const char *stream_name; /* Stream name */
/*
* You MAY specify the link's CPU-side device, either by device name,
* or by DT/OF node, but not both. If this information is omitted,
* the CPU-side DAI is matched using .cpu_dai_name only, which hence
* must be globally unique. These fields are currently typically used
* only for codec to codec links, or systems using device tree.
*/
const char *cpu_name;
struct device_node *cpu_of_node;
/*
* You MAY specify the DAI name of the CPU DAI. If this information is
* omitted, the CPU-side DAI is matched using .cpu_name/.cpu_of_node
* only, which only works well when that device exposes a single DAI.
*/
const char *cpu_dai_name;
/*
* You MUST specify the link's codec, either by device name, or by
* DT/OF node, but not both.
*/
const char *codec_name;
struct device_node *codec_of_node;
/* You MUST specify the DAI name within the codec */
const char *codec_dai_name;
struct snd_soc_dai_link_component *codecs;
unsigned int num_codecs;
/*
* You MAY specify the link's platform/PCM/DMA driver, either by
* device name, or by DT/OF node, but not both. Some forms of link
* do not need a platform.
*/
const char *platform_name;
struct device_node *platform_of_node;
int be_id; /* optional ID for machine driver BE identification */
const struct snd_soc_pcm_stream *params;
unsigned int dai_fmt; /* format to set on init */
enum snd_soc_dpcm_trigger trigger[2]; /* trigger type for DPCM */
/* Keep DAI active over suspend */
unsigned int ignore_suspend:1;
/* Symmetry requirements */
unsigned int symmetric_rates:1;
unsigned int symmetric_channels:1;
unsigned int symmetric_samplebits:1;
/* Do not create a PCM for this DAI link (Backend link) */
unsigned int no_pcm:1;
/* This DAI link can route to other DAI links at runtime (Frontend)*/
unsigned int dynamic:1;
/* DPCM capture and Playback support */
unsigned int dpcm_capture:1;
unsigned int dpcm_playback:1;
/* pmdown_time is ignored at stop */
unsigned int ignore_pmdown_time:1;
/* codec/machine specific init - e.g. add machine controls */
int (*init)(struct snd_soc_pcm_runtime *rtd);
/* optional hw_params re-writing for BE and FE sync */
int (*be_hw_params_fixup)(struct snd_soc_pcm_runtime *rtd,
struct snd_pcm_hw_params *params);
/* machine stream operations */
const struct snd_soc_ops *ops;
const struct snd_soc_compr_ops *compr_ops;
/* For unidirectional dai links */
bool playback_only;
bool capture_only;
};.cpu_dai_name: 用于指定cpu侧的dai名字，也就是所谓的cpu侧的数字音频接口，一般都是i2S接口。如果省略则会使用cpu_name/cou_of_name。
.codec_dai_name: 用于codec侧的dai名字，不可以省略。
.codec_name: 用于指定codec芯片。不可以省略。
.platform_name: 用于指定cpu侧平台驱动，通常都是DMA驱动，用于传输。
.ops: audio的相关操作函数集合。
再次回到snd_soc_register_card函数中，继续分析Machine的作用。
1. 根据struct snd_soc_dai_link结构体的个数，此处是一个，检测下需要设置的name是否已经设置。
代码语言：javascript复制if (link->platform_name && link->platform_of_node) {
dev_err(card->dev,
"ASoC: Both platform name/of_node are set for %sn",link->name);
return -EINVAL;
