ALSA ASOC Machine Driver

一、Machine 简介
二、ASoC Machine Driver
- 2.1 Machine Driver 的 Platform Driver & Platform Device 驱动模型
- 2.2 在 Probe() 中注册声卡
三、snd_soc_register_card 函数
- 3.1 bind DAIs
- 3.2 New a sound card
- 3.3 Create card new widgets
- 3.4 Probe all component used by DAI links
- 3.5 Probe all DAI links on this card
- 3.6 DAPM 相关操作
- 3.7 Register Sound Card
四、创建 Pcm Device 节点

一、Machine 简介

在前面的章节中已经有提到，ASoC 被分为 Machine、Platform 和 Codec 三大部分，其中 Machine 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。Machine 驱动负责处理机器特有的一些控件和音频事件，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。

二、ASoC Machine Driver

# Note下面均以 mt2701-wm8960.c 为例进行讲解

2.1 Machine Driver 的 Platform Driver & Platform Device 驱动模型

该部分其实就是 /sound/soc/mediate/mt2701/mt2701-wm8960.c 中的 platform driver 与 /arch/arm/boot/dts/mt7623a-rfb-emmc.dts 中的 platform device 进行匹配，匹配成功后调用 mt2701_wm8960_machine_probe() 函数。

1）mt2701-wm8960.c

#ifdef CONFIG_OF
static const struct of_device_id mt2701_wm8960_machine_dt_match[] = {{.compatible = "mediatek,mt2701-wm8960-machine",},{}
};
#endifstatic struct platform_driver mt2701_wm8960_machine = {.driver = {.name = "mt2701-wm8960",
#ifdef CONFIG_OF.of_match_table = mt2701_wm8960_machine_dt_match,
#endif},.probe = mt2701_wm8960_machine_probe,
};module_platform_driver(mt2701_wm8960_machine);

在 platform driver 中有注册名为 "ediatek,mt2701-wm8960-machine"；

2）mt7623a-rfb-emmc.dts

...
sound {compatible = "mediatek,mt2701-wm8960-machine";mediatek,platform = <&afe>;audio-routing ="Headphone", "HP_L","Headphone", "HP_R","LINPUT1", "AMIC","RINPUT1", "AMIC";mediatek,audio-codec = <&wm8960>;pinctrl-names = "default";pinctrl-0 = <&i2s0_pins_a>;};
...

在设备树文件中有注册名为 "mediatek,mt2701-wm8960-machine" 的 platform device，当 platform driver & platform device 匹配之后则会调用 platform_driver 下的 probe() 函数。

2.2 在 Probe() 中注册声卡

devm_snd_soc_register_card(&pdev->dev, card); // 注册声卡

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。在 mt2701_wm8960_machine__probe() 函数会调用 devm_snd_soc_register_card() 函数在 ASoC Core 中注册一个 card，即 snd_soc_card，代码如下：

static struct snd_soc_card mt2701_wm8960_card = {.name = "mt2701-wm8960",.owner = THIS_MODULE,.dai_link = mt2701_wm8960_dai_links,.num_links = ARRAY_SIZE(mt2701_wm8960_dai_links),.controls = mt2701_wm8960_controls,.num_controls = ARRAY_SIZE(mt2701_wm8960_controls),.dapm_widgets = mt2701_wm8960_widgets,.num_dapm_widgets = ARRAY_SIZE(mt2701_wm8960_widgets),
};static int mt2701_wm8960_machine_probe(struct platform_device *pdev)
{struct snd_soc_card *card = &mt2701_wm8960_card;...ret = devm_snd_soc_register_card(&pdev->dev, card);if (ret)dev_err(&pdev->dev, "%s snd_soc_register_card fail %d\n",__func__, ret);...return ret;
}

如上代码所示，在 snd_soc_card 中会定义 num_links 个 snd_soc_dai_link 实例 dai_link，其中 dai_link 根据 DPCM 可以分为 Front End & Back End（当然也还有在两者之外的），并且会指定 cpu_name/cpu_of_node/cpu_dai_name、codec_name/codec_of_node/codec_dai_name、platform_name/platform_of_node 等，稍后 Machine 驱动将会利用这些属性去匹配系统中已经注册的 platform、cpu_dai、codec component。

