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我們知道，asoc框架里面主要包含machine codec platform 這三大部分，
（1）machine:單板相關(guān)內(nèi)容，表明聲卡中所用的主芯片（Platform是指Soc）、編解碼芯片(codec)是哪一個(gè)。
主芯片里的接口（DAI(全稱Digital Audio Interface)接口）接到哪里去.CPU DAI是哪一個(gè)，codec DAI是哪一個(gè)，DMA是哪個(gè)。
（2）platform:用于實(shí)現(xiàn)平臺(tái)相關(guān)內(nèi)容，如IIS（DAI）（設(shè)置接口）和DMA（傳輸數(shù)據(jù)）。
（3）codec:用于實(shí)現(xiàn)平臺(tái)無(wú)關(guān)的功能，即編解碼芯片驅(qū)動(dòng)， DAI和控制接口（控制音量）。
音頻子系統(tǒng)(ASOC框架)之Machine
音頻子系統(tǒng)(ASOC框架)之Codec

但是有些場(chǎng)合，我們是不需要一個(gè)“真實(shí)”的codec做處理的，例如藍(lán)牙通話，這時(shí)候只要一個(gè)虛擬聲卡即可。這里提供一個(gè)虛擬聲卡的驅(qū)動(dòng)：

/** Driver for generic Bluetooth SCO link* Copyright 2011 Lars-Peter Clausen <lars@metafoo.de>** This program is free software; you can redistribute it and/or modify it* under the terms of the GNU General Public License as published by the* Free Software Foundation; either version 2 of the License, or (at your* option) any later version.**/ #include <linux/init.h> #include <linux/module.h> #include <linux/platform_device.h> #include <sound/soc.h>static const struct snd_soc_dapm_widget bt_sco_widgets[] = {SND_SOC_DAPM_INPUT("RX"),SND_SOC_DAPM_OUTPUT("TX"), };static const struct snd_soc_dapm_route bt_sco_routes[] = {{ "Capture", NULL, "RX" },{ "TX", NULL, "Playback" }, };static int btsco_set_sysclk(struct snd_soc_dai *dai, int clk_id, unsigned int freq, int dir){pr_debug("%s: clk_id=%d, freq=%d.\n", __func__, clk_id, freq);return 0; }static int btsco_set_pll(struct snd_soc_dai *dai, int pll_id, int source, unsigned int freq_in, unsigned int freq_out){pr_debug("%s: pll_id=%d, freq_in=%d, freq_out=%d.\n", __func__, pll_id, freq_in, freq_out);return 0; }static int btsco_set_clkdiv(struct snd_soc_dai *dai, int div_id, int div){pr_debug("%s: div_id=%d, div=%d.\n", __func__, div_id, div);return 0; }static int btsco_hw_params(struct snd_pcm_substream *substream, struct snd_pcm_hw_params *params, struct snd_soc_dai *dai){pr_debug("%s .\n", __func__);return 0; }static int btsco_hw_free(struct snd_pcm_substream *substream, struct snd_soc_dai *dai){pr_debug("%s .\n", __func__);return 0; }static int btsco_set_fmt(struct snd_soc_dai *dai, unsigned int fmt){pr_debug("%s: fmt=%d.\n", __func__, fmt);return 0; }static const struct snd_soc_dai_ops btsco_dai_ops = {.set_sysclk = btsco_set_sysclk,.set_pll = btsco_set_pll,.set_clkdiv = btsco_set_clkdiv,.hw_params = btsco_hw_params,.hw_free = btsco_hw_free,.set_fmt = btsco_set_fmt, };static struct snd_soc_dai_driver bt_sco_dai[] = {{.name = "bt-sco-pcm",.capture = {.stream_name = "Capture",.channels_min = 1,.channels_max = 2,.rates = SNDRV_PCM_RATE_8000_48000 | SNDRV_PCM_RATE_KNOT,.formats = SNDRV_PCM_FMTBIT_S16_LE|SNDRV_PCM_FMTBIT_S20_3LE | SNDRV_PCM_FMTBIT_S24_LE | SNDRV_PCM_FMTBIT_S32_LE,},.ops = &btsco_dai_ops,}, };static struct snd_soc_codec_driver soc_codec_dev_bt_sco = {.component_driver = {.dapm_widgets = bt_sco_widgets,.num_dapm_widgets = ARRAY_SIZE(bt_sco_widgets),.dapm_routes = bt_sco_routes,.num_dapm_routes = ARRAY_SIZE(bt_sco_routes),}, };static int bt_sco_probe(struct platform_device *pdev) {pr_debug("bt_sco_probe.\n");return snd_soc_register_codec(&pdev->dev, &soc_codec_dev_bt_sco,&bt_sco_dai[0], 1); }static int bt_sco_remove(struct platform_device *pdev) {snd_soc_unregister_codec(&pdev->dev);return 0; }static const struct platform_device_id bt_sco_driver_ids[] = {{.name = "dfbmcs320",},{.name = "bt-sco",},{}, }; MODULE_DEVICE_TABLE(platform, bt_sco_driver_ids);#if defined(CONFIG_OF) static const struct of_device_id bt_sco_codec_of_match[] = {{ .compatible = "delta,dfbmcs320", },{ .compatible = "linux,bt-sco", },{}, }; MODULE_DEVICE_TABLE(of, bt_sco_codec_of_match); #endifstatic struct platform_driver bt_sco_driver = {.driver = {.name = "bt-sco",.of_match_table = of_match_ptr(bt_sco_codec_of_match),},.probe = bt_sco_probe,.remove = bt_sco_remove,.id_table = bt_sco_driver_ids, };module_platform_driver(bt_sco_driver);MODULE_DESCRIPTION("ASoC generic bluetooth sco link driver"); MODULE_LICENSE("GPL");