}2. 分配一个struct snd_soc_pcm_runtime结构，然后根据num_links，设置card，复制dai_link等。
代码语言：javascript复制card->rtd = devm_kzalloc(card->dev,sizeof(struct snd_soc_pcm_runtime) *
(card->num_links card->num_aux_devs),GFP_KERNEL);
if (card->rtd == NULL)
return -ENOMEM;
card->num_rtd = 0;
card->rtd_aux = &card->rtd[card->num_links];
for (i = 0; i < card->num_links; i ) {
card->rtd[i].card = card;
card->rtd[i].dai_link = &card->dai_link[i];
card->rtd[i].codec_dais = devm_kzalloc(card->dev,
sizeof(struct snd_soc_dai *) *
(card->rtd[i].dai_link->num_codecs),
GFP_KERNEL);
if (card->rtd[i].codec_dais == NULL)
return -ENOMEM;
}3. 然后所有的重点工作全部在snd_soc_instantiate_card函数中实现。
分析snd_soc_instantiate_card函数的实际操作:
1. 根据num_links的值，进行DAIs的bind工作。第一步先bind cpu侧的dai
代码语言：javascript复制 cpu_dai_component.name = dai_link->cpu_name;
cpu_dai_component.of_node = dai_link->cpu_of_node;
cpu_dai_component.dai_name = dai_link->cpu_dai_name;
rtd->cpu_dai = snd_soc_find_dai(&cpu_dai_component);
if (!rtd->cpu_dai) {
dev_err(card->dev, "ASoC: CPU DAI %s not registeredn",
dai_link->cpu_dai_name);
return -EPROBE_DEFER;
}此处dai_link就是在machine中注册的struct snd_soc_dai_link结构体，cpu_dai_name也就是注册的name，最后通过snd_soc_find_dai接口出查找。
代码语言：javascript复制static struct snd_soc_dai *snd_soc_find_dai(
const struct snd_soc_dai_link_component *dlc)
{
struct snd_soc_component *component;
struct snd_soc_dai *dai;
/* Find CPU DAI from registered DAIs*/
list_for_each_entry(component, &component_list, list) {
if (dlc->of_node && component->dev->of_node != dlc->of_node)
continue;
if (dlc->name && strcmp(component->name, dlc->name))
continue;
list_for_each_entry(dai, &component->dai_list, list) {
if (dlc->dai_name && strcmp(dai->name, dlc->dai_name))
continue;
return dai;
}
}
return NULL;
}此函数会在component_list链表中先找到相同的name，然后在component->dai_list中查找是否有相同的dai_name。此处的component_list是在注册codec和platform中的时候设置的。会在codec和platform的时候会详细介绍。在此处找到注册的cpu_dai之后，存在snd_soc_pcm_runtime中的cpu_dai中。
2. 然后根据codec的数据，寻找codec侧的dai。
代码语言：javascript复制/* Find CODEC from registered CODECs */
for (i = 0; i < rtd->num_codecs; i ) {
codec_dais[i] = snd_soc_find_dai(&codecs[i]);
if (!codec_dais[i]) {
dev_err(card->dev, "ASoC: CODEC DAI %s not registeredn",
codecs[i].dai_name);
return -EPROBE_DEFER;
}
}然后将找到的codec侧的dai也同样赋值给snd_soc_pcm_runtime中的codec_dai中。
3. 在platform_list链表中查找platfrom，根据dai_link中的platform_name域。如果没有platform_name，则设置为"snd-soc-dummy"
代码语言：javascript复制/* if there's no platform we match on the empty platform */
platform_name = dai_link->platform_name;
if (!platform_name && !dai_link->platform_of_node)
platform_name = "snd-soc-dummy";
/* find one from the set of registered platforms */
list_for_each_entry(platform, &platform_list, list) {
if (dai_link->platform_of_node) {
if (platform->dev->of_node !=
dai_link->platform_of_node)
continue;
} else {
if (strcmp(platform->component.name, platform_name))
continue;
}
rtd->platform = platform;
}这样查找完毕之后，snd_soc_pcm_runtime中存储了查找到的codec, dai, platform。
4. 接着初始化注册的codec cache，cache_init代表是否已经初始化过。