三、snd_soc_register_card 函数

snd_soc_register_card() 函数中大部分工作都是在 snd_soc_instantiate_card() 函数中实现，其主要实现的内容如下：（涉及 DAPM 此处省略，待后面 DAPM 部分再解析）

3.1 bind DAIs

在 snd_soc_instantiate_card() 函数首先会进行 bind dais 操作，代码如下：

/* bind DAIs *///    Machine://      For FE dai_link: bind platform FE dai//     For BE dai_link: bind platform BE dai & Codec dai////   FE  --------  BE   -------//      _|        |_   _|       |_ HP//     _|platform|_    | Codec |//    _|     |_   _|       |_ SPK//     |        |     |       |//       --------       -------//for_each_card_prelinks(card, i, dai_link) {ret = soc_bind_dai_link(card, dai_link);if (ret != 0)goto base_error;}

如前面所述，在定义 snd_soc_card 时有定义 dai_links，如上述代码则会循环为每个 dai_link 均调用 soc_bind_dai_link() 函数进行匹配绑定 DAI 操作，其中 soc_bind_dai_link() 函数定义如下：

static int soc_bind_dai_link(struct snd_soc_card *card,struct snd_soc_dai_link *dai_link)
{struct snd_soc_pcm_runtime *rtd;struct snd_soc_dai_link_component *codecs = dai_link->codecs;struct snd_soc_dai_link_component cpu_dai_component;struct snd_soc_component *component;struct snd_soc_dai **codec_dais;int i;if (dai_link->ignore)return 0;dev_dbg(card->dev, "ASoC: binding %s\n", dai_link->name);//3.1.1 Check the dai_link 是否已经被绑定if (soc_is_dai_link_bound(card, dai_link)) {dev_dbg(card->dev, "ASoC: dai link %s already bound\n",dai_link->name);return 0;}//3.1.2 为该 dai_link 分配一个 snd_soc_pcm_runtime 实例 rtdrtd = soc_new_pcm_runtime(card, dai_link);if (!rtd)return -ENOMEM;//3.1.3 Find cpu_dai from dai_link cpu params & component_list cpu paramscpu_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_info(card->dev, "ASoC: CPU DAI %s not registered\n",dai_link->cpu_dai_name);goto _err_defer;}snd_soc_rtdcom_add(rtd, rtd->cpu_dai->component);//3.1.4 FInd codec_dais[] from dai_link codec params & component_list codec paramsrtd->num_codecs = dai_link->num_codecs;/* Find CODEC from registered CODECs *//* we can use for_each_rtd_codec_dai() after this */codec_dais = rtd->codec_dais;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 registered\n",codecs[i].dai_name);goto _err_defer;}snd_soc_rtdcom_add(rtd, codec_dais[i]->component);}/* Single codec links expect codec and codec_dai in runtime data */rtd->codec_dai = codec_dais[0];//3.1.5 Find platform component from dai_link platform params & component_list platform params/* find one from the set of registered platforms */for_each_component(component) {if (!snd_soc_is_matching_component(dai_link->platform,component))continue;snd_soc_rtdcom_add(rtd, component);}//3.1.6 Add rtd -> card->rtd_listsoc_add_pcm_runtime(card, rtd);return 0;_err_defer:soc_free_pcm_runtime(rtd);return -EPROBE_DEFER;
}

如代码所示，该函数首先会为 dai_link 分配一个 snd_soc_pcm_runtime 实例 rtd，在前面小节中已经介绍 platform & codec 驱动最终都会创建相应的 component 实例并插入到全局链表 component_list 中，此处则会根据 dai_link 参数匹配相应的 platform & codec，

① 根据 cpu_name/cpu_of_node/cpu_dai_name 从 component(cpu_dai_component) 中找到匹配的 snd_soc_dai cpu_dai(rtd->cpu_dai)，并将该 cpu_dai 对应的 component 插入到 rtd->component_list 链表中；
② 根据 codec_name/codec_of_node/codec_dai_name 从 component(codec_dai component) 中找到匹配的 snd_soc_dai codec_dais(rtd->codec_dais[])，并将该 codec_dais 对应的 component 插入到 rtd->component_list 链表中；
③ 根据 platform_name/platform_of_node 从 component(platform driver component) 【例如前面的 mtk_afe_pcm_platform】中找到匹配的 component，并将该 component 插入到 rtd->component_list 链表中。

