Linux内核4.14版本——alsa框架分析(10)——PCM DMA的使用

1. dmaengine_pcm_new

2. dmaengine_pcm_open

2.1 dmaengine_pcm_set_runtime_hwparams

2.1.1 dma_get_slave_caps

2.1.2 snd_soc_set_runtime_hwparams

2.2 snd_dmaengine_pcm_open

3. dmaengine_pcm_hw_params

3.1 snd_dmaengine_pcm_prepare_slave_config

3.1.1 snd_soc_dai_get_dma_data

3.1.2 snd_hwparams_to_dma_slave_config

3.1.3 snd_dmaengine_pcm_set_config_from_dai_data

3.2 dmaengine_slave_config

3.3 snd_pcm_lib_malloc_pages

4. snd_dmaengine_pcm_trigger

4.1 SNDRV_PCM_TRIGGER_START

4.1.1  dmaengine_pcm_prepare_and_submit

4.1.2 dma_async_issue_pending

4.2 SNDRV_PCM_TRIGGER_STOP

---

源码：sound\soc\soc-generic-dmaengine-pcm.c

static const struct snd_pcm_ops dmaengine_pcm_ops = {
	.open		= dmaengine_pcm_open,
	.close		= snd_dmaengine_pcm_close,
	.ioctl		= snd_pcm_lib_ioctl,
	.hw_params	= dmaengine_pcm_hw_params,
	.hw_free	= snd_pcm_lib_free_pages,
	.trigger	= snd_dmaengine_pcm_trigger,
	.pointer	= dmaengine_pcm_pointer,
};

static const struct snd_soc_platform_driver dmaengine_pcm_platform = {
	.component_driver = {
		.probe_order = SND_SOC_COMP_ORDER_LATE,
	},
	.ops		= &dmaengine_pcm_ops,
	.pcm_new	= dmaengine_pcm_new,
};

      所有的关于PCM DMA的使用都在这几个操作函数里面，简单分析一下。至于什么时候调用暂时先不管，后面再讲。这几个函数中，最早被调用的是dmaengine_pcm_new。

1. dmaengine_pcm_new

static int dmaengine_pcm_new(struct snd_soc_pcm_runtime *rtd)
{
	struct dmaengine_pcm *pcm = soc_platform_to_pcm(rtd->platform);
	const struct snd_dmaengine_pcm_config *config = pcm->config;
	struct device *dev = rtd->platform->dev;
	struct snd_dmaengine_dai_dma_data *dma_data;
	struct snd_pcm_substream *substream;
	size_t prealloc_buffer_size;
	size_t max_buffer_size;
	unsigned int i;
	int ret;

	if (config && config->prealloc_buffer_size) {
		prealloc_buffer_size = config->prealloc_buffer_size;
		max_buffer_size = config->pcm_hardware->buffer_bytes_max;
	} else {
		prealloc_buffer_size = 512 * 1024;
		max_buffer_size = SIZE_MAX;
	}

	for (i = SNDRV_PCM_STREAM_PLAYBACK; i <= SNDRV_PCM_STREAM_CAPTURE; i++) {
		substream = rtd->pcm->streams[i].substream;
		if (!substream)
			continue;

		dma_data = snd_soc_dai_get_dma_data(rtd->cpu_dai, substream);

		if (!pcm->chan[i] &&
		    (pcm->flags & SND_DMAENGINE_PCM_FLAG_CUSTOM_CHANNEL_NAME))
			pcm->chan[i] = dma_request_slave_channel(dev,
				dma_data->chan_name);

		if (!pcm->chan[i] && (pcm->flags & SND_DMAENGINE_PCM_FLAG_COMPAT)) {
			pcm->chan[i] = dmaengine_pcm_compat_request_channel(rtd,
				substream);
		}

		if (!pcm->chan[i]) {
			dev_err(rtd->platform->dev,
				"Missing dma channel for stream: %d\n", i);
			return -EINVAL;
		}

		ret = snd_pcm_lib_preallocate_pages(substream,
				SNDRV_DMA_TYPE_DEV_IRAM,
				dmaengine_dma_dev(pcm, substream),
				prealloc_buffer_size,
				max_buffer_size);
		if (ret)
			return ret;

		if (!dmaengine_pcm_can_report_residue(dev, pcm->chan[i]))
			pcm->flags |= SND_DMAENGINE_PCM_FLAG_NO_RESIDUE;

		if (rtd->pcm->streams[i].pcm->name[0] == '\0') {
			strncpy(rtd->pcm->streams[i].pcm->name,
				rtd->pcm->streams[i].pcm->id,
				sizeof(rtd->pcm->streams[i].pcm->name));
		}
	}

	return 0;
}

      dmaengine_pcm_new这个函数主要是进行playback和capture substream的DMA内存的预分配，注意是预分配，真正的内存还没有分配。

