platform driver注册PCM driver、CPU DAI driver以及相关的操作函数、为PCM components分配缓存和设定playback、capture操作。platform driver主要包括audio DMA配置和音频接口驱动。

platform driver中有两个重要的数据结构：struct snd_soc_component_driver和struct snd_soc_dai_driver。前者主要管理PCM和DMA配置、后者则用于DAI的参数配置。

1.platform class中的snd_soc_component_driver

在platform部分的struct snd_soc_component_driver中定义的功能很少，DMA相关的配置由独立的soc generic dma framework完成，所以platform对应的struct snd_soc_component_driver中一般只需要定义name项以及部分kcontrol定义即可。

elf2中的i2s部分的定义如下，只定义了几个成员变量。

而rockchip中对应的platform中的定义为
位于sound/soc/rockchip/rockchip_i2s_tdm.c中

static const struct snd_soc_component_driver rockchip_i2s_tdm_component = {
	.name = DRV_NAME,
	.controls = rockchip_i2s_tdm_snd_controls,
	.num_controls = ARRAY_SIZE(rockchip_i2s_tdm_snd_controls),
};

1.1 ASoC中的generic PCM DMA engine framework

ALSA中PCM设备是其音频处理的核心，PCM layer(属于ALSA core)负责所有的数字音频处理工作，比如初始化playback声卡和caputure声卡、启动设备的数据传输等。

PCM driver通过struct snd_pcm_ops接口中函数指针指向的函数来实现DMA操作。这部分与ASoC中的platform是无关的，仅与SOC DMA engine中的上行接口(upstream API)交互。DMA engine然后与platform相关的DMA驱动进行交互，进行相应的DMA设置。struct snd_pcm_ops中包含对PCM接口不同事件的回调函数。

不过在ASoC中，只要用户使用ASoC中generic PCM DMA engine framework，用户无需实例化上述结构体。ASoC core会帮助用户完成上述操作。相关的调用栈为：snd_soc_register_card->snd_soc_instantiate_card->soc_probe_link_dais->soc_new_pcm。用户只需提供必要的硬件信息，比如I2S数据收发寄存器的地址、DMA通道配置等具体的硬件参数和特定的函数即可，减少代码量。

1.2 The audio DMA interface

SoC中使用audio DMA interface提供DMA功能。audio DMA driver通过devm_snd_damengine_pcm_register()进行注册。其函数原型为

int devm_snd_dmaengine_pcm_register(
    struct device *dev,
    const struct snd_dmaengine_pcm_config *config,
    unsigned int flag
);

elf2中rockchip_i2s_tdm_register_platform函数中会调用上述函数。

static int rockchip_i2s_tdm_register_platform(struct device *dev)
{
	struct rk_i2s_tdm_dev *i2s_tdm = dev_get_drvdata(dev);
	struct snd_soc_component *comp;
	int ret = 0;

	if (device_property_read_bool(dev, "rockchip,no-dmaengine")) {
		dev_info(dev, "Used for Multi-DAI\n");
		return 0;
	}

	if (device_property_read_bool(dev, "rockchip,digital-loopback")) {
		ret = devm_snd_dmaengine_dlp_register(dev, &dconfig);
		if (ret)
			dev_err(dev, "Could not register DLP\n");
		return ret;
	}

	if (i2s_tdm->clk_trcm) {
		ret =  devm_snd_dmaengine_trcm_register(dev);
		if (ret) {
			dev_err(dev, "Could not register TRCM PCM\n");
			return ret;
		}

		comp = snd_soc_lookup_component(i2s_tdm->dev,
						SND_DMAENGINE_TRCM_DRV_NAME);
		if (!comp) {
			dev_err(dev, "Could not find TRCM PCM\n");
			ret = -ENODEV;
		}

		i2s_tdm->pcm_comp = comp;

		return ret;
	}

	ret = devm_snd_dmaengine_pcm_register(dev, NULL, 0);
	if (ret)
		dev_err(dev, "Could not register PCM\n");

	return ret;
}

该函数会注册strcut snd_dmaengine_pcm_config到设备中。上述函数原型中，dev是PCM设备的父类设备，通常来说&pdev->dev.config是platform-specific PCM configuration，其类型为struct snd_dmaengine_pcm_config。flags用于表示如何进行DMA channel的相关处理，一般其值设定为0。