最后调用 soc_add_pcm_runtime() 函数将该 rtd 插入到 card->rtd_list 中。

3.2 New a sound card

紧接着调用标准 alsa core 函数创建声卡实例，代码如下：

/* 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: %d\n",card->name, ret);goto base_error;}

3.3 Create card new widgets

紧接着调用 snd_soc_dapm_new_controls() 函数创建 card->dapm_widgets，如下代码可知，定义该 widget 可以通知直接在代码中定义 dapm_widgets，也可以通过在 dts 文件中创建（该函数详解解析属于 DAPM，见后面章节）

if (card->dapm_widgets)snd_soc_dapm_new_controls(&card->dapm, card->dapm_widgets,card->num_dapm_widgets);if (card->of_dapm_widgets)snd_soc_dapm_new_controls(&card->dapm, card->of_dapm_widgets,card->num_of_dapm_widgets);

3.4 Probe all component used by DAI links

紧接着遍历 card->rtd_list 中的所有 rtd 调用 soc_probe_link_components() 函数，代码如下：

/* probe all components used by DAI links on this card */for_each_comp_order(order) {for_each_card_rtds(card, rtd) {ret = soc_probe_link_components(card, rtd, order);if (ret < 0) {dev_err(card->dev,"ASoC: failed to instantiate card %d\n",ret);goto probe_dai_err;}}}

其中 soc_probe_link_components() 函数定义如下：

static int soc_probe_link_components(struct snd_soc_card *card,struct snd_soc_pcm_runtime *rtd, int order)
{struct snd_soc_component *component;struct snd_soc_rtdcom_list *rtdcom;int ret;// 3.4.1 遍历该 rtd->component_list, 为所有用到的 components 调用 soc_probe_component() 函数for_each_rtdcom(rtd, rtdcom) {component = rtdcom->component;if (component->driver->probe_order == order) {ret = soc_probe_component(card, component);if (ret < 0)return ret;}}return 0;
}

如上代码所示，该函数会遍历该 rtd->component_list, 为所有用到的 components 调用 soc_probe_component() 函数，定义如下：

static int soc_probe_component(struct snd_soc_card *card,struct snd_soc_component *component)
{// dapm context 详见后面章节struct snd_soc_dapm_context *dapm =snd_soc_component_get_dapm(component);struct snd_soc_dai *dai;int ret;if (!strcmp(component->name, "snd-soc-dummy"))return 0;...if (!try_module_get(component->dev->driver->owner))return -ENODEV;component->card = card;dapm->card = card;soc_set_name_prefix(card, component);soc_init_component_debugfs(component);// 3.4.1.1 创建 component->driver->dai_widgetsif (component->driver->dapm_widgets) {ret = snd_soc_dapm_new_controls(dapm,component->driver->dapm_widgets,component->driver->num_dapm_widgets);if (ret != 0) {dev_err(component->dev,"Failed to create new controls %d\n", ret);goto err_probe;}}// 3.4.1.2 遍历 component->dai_list 创建 dai widgetsfor_each_component_dais(component, dai) {ret = snd_soc_dapm_new_dai_widgets(dapm, dai);if (ret != 0) {dev_err(component->dev,"Failed to create DAI widgets %d\n", ret);goto err_probe;}}// 3.4.1.3 调用 component->driver->probe() 函数if (component->driver->probe) {ret = component->driver->probe(component);if (ret < 0) {dev_err(component->dev,"ASoC: failed to probe component %d\n", ret);goto err_probe;}WARN(dapm->idle_bias_off &&dapm->bias_level != SND_SOC_BIAS_OFF,"codec %s can not start from non-off bias with idle_bias_off==1\n",component->name);}...// 3.4.1.4 创建 component->driver->controls & dapm_routesif (component->driver->controls)snd_soc_add_component_controls(component,component->driver->controls,component->driver->num_controls);if (component->driver->dapm_routes)snd_soc_dapm_add_routes(dapm,component->driver->dapm_routes,component->driver->num_dapm_routes);// 3.4.1.5 将该 dapm text(即 component dapm text) 插入到 card->dapm_list 中list_add(&dapm->list, &card->dapm_list);/* see for_each_card_components */list_add(&component->card_list, &card->component_dev_list);return 0;...return ret;
}