這個(gè)虛擬到聲卡驅(qū)動(dòng)是通用的，就一個(gè)虛擬codec，里面啥都沒(méi)做，就規(guī)定了一些參數(shù)：最大兩通道，采樣率在8k~48k，支持16、20、24、32bit位寬。

既然codec是通用的，那么machine是否也有通用的例子的？
還真有！！！就是simple card framework

Simple card顧名思義，簡(jiǎn)單通用的machine driver，代碼實(shí)現(xiàn)路徑如下：sound/soc/generic/simple-card.c
需要在內(nèi)核開(kāi)啟如下配置才能使用：

Device Drivers ---> <*> Sound card support ---> <*> Advanced Linux Sound Architecture ---> <*> ALSA for SoC audio support ---> <*> ASoC Simple sound card support

同時(shí)，我們還需要在設(shè)備樹(shù)中進(jìn)行相應(yīng)到配置：

sound {compatible = "simple-audio-card";simple-audio-card,name = "Dummy-Audio-Card";simple-audio-card,format = "dsp_a";simple-audio-card,bitclock-master = <&dailink0_master>;simple-audio-card,frame-master = <&dailink0_master>;simple-audio-card,mclk-fs = <512>; //codec做主,主控供給編解碼芯片用的時(shí)鐘 512FSsimple-audio-card,cpu {sound-dai = <&i2s>;};dailink0_master: simple-audio-card,codec {sound-dai = <&bt_sco>;};}; &i2s {#sound-dai-cells = <0>;status = "okay"; };&bt_sco {#sound-dai-cells = <0>;compatible = "linux,bt-sco";status = "okay"; };

這里設(shè)備數(shù)配置到時(shí)：選擇藍(lán)牙做主設(shè)備，pcm格式。

關(guān)于simple-card里的描述，差不多可以歸納為：

• simple-audio-card,name：用戶指定的音頻聲卡名稱。
• simple-audio-card,widgets：指定音頻編解碼器DAPM小部件(widgets),
  每個(gè)條目都是一對(duì)字符串:“template-wname”，“user-supplied-wname”。
  “ template-wname”是模板小部件名稱，可選為：“Microphone”, “Line”, “Headphone”, “Speaker”.
  “user-supplied-wname”是用戶指定的窗口小部件名稱。
• simple-audio-card,routing：音頻組件之間的連接列表。
  每個(gè)條目都是一對(duì)字符串，第一個(gè)是目的(sink)，第二個(gè)是源(source)。
• simple-audio-card,pin-switches：包含以下組件的窗口小部件名稱的字符串列表必須創(chuàng)建哪個(gè)引腳開(kāi)關(guān)。
• simple-audio-card,dai-link：用于dai-link里指定CPU和CODEC子節(jié)點(diǎn)。
• simple-audio-card,format：CPU / CODEC通用音頻格式。
  可選格式為：
  “ i2s”，“ right_j”，“ left_j”，“ dsp_a”
  “ dsp_b”，“ ac97”，“ pdm”，“ msb”，“ lsb”。
• frame-master：指向CPU或者CODEC，表示dai-link fram 主機(jī)。
• bitclock-master：指向CPU或者CODEC，表示dai-link的位時(shí)鐘主控。
• bitclock-inversion：布爾屬性，如果添加則表明dai-link使用位時(shí)鐘反轉(zhuǎn)。
• frame-inversion：布爾屬性。 如果添加則表明dai-link使用幀時(shí)鐘反轉(zhuǎn)。

舉幾個(gè)具體到例子：

simple-audio-card,pin-switches = "Headphone","HpSpeaker","LINEOUT"; 等價(jià)于驅(qū)動(dòng)里 static const struct snd_kcontrol_new sunxi_card_controls[] = {SOC_DAPM_PIN_SWITCH("Headphone"),SOC_DAPM_PIN_SWITCH("HpSpeaker"),SOC_DAPM_PIN_SWITCH("LINEOUT"), }; snd_soc_add_card_controls(card, sunxi_card_controls,ARRAY_SIZE(sunxi_card_controls)); simple-audio-card,widgets = "Microphone", "HeadphoneMic","Microphone", "Main Mic"; 等價(jià)于驅(qū)動(dòng)里 static const struct snd_soc_dapm_widget sunxi_card_dapm_widgets[] = {SND_SOC_DAPM_MIC("HeadphoneMic", NULL),SND_SOC_DAPM_MIC("Main Mic", NULL), }; snd_soc_dapm_new_controls(dapm, sunxi_card_dapm_widgets,ARRAY_SIZE(sunxi_card_dapm_widgets)); simple-audio-card,routing = "MainMic Bias", "Main Mic","MIC1", "MainMic Bias","MIC2", "HeadphoneMic"; 等價(jià)于驅(qū)動(dòng)里 static const struct snd_soc_dapm_route sunxi_card_routes[] = {{"MainMic Bias", NULL, "Main Mic"},{"MIC1", NULL, "MainMic Bias"},{"MIC2", NULL, "HeadphoneMic"}, }; snd_soc_dapm_add_routes(dapm, sunxi_card_routes,ARRAY_SIZE(sunxi_card_routes));

最后，更多實(shí)現(xiàn)詳情可以參考內(nèi)核文檔：Documentation/devicetree/bindings/sound/simple-card.txt
或者網(wǎng)上也可以瀏覽在線文件：https://www.kernel.org/doc/Documentation/devicetree/bindings/sound/simple-card.txt

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