/*
 * Try to request the DMA channel using compat_request_channel or
 * compat_filter_fn if it couldn't be requested through devicetree.
 */
#define SND_DMAENGINE_PCM_FLAG_COMPAT BIT(0)
/*
 * Don't try to request the DMA channels through devicetree. This flag only
 * makes sense if SND_DMAENGINE_PCM_FLAG_COMPAT is set as well.
 */
#define SND_DMAENGINE_PCM_FLAG_NO_DT BIT(1)
/*
 * The PCM is half duplex and the DMA channel is shared between capture and
 * playback.
 */
#define SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX BIT(3)

在注册完成后，generic PCM DMA engine framework会生成相应的snd_pcm_ops，并将其赋予component driver的的.ops域。

Linux中DMA操作流程一般为：

1. dma_request_channel:用于分配slave channel，这里的slave channel为DMA控制器的UART、SPI、I2S等外设通道。
2. dmaengine_slave_config：设定slave和controller的具体参数；
3. dma_prep_xxxx：生成用于传输的描述符(descriptor);
4. dma_cookie=dmaengine_submit(tx):提交传输请求并获取DMA cookie;
5. dma_async_issue_pending(chan):启动数据传输并等待完成的回调通知；

在ASoc中，设备树用于将DMA channel映射到PCM设备中。devm_snd_dmaengine_pcm_regisiter()中通过调用dmaengine_pcm_request_chan_of()(该函数是基于设备树的接口，dma_request_chan属于Linux中DMA Engine申请DMA channel的函数)申请DMA channel。为完成步骤1到3，需要向PCM DMA engine core提供额外的信息。这里有两种方式，一种是通过填充strcut snd_dmaengine_pcm_config中的数据项，传递到devm_snd_damengine_pcm_register()函数中；另一种是编程使PCM DMA engine framework从系统的DMA engine core中提取信息。步骤4到5由PCM DMA engine core自行处理。

位于"include\sound\dmaengine_pcm.h"对struct snd_dmaengine_pcm_config进行了定义。

/**
 * struct snd_dmaengine_pcm_config - Configuration data for dmaengine based PCM
 * @prepare_slave_config: Callback used to fill in the DMA slave_config for a
 *   PCM substream. Will be called from the PCM drivers hwparams callback.
 * @compat_request_channel: Callback to request a DMA channel for platforms
 *   which do not use devicetree.
 * @process: Callback used to apply processing on samples transferred from/to
 *   user space.
 * @compat_filter_fn: Will be used as the filter function when requesting a
 *  channel for platforms which do not use devicetree. The filter parameter
 *  will be the DAI's DMA data.
 * @dma_dev: If set, request DMA channel on this device rather than the DAI
 *  device.
 * @chan_names: If set, these custom DMA channel names will be requested at
 *  registration time.
 * @pcm_hardware: snd_pcm_hardware struct to be used for the PCM.
 * @prealloc_buffer_size: Size of the preallocated audio buffer.
 *
 * Note: If both compat_request_channel and compat_filter_fn are set
 * compat_request_channel will be used to request the channel and
 * compat_filter_fn will be ignored. Otherwise the channel will be requested
 * using dma_request_channel with compat_filter_fn as the filter function.
 */
struct snd_dmaengine_pcm_config {
	int (*prepare_slave_config)(struct snd_pcm_substream *substream,
			struct snd_pcm_hw_params *params,
			struct dma_slave_config *slave_config);
	struct dma_chan *(*compat_request_channel)(
			struct snd_soc_pcm_runtime *rtd,
			struct snd_pcm_substream *substream);
	int (*process)(struct snd_pcm_substream *substream,
		       int channel, unsigned long hwoff,
		       void *buf, unsigned long bytes);
	dma_filter_fn compat_filter_fn;
	struct device *dma_dev;
	const char *chan_names[SNDRV_PCM_STREAM_LAST + 1];

	const struct snd_pcm_hardware *pcm_hardware;
	unsigned int prealloc_buffer_size;
};

上述结构体主要用于解决DMA channel management,buffer management以及通道配置。目前看到的瑞芯微的代码中，没有使用该结构体进行管理，而是采用第二种办法进行处理，第二种办法更为灵活，不过理解这个结构体的定义，便于学习相关概念，理解第二种方式即generic DMA framework的回调函数的方式