如代码所示，该函数主要是实现对匹配的 platform/codec component 涉及的 DAPM、kcontrol 相关操作(详见后面 DAPM 章节)，并调用 component->driver->probe()，该 probe() 函数主要也是涉及 DAPM、kcontrol 操作等。

3.5 Probe all DAI links on this card

紧接着遍历 card->rtd_list 中的所有 rtd 调用 soc_probe_link_dais() 函数，代码如下：

/* probe all DAI links on this card */for_each_comp_order(order) {for_each_card_rtds(card, rtd) {ret = soc_probe_link_dais(card, rtd, order);if (ret < 0) {dev_err(card->dev,"ASoC: failed to instantiate card %d\n",ret);goto probe_dai_err;}}}

其中 soc_probe_link_dais() 函数定义如下：

static int soc_probe_link_dais(struct snd_soc_card *card,struct snd_soc_pcm_runtime *rtd, int order)
{struct snd_soc_dai_link *dai_link = rtd->dai_link;struct snd_soc_dai *cpu_dai = rtd->cpu_dai;struct snd_soc_rtdcom_list *rtdcom;struct snd_soc_component *component;struct snd_soc_dai *codec_dai;int i, ret, num;...// 3.5.1 调用 rtd->cpu_dai->driver->probe() 函数ret = soc_probe_dai(cpu_dai, order);if (ret)return ret;// 3.5.2 调用 rtd->codec_dais[]->driver->probe() 函数/* probe the CODEC DAI */for_each_rtd_codec_dai(rtd, i, codec_dai) {ret = soc_probe_dai(codec_dai, order);if (ret)return ret;}...// 3.5.3 创建 pcm deviceif (!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 :%d\n",dai_link->stream_name, ret);return ret;}// 3.5.4 调用 cpu_dai/codec_dais[]->driver->pcm_new() 函数ret = soc_link_dai_pcm_new(&cpu_dai, 1, rtd);if (ret < 0)return ret;ret = soc_link_dai_pcm_new(rtd->codec_dais,rtd->num_codecs, rtd);if (ret < 0)return ret;} else {INIT_DELAYED_WORK(&rtd->delayed_work,codec2codec_close_delayed_work);}return 0;
}

如上述函数定义所示，该函数首先会调用各个 rtd->cpu_dai/codec_dais[]->driver->probe() 函数，接着调用 soc_new_pcm() 函数用于 create the pcm，该函数定义如下：