2. dmaengine_pcm_open

static int dmaengine_pcm_open(struct snd_pcm_substream *substream)
{
	struct snd_soc_pcm_runtime *rtd = substream->private_data;
	struct dmaengine_pcm *pcm = soc_platform_to_pcm(rtd->platform);
	struct dma_chan *chan = pcm->chan[substream->stream];
	int ret;

	ret = dmaengine_pcm_set_runtime_hwparams(substream);
	if (ret)
		return ret;

	return snd_dmaengine_pcm_open(substream, chan);
}

      dmaengine_pcm_open对参数进行一些设置，还有重要的一点是对用到的DMA控制器的能力集进行校验，如是否支持DMA_SLAVE、DMA_CYCLIC传输等等。这些都在dmaengine_pcm_set_runtime_hwparams函数里面的dma_get_slave_caps做的。

2.1 dmaengine_pcm_set_runtime_hwparams

static int dmaengine_pcm_set_runtime_hwparams(struct snd_pcm_substream *substream)
{
	struct snd_soc_pcm_runtime *rtd = substream->private_data;
.............

	ret = dma_get_slave_caps(chan, &dma_caps);
	if (ret == 0) {
		if (dma_caps.cmd_pause)
			hw.info |= SNDRV_PCM_INFO_PAUSE | SNDRV_PCM_INFO_RESUME;
		if (dma_caps.residue_granularity <= DMA_RESIDUE_GRANULARITY_SEGMENT)
			hw.info |= SNDRV_PCM_INFO_BATCH;

		if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
			addr_widths = dma_caps.dst_addr_widths;
		else
			addr_widths = dma_caps.src_addr_widths;
	}

.............

	return snd_soc_set_runtime_hwparams(substream, &hw);
}

2.1.1 dma_get_slave_caps

int dma_get_slave_caps(struct dma_chan *chan, struct dma_slave_caps *caps)
{
	struct dma_device *device;

	if (!chan || !caps)
		return -EINVAL;

	device = chan->device;

	/* check if the channel supports slave transactions */
	if (!(test_bit(DMA_SLAVE, device->cap_mask.bits) ||
	      test_bit(DMA_CYCLIC, device->cap_mask.bits)))
		return -ENXIO;

	/*
	 * Check whether it reports it uses the generic slave
	 * capabilities, if not, that means it doesn't support any
	 * kind of slave capabilities reporting.
	 */
	if (!device->directions)
		return -ENXIO;

	caps->src_addr_widths = device->src_addr_widths;
	caps->dst_addr_widths = device->dst_addr_widths;
	caps->directions = device->directions;
	caps->max_burst = device->max_burst;
	caps->residue_granularity = device->residue_granularity;
	caps->descriptor_reuse = device->descriptor_reuse;

	/*
	 * Some devices implement only pause (e.g. to get residuum) but no
	 * resume. However cmd_pause is advertised as pause AND resume.
	 */
	caps->cmd_pause = !!(device->device_pause && device->device_resume);
	caps->cmd_terminate = !!device->device_terminate_all;

	return 0;
}

      dma_get_slave_caps对参数进行一些设置，还有重要的一点是对用到的DMA控制器的能力集进行校验，如是否支持DMA_SLAVE、DMA_CYCLIC传输等等。如果需要支持暂停等功能，需要DMA控制器实现相关函数，如device_pause等。

2.1.2 snd_soc_set_runtime_hwparams

/**
 * snd_soc_set_runtime_hwparams - set the runtime hardware parameters
 * @substream: the pcm substream
 * @hw: the hardware parameters
 *
 * Sets the substream runtime hardware parameters.
 */
int snd_soc_set_runtime_hwparams(struct snd_pcm_substream *substream,
	const struct snd_pcm_hardware *hw)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	runtime->hw.info = hw->info;
	runtime->hw.formats = hw->formats;
	runtime->hw.period_bytes_min = hw->period_bytes_min;
	runtime->hw.period_bytes_max = hw->period_bytes_max;
	runtime->hw.periods_min = hw->periods_min;
	runtime->hw.periods_max = hw->periods_max;
	runtime->hw.buffer_bytes_max = hw->buffer_bytes_max;
	runtime->hw.fifo_size = hw->fifo_size;
	return 0;
}