• prepare_slave_config:该回调函数用于填充对于单个PCM sub-stream的DMAslave_config(其为struct dma_slave_config，用于运行时对DMA slave channel)。其在PCM driver的hwparams回调函数中调用。另外，用户可以使用snd_dmaengine_pcm_prepare_slave_config,对于使用struct snd_dmaengine_dai_dma_data表示DAI DMA数据的platform驱动，这一函数是prepare_slave_config通用版本。该函数内部会调用snd_hwparams_to_dma_slave_config基于hw_params参数填充slave config的相关项，然后调用snd_dmaengine_set_config_from_dai_data基于DAI DMA数据填充剩余的数据项。

在使用generic DMA framework提供的回调方法时，用户需要在CPU侧的DAI驱动的.probe回调函数中调用snd_soc_dai_init_dma_data()(给定DAI相关的capture和playback通道的DMA数据配置，用数据类型struct snd_dmaengine_dai_dma_data表示),该函数会设定cpu_dai->playback_dma_data和cpu_dai->capture_dma_data数据项，或者类似以下代码片段直接赋值。目的是通过填充struct snd_dmaengine_dai_dma_data配置playback和capture的参数。

snd_soc_dai_init_dma_data()的定义如下，

static inline void snd_soc_dai_init_dma_data(struct snd_soc_dai *dai,void *playback,void *capture)
{
    dai->playback_dma_data = playback;
    dai->capture_dma_data = capture;
}

其中playback和capture虽然传参时是void类型，但是其数据类型实际为strcut snd_dmaengine_dai_dma_data，其定义位于"include\sound\dmaengine_pcm.h"

/**
 * struct snd_dmaengine_dai_dma_data - DAI DMA configuration data
 * @addr: Address of the DAI data source or destination register.
 * @addr_width: Width of the DAI data source or destination register.
 * @maxburst: Maximum number of words(note: words, as in units of the
 * src_addr_width member, not bytes) that can be send to or received from the
 * DAI in one burst.
 * @slave_id: Slave requester id for the DMA channel.
 * @filter_data: Custom DMA channel filter data, this will usually be used when
 * requesting the DMA channel.
 * @chan_name: Custom channel name to use when requesting DMA channel.
 * @fifo_size: FIFO size of the DAI controller in bytes
 * @flags: PCM_DAI flags, only SND_DMAENGINE_PCM_DAI_FLAG_PACK for now
 * @peripheral_config: peripheral configuration for programming peripheral
 * for dmaengine transfer
 * @peripheral_size: peripheral configuration buffer size
 */
struct snd_dmaengine_dai_dma_data {
	dma_addr_t addr;
	enum dma_slave_buswidth addr_width;
	u32 maxburst;
	unsigned int slave_id;
	void *filter_data;
	const char *chan_name;
	unsigned int fifo_size;
	unsigned int flags;
	void *peripheral_config;
	size_t peripheral_size;
};

在elf2中，相关的部分代码位于rockchip-i2s-tdm.c中，这里没有使用snd_soc_dai_init_dma_data()，而是直接赋值。

static int rockchip_i2s_tdm_dai_probe(struct snd_soc_dai *dai)
{
	struct rk_i2s_tdm_dev *i2s_tdm = snd_soc_dai_get_drvdata(dai);

	dai->capture_dma_data = &i2s_tdm->capture_dma_data;
	dai->playback_dma_data = &i2s_tdm->playback_dma_data;

	if (i2s_tdm->mclk_calibrate)
		snd_soc_add_component_controls(dai->component,
					       &rockchip_i2s_tdm_compensation_control,
					       1);

	return 0;
}

在实际的代码中，PCM DMA config存在调用devm_snd_dmaengine_pcm_regisiter()函数时不传递参数的情况，其值为NULL，注册函数的调用形式为ret=devm_snd_dmaengine_pcm_regisiter(&pdev->dev,NULL,0)，elf2中采用的就是这种注册方法。

