这里以 Wolfson 公司的编解码芯片 WM8960 为例来说明上篇介绍的相关内容。

定义 widget 所需的 DAPM Kcontrol

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static const struct snd_kcontrol_new wm8960_loutput_mixer[] = {
    SOC_DAPM_SINGLE("PCM Playback Switch", WM8960_LOUTMIX, 8, 1, 0),
    SOC_DAPM_SINGLE("LINPUT3 Switch", WM8960_LOUTMIX, 7, 1, 0),
    SOC_DAPM_SINGLE("Boost Bypass Switch", WM8960_BYPASS1, 7, 1, 0),
};
static const struct snd_kcontrol_new wm8960_routput_mixer[] = {
    SOC_DAPM_SINGLE("PCM Playback Switch", WM8960_ROUTMIX, 8, 1, 0),
    SOC_DAPM_SINGLE("RINPUT3 Switch", WM8960_ROUTMIX, 7, 1, 0),
    SOC_DAPM_SINGLE("Boost Bypass Switch", WM8960_BYPASS2, 7, 1, 0),
};
static const struct snd_kcontrol_new wm8960_mono_out[] = {
    SOC_DAPM_SINGLE("Left Switch", WM8960_MONOMIX1, 7, 1, 0),
    SOC_DAPM_SINGLE("Right Switch", WM8960_MONOMIX2, 7, 1, 0),
};

在上述代码中定义了左右输出通道的混音器控件，以及单声道输出混音器：wm8960_loutput_mixer、wm8960_routput_mixer、wm8960_mono_out。

定义真实的 widget，包括 DAMP 控件

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static const struct snd_soc_dapm_widget wm8960_dapm_widgets[] = {
    SND_SOC_DAPM_INPUT("LINPUT1"),
    SND_SOC_DAPM_INPUT("RINPUT1"),
    SND_SOC_DAPM_INPUT("LINPUT2"),
    SND_SOC_DAPM_INPUT("RINPUT2"),
    SND_SOC_DAPM_INPUT("LINPUT3"),
    SND_SOC_DAPM_INPUT("RINPUT3"),

    SND_SOC_DAPM_SUPPLY("MICB", WM8960_POWER1, 1, 0, NULL, 0),

    SND_SOC_DAPM_MIXER("Left Boost Mixer", WM8960_POWER1, 5, 0,
    		   wm8960_lin_boost, ARRAY_SIZE(wm8960_lin_boost)),
    SND_SOC_DAPM_MIXER("Right Boost Mixer", WM8960_POWER1, 4, 0,
    		   wm8960_rin_boost, ARRAY_SIZE(wm8960_rin_boost)),

    SND_SOC_DAPM_MIXER("Left Input Mixer", WM8960_POWER3, 5, 0,
    		   wm8960_lin, ARRAY_SIZE(wm8960_lin)),
    SND_SOC_DAPM_MIXER("Right Input Mixer", WM8960_POWER3, 4, 0,
    		   wm8960_rin, ARRAY_SIZE(wm8960_rin)),

    SND_SOC_DAPM_ADC("Left ADC", "Capture", WM8960_POWER1, 3, 0),
    SND_SOC_DAPM_ADC("Right ADC", "Capture", WM8960_POWER1, 2, 0),

    SND_SOC_DAPM_DAC("Left DAC", "Playback", WM8960_POWER2, 8, 0),
    SND_SOC_DAPM_DAC("Right DAC", "Playback", WM8960_POWER2, 7, 0),

    SND_SOC_DAPM_MIXER("Left Output Mixer", WM8960_POWER3, 3, 0,
    	&wm8960_loutput_mixer[0],
    	ARRAY_SIZE(wm8960_loutput_mixer)),
    SND_SOC_DAPM_MIXER("Right Output Mixer", WM8960_POWER3, 2, 0,
    	&wm8960_routput_mixer[0],
    	ARRAY_SIZE(wm8960_routput_mixer)),

    SND_SOC_DAPM_PGA("LOUT1 PGA", WM8960_POWER2, 6, 0, NULL, 0),
    SND_SOC_DAPM_PGA("ROUT1 PGA", WM8960_POWER2, 5, 0, NULL, 0),