2.2 snd_dmaengine_pcm_open

/**
 * snd_dmaengine_pcm_open - Open a dmaengine based PCM substream
 * @substream: PCM substream
 * @chan: DMA channel to use for data transfers
 *
 * Returns 0 on success, a negative error code otherwise.
 *
 * The function should usually be called from the pcm open callback. Note that
 * this function will use private_data field of the substream's runtime. So it
 * is not available to your pcm driver implementation.
 */
int snd_dmaengine_pcm_open(struct snd_pcm_substream *substream,
	struct dma_chan *chan)
{
	struct dmaengine_pcm_runtime_data *prtd;
	int ret;

	if (!chan)
		return -ENXIO;

	ret = snd_pcm_hw_constraint_integer(substream->runtime,
					    SNDRV_PCM_HW_PARAM_PERIODS);
	if (ret < 0)
		return ret;

	prtd = kzalloc(sizeof(*prtd), GFP_KERNEL);
	if (!prtd)
		return -ENOMEM;

	prtd->dma_chan = chan;

	substream->runtime->private_data = prtd;

	return 0;
}

3. dmaengine_pcm_hw_params

static int dmaengine_pcm_hw_params(struct snd_pcm_substream *substream,
	struct snd_pcm_hw_params *params)
{
	struct snd_soc_pcm_runtime *rtd = substream->private_data;
	struct dmaengine_pcm *pcm = soc_platform_to_pcm(rtd->platform);
	struct dma_chan *chan = snd_dmaengine_pcm_get_chan(substream);
	int (*prepare_slave_config)(struct snd_pcm_substream *substream,
			struct snd_pcm_hw_params *params,
			struct dma_slave_config *slave_config);
	struct dma_slave_config slave_config;
	int ret;

	memset(&slave_config, 0, sizeof(slave_config));

	if (!pcm->config)
		prepare_slave_config = snd_dmaengine_pcm_prepare_slave_config;
	else
		prepare_slave_config = pcm->config->prepare_slave_config;

	if (prepare_slave_config) {
		ret = prepare_slave_config(substream, params, &slave_config);
		if (ret)
			return ret;

		ret = dmaengine_slave_config(chan, &slave_config);
		if (ret)
			return ret;
	}

	return snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(params));
}

      dmaengine_pcm_hw_params做了两件事，一是获取DMA SLAVE的参数，并设置DMA channel。二是申请内存。

3.1 snd_dmaengine_pcm_prepare_slave_config

/**
 * snd_dmaengine_pcm_prepare_slave_config() - Generic prepare_slave_config callback
 * @substream: PCM substream
 * @params: hw_params
 * @slave_config: DMA slave config to prepare
 *
 * This function can be used as a generic prepare_slave_config callback for
 * platforms which make use of the snd_dmaengine_dai_dma_data struct for their
 * DAI DMA data. Internally the function will first call
 * snd_hwparams_to_dma_slave_config to fill in the slave config based on the
 * hw_params, followed by snd_dmaengine_set_config_from_dai_data to fill in the
 * remaining fields based on the DAI DMA data.
 */
int snd_dmaengine_pcm_prepare_slave_config(struct snd_pcm_substream *substream,
	struct snd_pcm_hw_params *params, struct dma_slave_config *slave_config)
{
	struct snd_soc_pcm_runtime *rtd = substream->private_data;
	struct snd_dmaengine_dai_dma_data *dma_data;
	int ret;

	dma_data = snd_soc_dai_get_dma_data(rtd->cpu_dai, substream);

	ret = snd_hwparams_to_dma_slave_config(substream, params, slave_config);
	if (ret)
		return ret;

	snd_dmaengine_pcm_set_config_from_dai_data(substream, dma_data,
		slave_config);

	return 0;
}

      使用snd_soc_dai_get_dma_data函数得到playback和capture substream的DMA的相关配置，这块很重要，需要我们在写驱动的时候提供，并使用函数snd_soc_dai_init_dma_data设置。例如;

static void s3c2440_i2s_init_pcm_config(struct tx511_tdm_info *tdm)
{
	struct snd_dmaengine_dai_dma_data *dma_data;