在这种情况下，按照上述描述在CPU DAI Driver的.probe函数中对capture和playback对应的DAI DMA channel进行配置。使用这种方式，其余的数据将从系统中提取。比如，申请DMA channel时，PCM DMA engine core将依赖device tree中的设备节点中特定的属性的值确定相关的配置，capture和playback对应的DMA channels属性值的name项设定为“rx”和“tx”，如果注册函数的flag项设定为SND_DMAENGINE_PCM_FLAG_HALF_DUPLEX，在这种情况下，capture和playback使用相同的DMA channel，在设备树节点的属性值的name项为“rx-tx”。

位于rk3588s.dts中可以看到其中的属性节点dmas和dma-names的值

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";
	};

1.3 PCM hardware configuration

DMA的配置信息无法自动由PCM DMA engine core从系统的设备树中提取时或者需要修改时，platform的PCM driver需要提供PCM hardware配置，描述硬件应当如何在内存中如何组织PCM数据。这些设置通过strcut snd_pcm_hardware数据类型传递。在rk3588源码中可以看到相关的应用代码。

位于"sound/soc/rockchip/rockchip_trcm.c"中

static int
dmaengine_pcm_set_runtime_hwparams(struct snd_soc_component *component,
				   struct snd_pcm_substream *substream)
{
	struct snd_soc_pcm_runtime *rtd = asoc_substream_to_rtd(substream);
	struct dmaengine_trcm *trcm = soc_component_to_trcm(component);
	struct device *dma_dev = dmaengine_dma_dev(trcm, substream);
	struct dma_chan *chan = trcm->chan[substream->stream];
	struct snd_dmaengine_dai_dma_data *dma_data;
	struct snd_pcm_hardware hw;

	if (rtd->num_cpus > 1) {
		dev_err(rtd->dev,
			"%s doesn't support Multi CPU yet\n", __func__);
		return -EINVAL;
	}

	dma_data = snd_soc_dai_get_dma_data(asoc_rtd_to_cpu(rtd, 0), substream);

	memset(&hw, 0, sizeof(hw));
	hw.info = SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID |
			SNDRV_PCM_INFO_INTERLEAVED;
	hw.periods_min = 2;
	hw.periods_max = UINT_MAX;
	hw.period_bytes_min = 256;
	hw.period_bytes_max = dma_get_max_seg_size(dma_dev);
	hw.buffer_bytes_max = SIZE_MAX;
	hw.fifo_size = dma_data->fifo_size;

	snd_dmaengine_pcm_refine_runtime_hwparams(substream,
						  dma_data,
						  &hw,
						  chan);

	return snd_soc_set_runtime_hwparams(substream, &hw);
}

完成配置后，如果使用通过填充strcut snd_dmaengine_pcm_config中的数据项，传递到devm_snd_damengine_pcm_register()函数中的方式注册，snd_dmaengine_pcm_config.pcm_hardware需要在调用注册函数前给定。或者参考rk3588的实现方式，调用其他接口函数，这里使用的函数是snd_dmaengine_pcm_refine_runtime_hwparams,在运行过程中对PCM hardware的配置进行修改。

2. platform class中的snd_soc_dai_driver

与codec class类似，platform一侧同样有snd_soc_dai_driver用于管理音频接口相关的功能。包括时钟参数的设定、音频格式设定以及事件触发处理函数等。主要的结构体为snd_soc_dai_driver。
elf2中的rockchip-i2s-tdm.c相关的代码如下

static const struct snd_soc_dai_ops rockchip_i2s_tdm_dai_ops = {
	.startup = rockchip_i2s_tdm_startup,
	.shutdown = rockchip_i2s_tdm_shutdown,
	.hw_params = rockchip_i2s_tdm_hw_params,
	.hw_free = rockchip_i2s_tdm_hw_free,
	.set_sysclk = rockchip_i2s_tdm_set_sysclk,
	.set_fmt = rockchip_i2s_tdm_set_fmt,
	.set_tdm_slot = rockchip_dai_tdm_slot,
	.trigger = rockchip_i2s_tdm_trigger,
};