    SND_SOC_DAPM_PGA("Left Speaker PGA", WM8960_POWER2, 4, 0, NULL, 0),
    SND_SOC_DAPM_PGA("Right Speaker PGA", WM8960_POWER2, 3, 0, NULL, 0),

    SND_SOC_DAPM_PGA("Right Speaker Output", WM8960_CLASSD1, 7, 0, NULL, 0),
    SND_SOC_DAPM_PGA("Left Speaker Output", WM8960_CLASSD1, 6, 0, NULL, 0),

    SND_SOC_DAPM_OUTPUT("SPK_LP"),
    SND_SOC_DAPM_OUTPUT("SPK_LN"),
    SND_SOC_DAPM_OUTPUT("HP_L"),
    SND_SOC_DAPM_OUTPUT("HP_R"),
    SND_SOC_DAPM_OUTPUT("SPK_RP"),
    SND_SOC_DAPM_OUTPUT("SPK_RN"),
    SND_SOC_DAPM_OUTPUT("OUT3"),
};

在这一步中为左右声道选择器定义了一个 MUX widget，他们是 Left Output Mixer、Right Output Mixer 和 Mono Output Mixer。 我们还为每个扬声器定义了一个混音器 widget：SPK_LP、SPK_LN、HP_L、HP_R、SPK_RP、SPK_RN、OUT3。具体的混音器控制由 wm8960_loutput_mixer、wm8960_routput_mixer、wm8960_mono_out 来完成，这三个 widget 都具有 power 属性，因此，当这些 widget 中的一个或多个位于一个有效的音频路径时，DAPM 框架可以通过其各自的寄存器的第 7 和第 8 位来控制电源状态。

定义 widget 的连接路径

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static const struct snd_soc_dapm_route audio_paths[] = {
	{ "Left Boost Mixer", "LINPUT1 Switch", "LINPUT1" },
	{ "Left Boost Mixer", "LINPUT2 Switch", "LINPUT2" },
	{ "Left Boost Mixer", "LINPUT3 Switch", "LINPUT3" },

	{ "Left Input Mixer", "Boost Switch", "Left Boost Mixer" },
	{ "Left Input Mixer", "Boost Switch", "LINPUT1" },  /* Really Boost Switch */
	{ "Left Input Mixer", NULL, "LINPUT2" },
	{ "Left Input Mixer", NULL, "LINPUT3" },

	{ "Right Boost Mixer", "RINPUT1 Switch", "RINPUT1" },
	{ "Right Boost Mixer", "RINPUT2 Switch", "RINPUT2" },
	{ "Right Boost Mixer", "RINPUT3 Switch", "RINPUT3" },

	{ "Right Input Mixer", "Boost Switch", "Right Boost Mixer" },
	{ "Right Input Mixer", "Boost Switch", "RINPUT1" },  /* Really Boost Switch */
	{ "Right Input Mixer", NULL, "RINPUT2" },
	{ "Right Input Mixer", NULL, "RINPUT3" },

	{ "Left ADC", NULL, "Left Input Mixer" },
	{ "Right ADC", NULL, "Right Input Mixer" },

	{ "Left Output Mixer", "LINPUT3 Switch", "LINPUT3" },
	{ "Left Output Mixer", "Boost Bypass Switch", "Left Boost Mixer" },
	{ "Left Output Mixer", "PCM Playback Switch", "Left DAC" },

	{ "Right Output Mixer", "RINPUT3 Switch", "RINPUT3" },
	{ "Right Output Mixer", "Boost Bypass Switch", "Right Boost Mixer" },
	{ "Right Output Mixer", "PCM Playback Switch", "Right DAC" },

	{ "LOUT1 PGA", NULL, "Left Output Mixer" },
	{ "ROUT1 PGA", NULL, "Right Output Mixer" },

	{ "HP_L", NULL, "LOUT1 PGA" },
	{ "HP_R", NULL, "ROUT1 PGA" },

	{ "Left Speaker PGA", NULL, "Left Output Mixer" },
	{ "Right Speaker PGA", NULL, "Right Output Mixer" },

