codec和platform分别对各自的设备构成和驱动进行描述，在ASoC中，需要将两者结合起来构成音频设备,machine class实现对如何将codec和platform结合构成音频设备进行描述，将不同部分(codec class和platform class)连接起来构成ALSA sound card device。其负责machine-specific controls和machine-level audio events的处理（比如在开始播放后打开放大器）。machine class drvier中涉及两个重要的结构体：strcut snd_soc_dai_link，用于描述CPU DAI和codec DAI之间的连接，strcut snd_soc_card用于表示最终组成的声卡，并将其注册到ALSA中。

platform和codec driver的代码是可以复用的，但是machine driver不是，具体的硬件特征是不可复用的，这主要是指codec DAI和platform DAI之间的连接。比如，通过GPIO打开放大器、通过GPIO检测音频设备插入、使用外部的OSC作为I2S总线的主时钟等等

1. Machine driver简介

machine driver负责的功能主要有：

• 根据开发板的硬件配置，填充strcut snd_soc_dai_link中对应的CPU和codec DAI；
• codec的时钟配置和codec的初始化；
• 补充DAPM widgets，从而完成DAPM链路的连接；
• 设定不同codec的具体工作参数，比如采样频率；

machine driver的开发从widgets、route的定义、dailink的配置到最后的声卡注册。

2. DAI Link以及Machine Class初始化

DAI Link用于表示CPU DAI和 codec DAI之间连接的逻辑关系，将codec和platform的DAI接口对应起来实现音频信号的完整传输。

2.1 snd_soc_dai_link数据含义

在内核中，使用strcut snd_soc_dai_link表示。其定义如下

位于“include/sound/soc.h”中

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.
	 */
	/*
	 * 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.
	 */
	struct snd_soc_dai_link_component *cpus;
	unsigned int num_cpus;

	/*
	 * You MUST specify the link's codec, either by device name, or by
	 * DT/OF node, but not both.
	 */
	/* You MUST specify the DAI name within the codec */
	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. In such case, platforms are not mandatory.
	 */
	struct snd_soc_dai_link_component *platforms;
	unsigned int num_platforms;

	int id;	/* optional ID for machine driver link identification */

	const struct snd_soc_pcm_stream *params;
	unsigned int num_params;

	unsigned int dai_fmt;           /* format to set on init */

	enum snd_soc_dpcm_trigger trigger[2]; /* trigger type for DPCM */

	/* codec/machine specific init - e.g. add machine controls */
	int (*init)(struct snd_soc_pcm_runtime *rtd);

	/* codec/machine specific exit - dual of init() */
	void (*exit)(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;

	/* Mark this pcm with non atomic ops */
	unsigned int nonatomic:1;

	/* For unidirectional dai links */
	unsigned int playback_only:1;
	unsigned int capture_only:1;

	/* 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;

	/* DPCM used FE & BE merged format */
	unsigned int dpcm_merged_format:1;
	/* DPCM used FE & BE merged channel */
	unsigned int dpcm_merged_chan:1;
	/* DPCM used FE & BE merged rate */
	unsigned int dpcm_merged_rate:1;

	/* pmdown_time is ignored at stop */
	unsigned int ignore_pmdown_time:1;

	/* Do not create a PCM for this DAI link (Backend link) */
	unsigned int ignore:1;

#ifdef CONFIG_SND_SOC_TOPOLOGY
	struct snd_soc_dobj dobj; /* For topology */
#endif

	ANDROID_KABI_RESERVE(1);
};

上述数据结构中指定了cpu、codec相关的设备树节点信息、cpu和codec的驱动信息，从而与开发板的实际硬件连接相对应，实现开发板的音频输出功能。

在一般情况下，开发板都有指定CPU DAI(比如I2S控制器)和codec DAI(比如nau8825)，DAI link应指定cpu_dai、codec_dai和对应的platform设备。machine driver将dai_link设定完成后，将strcut snd_soc_dai_link变量传递给数据类型为strcut snd_soc_card的声卡变量。

strcut snd_soc_dai_link中的struct snd_soc_dai_link_component成员cpus、codecs和platforms中存储与设备树相关的信息，用于寻找相应的设备配置。
位于“include/sound/soc.h”中

struct snd_soc_dai_link_component {
	const char *name;
	struct device_node *of_node;
	const char *dai_name;
};