/* create a new pcm */
int soc_new_pcm(struct snd_soc_pcm_runtime *rtd, int num)
{struct snd_soc_dai *codec_dai;struct snd_soc_dai *cpu_dai = rtd->cpu_dai;struct snd_soc_component *component;struct snd_soc_rtdcom_list *rtdcom;struct snd_pcm *pcm;char new_name[64];int ret = 0, playback = 0, capture = 0;int i;// 3.5.3.1 Check rtd 是否支持 playback & capture// DPCM://       FE: rtd->dai_link->dynamic//      BE: rtd->dai_link->no_pcm// No DPCM://        codec_dai&cpu_daiif (rtd->dai_link->dynamic || rtd->dai_link->no_pcm) {playback = rtd->dai_link->dpcm_playback;capture = rtd->dai_link->dpcm_capture;} else {for_each_rtd_codec_dai(rtd, i, codec_dai) {if (codec_dai->driver->playback.channels_min)playback = 1;if (codec_dai->driver->capture.channels_min)capture = 1;}capture = capture && cpu_dai->driver->capture.channels_min;playback = playback && cpu_dai->driver->playback.channels_min;}if (rtd->dai_link->playback_only) {playback = 1;capture = 0;}if (rtd->dai_link->capture_only) {playback = 0;capture = 1;}// 3.5.3.2 create the pcm// 由于 FE->BE ==> BE 调用 snd_pcm_new_internal() 不会创建 user device, 自然后面也就不需要填充 ops 操作集给到 user/* create the PCM */if (rtd->dai_link->no_pcm) {snprintf(new_name, sizeof(new_name), "(%s)",rtd->dai_link->stream_name);ret = snd_pcm_new_internal(rtd->card->snd_card, new_name, num,playback, capture, &pcm);} else {if (rtd->dai_link->dynamic)snprintf(new_name, sizeof(new_name), "%s (*)",rtd->dai_link->stream_name);elsesnprintf(new_name, sizeof(new_name), "%s %s-%d",rtd->dai_link->stream_name,(rtd->num_codecs > 1) ?"multicodec" : rtd->codec_dai->name, num);ret = snd_pcm_new(rtd->card->snd_card, new_name, num, playback,capture, &pcm);}if (ret < 0) {dev_err(rtd->card->dev, "ASoC: can't create pcm for %s\n",rtd->dai_link->name);return ret;}...pcm->nonatomic = rtd->dai_link->nonatomic;rtd->pcm = pcm;pcm->private_data = rtd;if (rtd->dai_link->no_pcm) {if (playback)pcm->streams[SNDRV_PCM_STREAM_PLAYBACK].substream->private_data = rtd;if (capture)pcm->streams[SNDRV_PCM_STREAM_CAPTURE].substream->private_data = rtd;goto out;}// 3.5.1.3 for BE/ No DPCM 设置 ops 操作集/* ASoC PCM operations */if (rtd->dai_link->dynamic) {rtd->ops.open      = dpcm_fe_dai_open;rtd->ops.hw_params   = dpcm_fe_dai_hw_params;rtd->ops.prepare    = dpcm_fe_dai_prepare;rtd->ops.trigger  = dpcm_fe_dai_trigger;rtd->ops.hw_free  = dpcm_fe_dai_hw_free;rtd->ops.close        = dpcm_fe_dai_close;rtd->ops.pointer    = soc_pcm_pointer;rtd->ops.ioctl        = soc_pcm_ioctl;} else {rtd->ops.open       = soc_pcm_open;rtd->ops.hw_params   = soc_pcm_hw_params;rtd->ops.prepare    = soc_pcm_prepare;rtd->ops.trigger  = soc_pcm_trigger;rtd->ops.hw_free  = soc_pcm_hw_free;rtd->ops.close        = soc_pcm_close;rtd->ops.pointer    = soc_pcm_pointer;rtd->ops.ioctl        = soc_pcm_ioctl;}for_each_rtdcom(rtd, rtdcom) {const struct snd_pcm_ops *ops = rtdcom->component->driver->ops;if (!ops)continue;if (ops->ack)rtd->ops.ack      = soc_rtdcom_ack;if (ops->copy_user)rtd->ops.copy_user   = soc_rtdcom_copy_user;if (ops->copy_kernel)rtd->ops.copy_kernel = soc_rtdcom_copy_kernel;if (ops->fill_silence)rtd->ops.fill_silence = soc_rtdcom_fill_silence;if (ops->page)rtd->ops.page        = soc_rtdcom_page;if (ops->mmap)rtd->ops.mmap        = soc_rtdcom_mmap;}if (playback)snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK, &rtd->ops);if (capture)snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE, &rtd->ops);//3.5.1.4 调用 rtd 下所有 component->driver_pcm_new()for_each_rtdcom(rtd, rtdcom) {component = rtdcom->component;if (!component->driver->pcm_new)continue;ret = component->driver->pcm_new(rtd);if (ret < 0) {dev_err(component->dev,"ASoC: pcm constructor failed: %d\n",ret);return ret;}}pcm->private_free = soc_pcm_private_free;
out:dev_info(rtd->card->dev, "%s <-> %s mapping ok\n",(rtd->num_codecs > 1) ? "multicodec" : rtd->codec_dai->name,cpu_dai->name);return ret;
}

如上所示该函数首先会调用标准 alsa core 函数，如下：

DPCM BE：由于 FE->BE，在操作 FE 时会同时操作与之 connect 的 BE，故不会对 BE 创建 user pcm device，即调用 snd_pcm_new_internal() 函数；
DPCM FE：调用 snd_pcm_new() 创建声卡 pcm device，并且 FE Device Name：("%s(*)", dai_link->stream_name) （故有 '*' 代表为 FE 可以动态连接 BE）；
No DPCM：调用 snd_pcm_new() 创建声卡 pcm device，并且 No DPCM Device Name：("%s %s-%d", dai_link->stream, "multicodec"/codec_dai->name, num)（故无 '*' 代表 No NDPCM）