	/* Playback */
	dma_data = &tdm->playback_dma_data;
	dma_data->maxburst = 16;
	dma_data->slave_id = TX511_DMA_SLAVE_ID_I2S0_TX;
	dma_data->addr = tdm->phy_addr + TDM_TXFIFO_DATA;

	/* Capture */
	dma_data = &tdm->capture_dma_data;
	dma_data->maxburst = 16;
	dma_data->slave_id = TX511_DMA_SLAVE_ID_I2S0_RX;
	dma_data->addr = tdm->phy_addr + TDM_RXFIFO_DATA;
}

static int s3c2440_i2s_dai_probe(struct snd_soc_dai *dai)
{
	struct tx511_tdm_info *tdm = snd_soc_dai_get_drvdata(dai);

	s3c2440_i2s_init_pcm_config(tdm);
	snd_soc_dai_init_dma_data(dai, &tdm->playback_dma_data,
					&tdm->capture_dma_data);
}

3.1.1 snd_soc_dai_get_dma_data

static inline void *snd_soc_dai_get_dma_data(const struct snd_soc_dai *dai,
					     const struct snd_pcm_substream *ss)
{
	return (ss->stream == SNDRV_PCM_STREAM_PLAYBACK) ?
		dai->playback_dma_data : dai->capture_dma_data;
}

      使用snd_soc_dai_get_dma_data函数得到playback和capture substream的DMA的相关配置。

static inline void snd_soc_dai_set_dma_data(struct snd_soc_dai *dai,
					    const struct snd_pcm_substream *ss,
					    void *data)
{
	if (ss->stream == SNDRV_PCM_STREAM_PLAYBACK)
		dai->playback_dma_data = data;
	else
		dai->capture_dma_data = data;
}

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;
}

      snd_soc_dai_init_dma_data在编写驱动的时候初始化，snd_soc_dai_set_dma_data在驱动运行时更改设置。

3.1.2 snd_hwparams_to_dma_slave_config

/**
 * snd_hwparams_to_dma_slave_config - Convert hw_params to dma_slave_config
 * @substream: PCM substream
 * @params: hw_params
 * @slave_config: DMA slave config
 *
 * This function can be used to initialize a dma_slave_config from a substream
 * and hw_params in a dmaengine based PCM driver implementation.
 */
int snd_hwparams_to_dma_slave_config(const struct snd_pcm_substream *substream,
	const struct snd_pcm_hw_params *params,
	struct dma_slave_config *slave_config)
{
	enum dma_slave_buswidth buswidth;
	int bits;

	bits = params_physical_width(params);
	if (bits < 8 || bits > 64)
		return -EINVAL;
	else if (bits == 8)
		buswidth = DMA_SLAVE_BUSWIDTH_1_BYTE;
	else if (bits == 16)
		buswidth = DMA_SLAVE_BUSWIDTH_2_BYTES;
	else if (bits == 24)
		buswidth = DMA_SLAVE_BUSWIDTH_3_BYTES;
	else if (bits <= 32)
		buswidth = DMA_SLAVE_BUSWIDTH_4_BYTES;
	else
		buswidth = DMA_SLAVE_BUSWIDTH_8_BYTES;

	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
		slave_config->direction = DMA_MEM_TO_DEV;
		slave_config->dst_addr_width = buswidth;
	} else {
		slave_config->direction = DMA_DEV_TO_MEM;
		slave_config->src_addr_width = buswidth;
	}

	slave_config->device_fc = false;