2.2 dai_link数据填充

在设备树文件中对相关的codec、cpu的设备节点定义如下
声卡设备节点

nau8822_sound: nau8822-sound {
		status = "okay";
		compatible = "rockchip,multicodecs-card";
		rockchip,card-name = "rockchip-nau8822";
		hp-det-gpio = <&gpio1 RK_PB2 GPIO_ACTIVE_HIGH>;
		io-channels = <&saradc 3>;
		io-channel-names = "adc-detect";
		keyup-threshold-microvolt = <1800000>;
		poll-interval = <100>;
//		spk-con-gpio = <&gpio1 RK_PD3 GPIO_ACTIVE_HIGH>;
//		hp-con-gpio = <&gpio1 RK_PD2 GPIO_ACTIVE_HIGH>;
		rockchip,format = "i2s";
		rockchip,mclk-fs = <256>;
		rockchip,cpu = <&i2s0_8ch>;
		rockchip,codec = <&nau8822>;
		rockchip,audio-routing =
			"Headphone", "RHP",
			"Headphone", "LHP",
			"Speaker", "LSPK",
			"Speaker", "RSPK",
			"RMICP", "Main Mic",
			"RMICN", "Main Mic",
			"RMICP", "Headset Mic",
			"RMICN", "Headset Mic",
			"LMICP", "Main Mic",
			"LMICN", "Main Mic",
			"LMICP", "Headset Mic",
			"LMICN", "Headset Mic";
		pinctrl-names = "default";
		pinctrl-0 = <&hp_det>;
		play-pause-key {
			label = "playpause";
			linux,code = <KEY_PLAYPAUSE>;
			press-threshold-microvolt = <2000>;
			poll-interval = <1000>;
		};
	};

codec设备节点

&i2c3 {
	status = "okay";
        nau8822: nau8822@1a {
                status = "okay";
                #sound-dai-cells = <0>;
                compatible = "nuvoton,nau8822";
                reg = <0x1a>;
                clocks = <&mclkout_i2s0>;
                clock-names = "mclk";
                assigned-clocks = <&mclkout_i2s0>;
                assigned-clock-rates = <12288000>;
                pinctrl-names = "default";
                pinctrl-0 = <&i2s0_mclk>;
        };

};

platform设备节点
位于rk3588s.dts中

i2s0_8ch: i2s@fe470000 {
		compatible = "rockchip,rk3588-i2s-tdm";
		reg = <0x0 0xfe470000 0x0 0x1000>;
		interrupts = <GIC_SPI 180 IRQ_TYPE_LEVEL_HIGH>;
		clocks = <&cru MCLK_I2S0_8CH_TX>, <&cru MCLK_I2S0_8CH_RX>, <&cru HCLK_I2S0_8CH>;
		clock-names = "mclk_tx", "mclk_rx", "hclk";
		assigned-clocks = <&cru CLK_I2S0_8CH_TX_SRC>, <&cru CLK_I2S0_8CH_RX_SRC>;
		assigned-clock-parents = <&cru PLL_AUPLL>, <&cru PLL_AUPLL>;
		dmas = <&dmac0 0>, <&dmac0 1>;
		dma-names = "tx", "rx";
		power-domains = <&power RK3588_PD_AUDIO>;
		resets = <&cru SRST_M_I2S0_8CH_TX>, <&cru SRST_M_I2S0_8CH_RX>;
		reset-names = "tx-m", "rx-m";
		rockchip,clk-trcm = <1>;
		i2s-lrck-gpio = <&gpio1 RK_PC5 GPIO_ACTIVE_HIGH>; /* i2s0_lrck */
		pinctrl-names = "default", "idle", "clk";
		pinctrl-0 = <&i2s0_sdi0
			     &i2s0_sdi1
			     &i2s0_sdi2
			     &i2s0_sdi3
			     &i2s0_sdo0
			     &i2s0_sdo1>;
		pinctrl-1 = <&i2s0_idle>;
		pinctrl-2 = <&i2s0_lrck
			     &i2s0_sclk>;
		#sound-dai-cells = <0>;
		status = "disabled";
	};