从上述代码中可以看出，对于 DPCM FE 和 No DPCM 对应 rtd->ops(snd_soc_ops) 字段赋值也不一样，由于 FE 的操作函数中需要涉及对 Connect’s BE 的操作；

在初始化 rtd->ops 时还会根据 platform driver 中的 snd_pcm_ops 填充相应的字段，如 ack、copy_user 等，最终会调用标准 alsa core 函数 snd_pcm_set_ops() 将 rtd->ops 设置为 substream->ops.

# Note：在 rtd->ops 字段中，如 open、hw_params、prepare 等回调函数中最终都会调用到 platform_driver & cpu_dai driver 以及 connect BE 对应的 codec driver 对应的 ops 字段相应的回调函数。

最后，该函数还会调用 rtd 下 component->driver->pcm_new() （主要是 platform driver pcm_new()）函数，如前面 platform driver 小节讲解，此函数主要是预分配 DMA Memory，最后真正分配 DMA Memory 是在 Platform Driver/Cpu Dai Driver hw_params 中。

3.6 DAPM 相关操作

接着会调用 card dapm、route 相关操作（详细见后面章节）

 snd_soc_dapm_link_dai_widgets(card);snd_soc_dapm_connect_dai_link_widgets(card);if (card->controls)snd_soc_add_card_controls(card, card->controls,card->num_controls);if (card->dapm_routes)snd_soc_dapm_add_routes(&card->dapm, card->dapm_routes,card->num_dapm_routes);if (card->of_dapm_routes)snd_soc_dapm_add_routes(&card->dapm, card->of_dapm_routes,card->num_of_dapm_routes);...snd_soc_dapm_new_widgets(card);

3.7 Register Sound Card

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;}}...ret = snd_card_register(card->snd_card);if (ret < 0) {dev_err(card->dev, "ASoC: failed to register soundcard %d\n",ret);goto probe_aux_dev_err;}card->instantiated = 1;

如上代码所示，在该函数最后先填充 snd_card 的重要参数，如 shortname、longname、driver 等，最后调用标准 alsa core 函数 snd_card_register() 来注册声卡以及注册该声卡下的所有 snd_devices.

至此，整个 Machine 驱动的初始化已经完成，通过各个子结构 probe 调用等，实际上，也完成了部分 Platform 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：

四、创建 Pcm Device 节点

snd_soc_dai_link mt2701_wm8960_dai_link 定义如下：

static struct snd_soc_dai_link mt2701_wm8960_dai_links[] = {/* FE */{.name = "wm8960-playback",.stream_name = "wm8960-playback",.cpu_dai_name = "PCMO0",.codec_name = "snd-soc-dummy",.codec_dai_name = "snd-soc-dummy-dai",.trigger = {SND_SOC_DPCM_TRIGGER_POST,SND_SOC_DPCM_TRIGGER_POST},.dynamic = 1,.dpcm_playback = 1,},{.name = "wm8960-capture",.stream_name = "wm8960-capture",.cpu_dai_name = "PCM0",.codec_name = "snd-soc-dummy",.codec_dai_name = "snd-soc-dummy-dai",.trigger = {SND_SOC_DPCM_TRIGGER_POST,SND_SOC_DPCM_TRIGGER_POST},.dynamic = 1,.dpcm_capture = 1,},/* BE */{.name = "wm8960-codec",.cpu_dai_name = "I2S0",.no_pcm = 1,.codec_dai_name = "wm8960-hifi",.dai_fmt = SND_SOC_DAIFMT_I2S | SND_SOC_DAIFMT_CBS_CFS| SND_SOC_DAIFMT_GATED,.ops = &mt2701_wm8960_be_ops,.dpcm_playback = 1,.dpcm_capture = 1,},
};

根据上面 Machine 驱动中调用 soc_new_pcm() 创建 pcm device 节点后如下：

参考链接：
linux-alsa详解7 ASOC-machine

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