	return 0;
}

3.1.3 snd_dmaengine_pcm_set_config_from_dai_data

/**
 * snd_dmaengine_pcm_set_config_from_dai_data() - Initializes a dma slave config
 *  using DAI DMA data.
 * @substream: PCM substream
 * @dma_data: DAI DMA data
 * @slave_config: DMA slave configuration
 *
 * Initializes the {dst,src}_addr, {dst,src}_maxburst, {dst,src}_addr_width and
 * slave_id fields of the DMA slave config from the same fields of the DAI DMA
 * data struct. The src and dst fields will be initialized depending on the
 * direction of the substream. If the substream is a playback stream the dst
 * fields will be initialized, if it is a capture stream the src fields will be
 * initialized. The {dst,src}_addr_width field will only be initialized if the
 * SND_DMAENGINE_PCM_DAI_FLAG_PACK flag is set or if the addr_width field of
 * the DAI DMA data struct is not equal to DMA_SLAVE_BUSWIDTH_UNDEFINED. If
 * both conditions are met the latter takes priority.
 */
void snd_dmaengine_pcm_set_config_from_dai_data(
	const struct snd_pcm_substream *substream,
	const struct snd_dmaengine_dai_dma_data *dma_data,
	struct dma_slave_config *slave_config)
{
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
		slave_config->dst_addr = dma_data->addr;
		slave_config->dst_maxburst = dma_data->maxburst;
		if (dma_data->flags & SND_DMAENGINE_PCM_DAI_FLAG_PACK)
			slave_config->dst_addr_width =
				DMA_SLAVE_BUSWIDTH_UNDEFINED;
		if (dma_data->addr_width != DMA_SLAVE_BUSWIDTH_UNDEFINED)
			slave_config->dst_addr_width = dma_data->addr_width;
	} else {
		slave_config->src_addr = dma_data->addr;
		slave_config->src_maxburst = dma_data->maxburst;
		if (dma_data->flags & SND_DMAENGINE_PCM_DAI_FLAG_PACK)
			slave_config->src_addr_width =
				DMA_SLAVE_BUSWIDTH_UNDEFINED;
		if (dma_data->addr_width != DMA_SLAVE_BUSWIDTH_UNDEFINED)
			slave_config->src_addr_width = dma_data->addr_width;
	}

	slave_config->slave_id = dma_data->slave_id;
}

3.2 dmaengine_slave_config

static inline int dmaengine_slave_config(struct dma_chan *chan,
					  struct dma_slave_config *config)
{
	if (chan->device->device_config)
		return chan->device->device_config(chan, config);

	return -ENOSYS;
}

      最终调用DMA 控制器实现的函数device_config。

3.3 snd_pcm_lib_malloc_pages

/**
 * snd_pcm_lib_malloc_pages - allocate the DMA buffer
 * @substream: the substream to allocate the DMA buffer to
 * @size: the requested buffer size in bytes
 *
 * Allocates the DMA buffer on the BUS type given earlier to
 * snd_pcm_lib_preallocate_xxx_pages().
 *
 * Return: 1 if the buffer is changed, 0 if not changed, or a negative
 * code on failure.
 */
int snd_pcm_lib_malloc_pages(struct snd_pcm_substream *substream, size_t size)
{
	struct snd_pcm_runtime *runtime;
	struct snd_dma_buffer *dmab = NULL;

	if (PCM_RUNTIME_CHECK(substream))
		return -EINVAL;
	if (snd_BUG_ON(substream->dma_buffer.dev.type ==
		       SNDRV_DMA_TYPE_UNKNOWN))
		return -EINVAL;
	runtime = substream->runtime;

	if (runtime->dma_buffer_p) {
		/* perphaps, we might free the large DMA memory region
		   to save some space here, but the actual solution
		   costs us less time */
		if (runtime->dma_buffer_p->bytes >= size) {
			runtime->dma_bytes = size;
			return 0;	/* ok, do not change */
		}
		snd_pcm_lib_free_pages(substream);
	}
	if (substream->dma_buffer.area != NULL &&
	    substream->dma_buffer.bytes >= size) {
		dmab = &substream->dma_buffer; /* use the pre-allocated buffer */
	} else {
		dmab = kzalloc(sizeof(*dmab), GFP_KERNEL);
		if (! dmab)
			return -ENOMEM;
		dmab->dev = substream->dma_buffer.dev;
		if (snd_dma_alloc_pages(substream->dma_buffer.dev.type,
					substream->dma_buffer.dev.dev,
					size, dmab) < 0) {
			kfree(dmab);
			return -ENOMEM;
		}
	}
	snd_pcm_set_runtime_buffer(substream, dmab);
	runtime->dma_bytes = size;
	return 1;			/* area was changed */
}

      实际分配DMA内存。

4. snd_dmaengine_pcm_trigger

/**
 * snd_dmaengine_pcm_trigger - dmaengine based PCM trigger implementation
 * @substream: PCM substream
 * @cmd: Trigger command
 *
 * Returns 0 on success, a negative error code otherwise.
 *
 * This function can be used as the PCM trigger callback for dmaengine based PCM
 * driver implementations.
 */
int snd_dmaengine_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
{
	struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
	struct snd_pcm_runtime *runtime = substream->runtime;
	int ret;