通过调用of_parse_phandle设备树帮助函数，获取对应的设备在设备树中的phandle。可以填充
snd_soc_dai_link_component中的of_node成员。

elf2位于“sound/soc/rockchip/multicodecs.c”中,通过调用of_parse_phandle函数，从而获得对应的device tree中的设备属性节点。

static int rk_multicodecs_probe(struct platform_device *pdev)
{
	//...

	link->codecs = codecs;
	link->num_codecs = idx;
	idx = 0;
	for (i = 0; i < count; i++) {
		node = of_parse_phandle(np, "rockchip,codec", i);
		if (!node)
			return -ENODEV;
		if (!of_device_is_available(node))
			continue;

		ret = of_parse_phandle_with_fixed_args(np, "rockchip,codec",
						       0, i, &args);
		if (ret)
			return ret;

		codecs[idx].of_node = node;
		ret = snd_soc_get_dai_name(&args, &codecs[idx].dai_name);
		if (ret)
			return ret;
		idx++;
	}

	/* Only reference the codecs[0].of_node which maybe as master. */
	rk_multicodecs_parse_daifmt(np, codecs[0].of_node, mc_data, prefix);

	link->cpus->of_node = of_parse_phandle(np, "rockchip,cpu", 0);
	if (!link->cpus->of_node)
		return -ENODEV;

	link->platforms->of_node = link->cpus->of_node;

	//...
}

3 Machine routing 设计

Machine driver中结合Codec class driver和Platform class driver中对DAPM的route设置，实现完整音频链路的描述。

3.1 Codec Pin

Codec Pin表示Codec连接到开发板上的耳机、喇叭等设备的引脚。这些引脚属于音频链路的末端，这些引脚在codec driver中使用SND_SOC_DAPM_INPUT和SND_SOC_DAPM_OUTPUT宏来进行定义。elf2开发板对应的nau8822的引脚定义如下。

位于“sound/soc/codecs/nau8822.c”中

static const struct snd_soc_dapm_widget nau8822_dapm_widgets[] = {
	SND_SOC_DAPM_DAC("Left DAC", "Left HiFi Playback",
		NAU8822_REG_POWER_MANAGEMENT_3, 0, 0),
	SND_SOC_DAPM_DAC("Right DAC", "Right HiFi Playback",
		NAU8822_REG_POWER_MANAGEMENT_3, 1, 0),
	SND_SOC_DAPM_ADC("Left ADC", "Left HiFi Capture",
		NAU8822_REG_POWER_MANAGEMENT_2, 0, 0),
	SND_SOC_DAPM_ADC("Right ADC", "Right HiFi Capture",
		NAU8822_REG_POWER_MANAGEMENT_2, 1, 0),

	SOC_MIXER_ARRAY("Left Output Mixer",
		NAU8822_REG_POWER_MANAGEMENT_3, 2, 0, nau8822_left_out_mixer),
	SOC_MIXER_ARRAY("Right Output Mixer",
		NAU8822_REG_POWER_MANAGEMENT_3,	3, 0, nau8822_right_out_mixer),
	SOC_MIXER_ARRAY("AUX1 Output Mixer",
		NAU8822_REG_POWER_MANAGEMENT_1, 7, 0, nau8822_auxout1_mixer),
	SOC_MIXER_ARRAY("AUX2 Output Mixer",
		NAU8822_REG_POWER_MANAGEMENT_1,	6, 0, nau8822_auxout2_mixer),

	SOC_MIXER_ARRAY("Left Input Mixer",
		NAU8822_REG_POWER_MANAGEMENT_2,
		2, 0, nau8822_left_input_mixer),
	SOC_MIXER_ARRAY("Right Input Mixer",
		NAU8822_REG_POWER_MANAGEMENT_2,
		3, 0, nau8822_right_input_mixer),

	SND_SOC_DAPM_PGA("Left Boost Mixer",
		NAU8822_REG_POWER_MANAGEMENT_2, 4, 0, NULL, 0),
	SND_SOC_DAPM_PGA("Right Boost Mixer",
		NAU8822_REG_POWER_MANAGEMENT_2, 5, 0, NULL, 0),