static int rockchip_i2s_tdm_dai_prepare(struct platform_device *pdev,
					struct snd_soc_dai_driver **soc_dai)
{
	struct snd_soc_dai_driver rockchip_i2s_tdm_dai = {
		.probe = rockchip_i2s_tdm_dai_probe,
		.playback = {
			.stream_name = "Playback",
			.channels_min = 2,
			.channels_max = 64,
			.rates = SNDRV_PCM_RATE_8000_192000,
			.formats = (SNDRV_PCM_FMTBIT_S8 |
				    SNDRV_PCM_FMTBIT_S16_LE |
				    SNDRV_PCM_FMTBIT_S20_3LE |
				    SNDRV_PCM_FMTBIT_S24_LE |
				    SNDRV_PCM_FMTBIT_S32_LE |
				    SNDRV_PCM_FMTBIT_IEC958_SUBFRAME_LE),
		},
		.capture = {
			.stream_name = "Capture",
			.channels_min = 2,
			.channels_max = 64,
			.rates = SNDRV_PCM_RATE_8000_192000,
			.formats = (SNDRV_PCM_FMTBIT_S8 |
				    SNDRV_PCM_FMTBIT_S16_LE |
				    SNDRV_PCM_FMTBIT_S20_3LE |
				    SNDRV_PCM_FMTBIT_S24_LE |
				    SNDRV_PCM_FMTBIT_S32_LE |
				    SNDRV_PCM_FMTBIT_IEC958_SUBFRAME_LE),
		},
		.ops = &rockchip_i2s_tdm_dai_ops,
	};

	*soc_dai = devm_kmemdup(&pdev->dev, &rockchip_i2s_tdm_dai,
				sizeof(rockchip_i2s_tdm_dai), GFP_KERNEL);
	if (!(*soc_dai))
		return -ENOMEM;

	return 0;
}

3 platform驱动注册

类似于codec驱动，通过调用devm_snd_soc_register_component()函数，将component和DAI 驱动注册到ASoC Core中，不同的地方在于，platform驱动所依赖的设备初始化函数中会额外调用devm_snd_damengine_pcm_register()函数用于设定PCM相关的dma操作。

elf2中相关的代码位于sound/soc/rockchip/rockchip_i2s_tdm.c中

static int rockchip_i2s_tdm_probe(struct platform_device *pdev)
{
	struct device_node *node = pdev->dev.of_node;
	const struct of_device_id *of_id;
	struct rk_i2s_tdm_dev *i2s_tdm;
	struct snd_soc_dai_driver *soc_dai;
	struct resource *res;
	void __iomem *regs;
#ifdef HAVE_SYNC_RESET
	bool sync;
#endif
	int ret, val, i, irq;
	//填充 snd_soc_dai_driver, 对snd_soc_dai_driver的实例soc_dai进行初始化
	ret = rockchip_i2s_tdm_dai_prepare(pdev, &soc_dai);
	if (ret)
		return ret;

	/*------- 

	其余设备初始化相关的代码

	-------*/
	//在该函数中调用 devm_snd_dmaengine_pcm_register
	ret = rockchip_i2s_tdm_register_platform(&pdev->dev);
	if (ret)
		goto err_suspend;

	//注册snd_soc_component_driver以及相应的snd_soc_dai_driver
	ret = devm_snd_soc_register_component(&pdev->dev,
					      &rockchip_i2s_tdm_component,
					      soc_dai, 1);
	if (ret) {
		dev_err(&pdev->dev, "Could not register DAI\n");
		goto err_suspend;
	}

	return 0;

err_suspend:
	if (!pm_runtime_status_suspended(&pdev->dev))
		i2s_tdm_runtime_suspend(&pdev->dev);
err_pm_disable:
	pm_runtime_disable(&pdev->dev);

#if defined(HAVE_SYNC_RESET) || defined(CONFIG_SND_SOC_ROCKCHIP_I2S_TDM_MULTI_LANES)
err_unmap:
	rockchip_i2s_tdm_unmap(i2s_tdm);
#endif

err_disable_hclk:
	clk_disable_unprepare(i2s_tdm->hclk);

	return ret;
}

static struct platform_driver rockchip_i2s_tdm_driver = {
	.probe = rockchip_i2s_tdm_probe,
	.remove = rockchip_i2s_tdm_remove,
	.shutdown = rockchip_i2s_tdm_platform_shutdown,
	.driver = {
		.name = DRV_NAME,
		.of_match_table = of_match_ptr(rockchip_i2s_tdm_match),
		.pm = &rockchip_i2s_tdm_pm_ops,
	},
};
module_platform_driver(rockchip_i2s_tdm_driver);