	switch (cmd) {
	case SNDRV_PCM_TRIGGER_START:
		ret = dmaengine_pcm_prepare_and_submit(substream);
		if (ret)
			return ret;
		dma_async_issue_pending(prtd->dma_chan);
		break;
	case SNDRV_PCM_TRIGGER_RESUME:
	case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
		dmaengine_resume(prtd->dma_chan);
		break;
	case SNDRV_PCM_TRIGGER_SUSPEND:
		if (runtime->info & SNDRV_PCM_INFO_PAUSE)
			dmaengine_pause(prtd->dma_chan);
		else
			dmaengine_terminate_async(prtd->dma_chan);
		break;
	case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
		dmaengine_pause(prtd->dma_chan);
		break;
	case SNDRV_PCM_TRIGGER_STOP:
		dmaengine_terminate_async(prtd->dma_chan);
		break;
	default:
		return -EINVAL;
	}

	return 0;
}

      前面已经做好了准备，snd_dmaengine_pcm_trigger函数就是根据上层传下的cmd命令，进行工作。如start、pause等，最终都调用到DMA 控制器实现的函数。

4.1 SNDRV_PCM_TRIGGER_START

	case SNDRV_PCM_TRIGGER_START:
		ret = dmaengine_pcm_prepare_and_submit(substream);
		if (ret)
			return ret;
		dma_async_issue_pending(prtd->dma_chan);
		break;

4.1.1  dmaengine_pcm_prepare_and_submit

static int dmaengine_pcm_prepare_and_submit(struct snd_pcm_substream *substream)
{
	struct dmaengine_pcm_runtime_data *prtd = substream_to_prtd(substream);
	struct dma_chan *chan = prtd->dma_chan;
	struct dma_async_tx_descriptor *desc;
	enum dma_transfer_direction direction;
	unsigned long flags = DMA_CTRL_ACK;

	direction = snd_pcm_substream_to_dma_direction(substream);

	if (!substream->runtime->no_period_wakeup)
		flags |= DMA_PREP_INTERRUPT;

	prtd->pos = 0;
	desc = dmaengine_prep_dma_cyclic(chan,
		substream->runtime->dma_addr,
		snd_pcm_lib_buffer_bytes(substream),
		snd_pcm_lib_period_bytes(substream), direction, flags);

	if (!desc)
		return -ENOMEM;

	desc->callback = dmaengine_pcm_dma_complete;
	desc->callback_param = substream;
	prtd->cookie = dmaengine_submit(desc);

	return 0;
}

       dmaengine_prep_dma_cyclic调用DAM控制器实现的device_prep_dma_cyclic，所以，如果使用PCM DMA，那么DMA控制器必须实现device_prep_dma_cyclic函数。

static inline struct dma_async_tx_descriptor *dmaengine_prep_dma_cyclic(
		struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
		size_t period_len, enum dma_transfer_direction dir,
		unsigned long flags)
{
	if (!chan || !chan->device || !chan->device->device_prep_dma_cyclic)
		return NULL;

	return chan->device->device_prep_dma_cyclic(chan, buf_addr, buf_len,
						period_len, dir, flags);
}

4.1.2 dma_async_issue_pending

/**
 * dma_async_issue_pending - flush pending transactions to HW
 * @chan: target DMA channel
 *
 * This allows drivers to push copies to HW in batches,
 * reducing MMIO writes where possible.
 */
static inline void dma_async_issue_pending(struct dma_chan *chan)
{
	chan->device->device_issue_pending(chan);
}

4.2 SNDRV_PCM_TRIGGER_STOP

	case SNDRV_PCM_TRIGGER_STOP:
		dmaengine_terminate_async(prtd->dma_chan);
		break;

/**
 * dmaengine_terminate_async() - Terminate all active DMA transfers
 * @chan: The channel for which to terminate the transfers
 *
 * Calling this function will terminate all active and pending descriptors
 * that have previously been submitted to the channel. It is not guaranteed
 * though that the transfer for the active descriptor has stopped when the
 * function returns. Furthermore it is possible the complete callback of a
 * submitted transfer is still running when this function returns.
 *
 * dmaengine_synchronize() needs to be called before it is safe to free
 * any memory that is accessed by previously submitted descriptors or before
 * freeing any resources accessed from within the completion callback of any
 * perviously submitted descriptors.
 *
 * This function can be called from atomic context as well as from within a
 * complete callback of a descriptor submitted on the same channel.
 *
 * If none of the two conditions above apply consider using
 * dmaengine_terminate_sync() instead.
 */
static inline int dmaengine_terminate_async(struct dma_chan *chan)
{
	if (chan->device->device_terminate_all)
		return chan->device->device_terminate_all(chan);

	return -EINVAL;
}