代码语言：javascript复制/* initialize the register cache for each available codec */
list_for_each_entry(codec, &codec_list, list) {
if (codec->cache_init)
continue;
ret = snd_soc_init_codec_cache(codec);
if (ret < 0)
goto base_error;
}5. 然后调用ALSA中的创建card的函数: snd_card_new创建一个card
代码语言：javascript复制/* card bind complete so register a sound card */
ret = snd_card_new(card->dev, SNDRV_DEFAULT_IDX1, SNDRV_DEFAULT_STR1,
card->owner, 0, &card->snd_card);
if (ret < 0) {
dev_err(card->dev,
"ASoC: can't create sound card for card %s: %dn",
card->name, ret);
goto base_error;
}6. 然后依次调用各个子部件的probe函数
代码语言：javascript复制/* initialise the sound card only once */
if (card->probe) {
ret = card->probe(card);
if (ret < 0)
goto card_probe_error;
}
/* probe all components used by DAI links on this card */
for (order = SND_SOC_COMP_ORDER_FIRST; order <= SND_SOC_COMP_ORDER_LAST;
order ) {
for (i = 0; i < card->num_links; i ) {
ret = soc_probe_link_components(card, i, order);
if (ret < 0) {
dev_err(card->dev,
"ASoC: failed to instantiate card %dn",
ret);
goto probe_dai_err;
}
}
}
/* probe all DAI links on this card */
for (order = SND_SOC_COMP_ORDER_FIRST; order <= SND_SOC_COMP_ORDER_LAST;
order ) {
for (i = 0; i < card->num_links; i ) {
ret = soc_probe_link_dais(card, i, order);
if (ret < 0) {
dev_err(card->dev,
"ASoC: failed to instantiate card %dn",
ret);
goto probe_dai_err;
}
}
}7. 在soc_probe_link_dais函数中依次调用了cpu_dai， codec_dai侧的probe函数
代码语言：javascript复制/* probe the cpu_dai */
if (!cpu_dai->probed &&
cpu_dai->driver->probe_order == order) {
if (cpu_dai->driver->probe) {
ret = cpu_dai->driver->probe(cpu_dai);
if (ret < 0) {
dev_err(cpu_dai->dev,
"ASoC: failed to probe CPU DAI %s: %dn",
cpu_dai->name, ret);
return ret;
}
}
cpu_dai->probed = 1;
}
/* probe the CODEC DAI */
for (i = 0; i < rtd->num_codecs; i ) {
ret = soc_probe_codec_dai(card, rtd->codec_dais[i], order);
if (ret)
return ret;8. 最终调用到soc_new_pcm函数创建pcm设备:
代码语言：javascript复制if (!dai_link->params) {
/* create the pcm */
ret = soc_new_pcm(rtd, num);
if (ret < 0) {
dev_err(card->dev, "ASoC: can't create pcm %s :%dn",
dai_link->stream_name, ret);
return ret;
}最中此函数会调用ALSA的标准创建pcm设备的接口: snd_pcm_new，然后会设置pcm相应的ops操作函数集合。然后调用到platform->driver->pcm_new的函数。此处不帖函数了。
9. 接着会在dapm和dai widget做相应的操作，后期会设置control参数，最终会调用到ALSA的注册card的函数snd_card_register。
代码语言：javascript复制ret = snd_card_register(card->snd_card);
if (ret < 0) {
dev_err(card->dev, "ASoC: failed to register soundcard %dn",
ret);
goto probe_aux_dev_err;
}总结： 经过Machine的驱动的注册，Machine会根据注册以"soc_audio"为名字的平台设备，然后在同名的平台的驱动的probe函数中，会根据snd_soc_dai_link结构体中的name，进行匹配查找相应的codec, codec_dai，platform, cpu_dai。找到之后将这些值全部放入结构体snd_soc_pcm_runtime的相应位置，然后注册card，依次调用codec, platform，cpu_dai侧相应的probe函数进行初始化，接着创建pcm设备，注册card到系统中。其实ASOC也就是在ALSA的基础上又再次封装了一次，让写驱动更方便，简便。
这样封装之后，就可以大大简化驱动的编写，关于Machine驱动需要做的:
1. 注册名为"soc-audio"的平台设备。
2. 分配一个struct snd_soc_card结构体，然后设置其中的dai_link。对其中的dai_link再次设置。
3. 将struct snd_soc_card结构放入到平台设备的dev的私有数据中。
4. 注册平台设备。