	SND_SOC_DAPM_PGA("Left Capture PGA",
		NAU8822_REG_LEFT_INP_PGA_CONTROL, 6, 1, NULL, 0),
	SND_SOC_DAPM_PGA("Right Capture PGA",
		NAU8822_REG_RIGHT_INP_PGA_CONTROL, 6, 1, NULL, 0),

	SND_SOC_DAPM_PGA("Left Headphone Out",
		NAU8822_REG_POWER_MANAGEMENT_2, 7, 0, NULL, 0),
	SND_SOC_DAPM_PGA("Right Headphone Out",
		NAU8822_REG_POWER_MANAGEMENT_2, 8, 0, NULL, 0),

	SND_SOC_DAPM_PGA("Left Speaker Out",
		 NAU8822_REG_POWER_MANAGEMENT_3, 6, 0, NULL, 0),
	SND_SOC_DAPM_PGA("Right Speaker Out",
		NAU8822_REG_POWER_MANAGEMENT_3,	5, 0, NULL, 0),

	SND_SOC_DAPM_PGA("AUX1 Out",
		NAU8822_REG_POWER_MANAGEMENT_3,	8, 0, NULL, 0),
	SND_SOC_DAPM_PGA("AUX2 Out",
		NAU8822_REG_POWER_MANAGEMENT_3,	7, 0, NULL, 0),

//	SND_SOC_DAPM_SUPPLY("Mic Bias",
//		NAU8822_REG_POWER_MANAGEMENT_1,	4, 0, NULL, 0),
	SND_SOC_DAPM_SUPPLY("PLL",
		NAU8822_REG_POWER_MANAGEMENT_1,	5, 0, NULL, 0),

	SND_SOC_DAPM_SWITCH("Digital Loopback", SND_SOC_NOPM, 0, 0,
		&nau8822_loopback),

	SND_SOC_DAPM_INPUT("LMICN"),
	SND_SOC_DAPM_INPUT("LMICP"),
	SND_SOC_DAPM_INPUT("RMICN"),
	SND_SOC_DAPM_INPUT("RMICP"),
	SND_SOC_DAPM_INPUT("LAUX"),
	SND_SOC_DAPM_INPUT("RAUX"),
	SND_SOC_DAPM_INPUT("L2"),
	SND_SOC_DAPM_INPUT("R2"),
	SND_SOC_DAPM_OUTPUT("LHP"),
	SND_SOC_DAPM_OUTPUT("RHP"),
	SND_SOC_DAPM_OUTPUT("LSPK"),
	SND_SOC_DAPM_OUTPUT("RSPK"),
	SND_SOC_DAPM_OUTPUT("AUXOUT1"),
	SND_SOC_DAPM_OUTPUT("AUXOUT2"),
};

3.2 Board Connectors

用于注册的声卡strcut snd_soc_card中的struct snd_soc_dapm_widget中定义相关的board connectors。这些引脚为板卡上的MIC、Speaker、LineIn的引脚，在现实中，这些引脚一般与codec上的引脚相连，由于machine class是针对开发板进行描述，所以在machine中定义这些引脚，完成整个音频链路。对于elf2开发板，相关的定义如下

位于“sound/soc/rockchip/rockchip_multicodecs.c”

static const struct snd_soc_dapm_widget mc_dapm_widgets[] = {

	SND_SOC_DAPM_HP("Headphone", NULL),
	SND_SOC_DAPM_SPK("Speaker", NULL),
	SND_SOC_DAPM_MIC("Main Mic", NULL),
	SND_SOC_DAPM_MIC("Headset Mic", NULL),
	SND_SOC_DAPM_SUPPLY("Speaker Power",
			    SND_SOC_NOPM, 0, 0,
			    mc_spk_event,
			    SND_SOC_DAPM_POST_PMU |
			    SND_SOC_DAPM_PRE_PMD),
	SND_SOC_DAPM_SUPPLY("Headphone Power",
			    SND_SOC_NOPM, 0, 0,
			    mc_hp_event,
			    SND_SOC_DAPM_POST_PMU |
			    SND_SOC_DAPM_PRE_PMD),
};

3.3 Machine routing

对codec和machine的相关引脚完成定义后。通过调用snd_soc_of_parse_audio_routing()，可以将Device Tree中声卡的"audio-route"属性解析为音频子系统的audio power map，存储在struct snd_soc_dapm_route中，elf2中的调用为。