	{ "Left Speaker Output", NULL, "Left Speaker PGA" },
	{ "Right Speaker Output", NULL, "Right Speaker PGA" },

	{ "SPK_LN", NULL, "Left Speaker Output" },
	{ "SPK_LP", NULL, "Left Speaker Output" },
	{ "SPK_RN", NULL, "Right Speaker Output" },
	{ "SPK_RP", NULL, "Right Speaker Output" },
};
static const struct snd_soc_dapm_route audio_paths_out3[] = {
	{ "Mono Output Mixer", "Left Switch", "Left Output Mixer" },
	{ "Mono Output Mixer", "Right Switch", "Right Output Mixer" },

	{ "OUT3", NULL, "Mono Output Mixer", }
};

通过这一步的定义我们知道 Left output Mux 和 Right output Mux 有三个输入引脚，分别是 Boost Bypass Switch、LINPUT3 Switch、PCM Playback Switch。Mono Output Mixer 只有两个输入引脚，Left Switch、Right Switch，所以，上述定义的含义如下：

  • Left Boost Mixer 通过 Boost Bypass Switch 连接到 Left Output Mixer
  • Left DAC 通过 PCM Playback Switch 连接到 Left Output Mixer
  • LINPUT3 通过 LINPUT3 Switch 连接到 Left Output Mixer
  • Left Output Mixer 连接到 LOUT1 PGA，但是此链接没有开关控制
  • Right xxx 同理这里不一一列举出来了

在编解码器驱动程序的 probe 回调函数中注册 widget 和路径

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static int wm8960_add_widgets(struct snd_soc_component *component)
{
	struct wm8960_priv *wm8960 = snd_soc_component_get_drvdata(component);
	struct wm8960_data *pdata = &wm8960->pdata;
	struct snd_soc_dapm_context *dapm = snd_soc_component_get_dapm(component);
	struct snd_soc_dapm_widget *w;

	snd_soc_dapm_new_controls(dapm, wm8960_dapm_widgets,
				  ARRAY_SIZE(wm8960_dapm_widgets));

	snd_soc_dapm_add_routes(dapm, audio_paths, ARRAY_SIZE(audio_paths));

	/* In capless mode OUT3 is used to provide VMID for the
	 * headphone outputs, otherwise it is used as a mono mixer.
	 */
	if (pdata && pdata->capless) {
		snd_soc_dapm_new_controls(dapm, wm8960_dapm_widgets_capless,
					  ARRAY_SIZE(wm8960_dapm_widgets_capless));

		snd_soc_dapm_add_routes(dapm, audio_paths_capless,
					ARRAY_SIZE(audio_paths_capless));
	} else {
		snd_soc_dapm_new_controls(dapm, wm8960_dapm_widgets_out3,
					  ARRAY_SIZE(wm8960_dapm_widgets_out3));

		snd_soc_dapm_add_routes(dapm, audio_paths_out3,
					ARRAY_SIZE(audio_paths_out3));
	}

	/* We need to power up the headphone output stage out of
	 * sequence for capless mode.  To save scanning the widget
	 * list each time to find the desired power state do so now
	 * and save the result.
	 */
	list_for_each_entry(w, &component->card->widgets, list) {
		if (w->dapm != dapm)
			continue;
		if (strcmp(w->name, "LOUT1 PGA") == 0)
			wm8960->lout1 = w;
		if (strcmp(w->name, "ROUT1 PGA") == 0)
			wm8960->rout1 = w;
		if (strcmp(w->name, "OUT3 VMID") == 0)
			wm8960->out3 = w;
	}
	
	return 0;
}
...
static int wm8960_probe(struct snd_soc_component *component)
{
    ...
	wm8960_add_widgets(component);
}

当 machine 驱动程序 probe 到这个编解码时，就会调用编解码器组件的 probe 回调函数 wm8960_probe，以完成编解码器驱动程序的初始化。编解码器需要绑定到 platform 驱动程序才能发挥作用，这是后面要介绍的内容。

流程

由于函数调用栈很深，没有办法一一列出，这里通过一个思维导图来将各函数调用关系列出来，感兴趣的读者可以自行阅读源码。

原图

参考文献

《Linux 设备驱动开发-约翰-马德奥》