位于“sound/soc/rockchip/rockchip_multicodecs.c”

snd_soc_of_parse_audio_routing(card, "rockchip,audio-routing");

4 Clocking and formatting 设计，时钟和音频格式设置

struct snd_soc_ops是ASoC中定义回调函数的接口，在DAI-Link的结构体struct snd_soc_dai_link中，这些回调函数表示machine-level的PCM操作，而在struct snd_soc_dai_driver中表示codec-dai或cpu-dai的相关操作。

位于“include/sound/soc.h”中

/* SoC audio ops */
struct snd_soc_ops {
	int (*startup)(struct snd_pcm_substream *);
	void (*shutdown)(struct snd_pcm_substream *);
	int (*hw_params)(struct snd_pcm_substream *, struct snd_pcm_hw_params *);
	int (*hw_free)(struct snd_pcm_substream *);
	int (*prepare)(struct snd_pcm_substream *);
	int (*trigger)(struct snd_pcm_substream *, int);
};

其中startup在打开playback或capture设备时调用，hw_params()在建立音频流时调用。Machine driver可以在两者调用时配置DAI link的数据格式。hw_params()可以接收音频流的相关参数(比如，channel count、format、sample rate等)，因此更适合进行配置。

CPU DAI和codec DAI间的数据格式配置应当保持一致，ASoC Core提供了相关的辅助函数用于修改相关的配置。这些函数原型为：

int snd_soc_dai_set_fmt(struct snd_soc_dai *dai,unsigned int fmt);
int snd_soc_dai_set_pll(struct snd_soc_dai *dai, int pll_id, int source,unsigned int freq_in, unsigned int freq_out);
int snd_soc_dai_set_sysclk(struct snd_soc_dai *dai, int clk_id,unsigned int freq, int dir);
int snd_soc_dai_set_clkdiv(struct snd_soc_dai *dai,int div_id, int div);

以上函数会根据输入参数中的dai的值不同，从而调用codec或者platform中对应的回调函数，比如

static int rk_multicodecs_hw_params(struct snd_pcm_substream *substream,
				    struct snd_pcm_hw_params *params)
{
	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
	struct snd_soc_dai *cpu_dai = asoc_rtd_to_cpu(rtd, 0);
	struct snd_soc_dai *codec_dai;
	struct multicodecs_data *mc_data = snd_soc_card_get_drvdata(rtd->card);
	unsigned int mclk;
	int ret, i;

	mclk = params_rate(params) * mc_data->mclk_fs;

	for_each_rtd_codec_dais(rtd, i, codec_dai) {
		ret = snd_soc_dai_set_sysclk(codec_dai, substream->stream, mclk,
					     SND_SOC_CLOCK_IN);
		if (ret && ret != -ENOTSUPP) {
			pr_err("Set codec_dai sysclk failed: %d\n", ret);
			goto out;
		}
	}

	ret = snd_soc_dai_set_sysclk(cpu_dai, substream->stream, mclk,
				     SND_SOC_CLOCK_OUT);
	if (ret && ret != -ENOTSUPP) {
		pr_err("Set cpu_dai sysclk failed: %d\n", ret);
		goto out;
	}

	return 0;

out:
	return ret;
}

5 声卡的注册

在完成struct snd_soc_card中相关成员的赋值后,调用devm_snd_soc_register_card()将声卡注册到ALSA中，该调用在machine class driver的.probe函数中，在machine class driver所在的模块加载时完成声卡注册，为后续的音频应用提供服务。

static int rk_multicodecs_probe(struct platform_device *pdev)
{
	struct snd_soc_card *card;
/*
dai_link数据填充、route设置等
*/

	snd_soc_card_set_drvdata(card, mc_data);
	platform_set_drvdata(pdev, card);

	ret = devm_snd_soc_register_card(&pdev->dev, card);
	if (ret) {
		dev_err(&pdev->dev, "Failed to register card: %d\n", ret);
		return ret;
	}

	irq = gpiod_to_irq(mc_data->hp_det_gpio);
	if (irq >= 0)
		queue_delayed_work(system_power_efficient_wq,
				   &mc_data->handler, msecs_to_jiffies(50));

	return ret;
}