Main process of audio data transfer in Linux kernel

Linux user space applications transfer data with the Linux kernel audio device driver and audio hardware through the sound card driver (generally involving multiple device drivers) and PCM device files exported by the Linux kernel ALSA framework, such as and /dev/snd/pcmC0D0c. /dev/snd/pcmC0D0pThe file operation definition of the PCM device file (located in sound/core/pcm_native.c ) is as follows:

const struct file_operations snd_pcm_f_ops[2] = {
	{
		.owner =		THIS_MODULE,
		.write =		snd_pcm_write,
		.write_iter =		snd_pcm_writev,
		.open =			snd_pcm_playback_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_poll,
		.unlocked_ioctl =	snd_pcm_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	},
	{
		.owner =		THIS_MODULE,
		.read =			snd_pcm_read,
		.read_iter =		snd_pcm_readv,
		.open =			snd_pcm_capture_open,
		.release =		snd_pcm_release,
		.llseek =		no_llseek,
		.poll =			snd_pcm_poll,
		.unlocked_ioctl =	snd_pcm_ioctl,
		.compat_ioctl = 	snd_pcm_ioctl_compat,
		.mmap =			snd_pcm_mmap,
		.fasync =		snd_pcm_fasync,
		.get_unmapped_area =	snd_pcm_get_unmapped_area,
	}
};

In most cases, audio devices provide both playback and recording functions. PCM device files for playback and recording are exported together. File operations for PCM device files for playback and recording are also defined together, where the index is, that SNDRV_PCM_STREAM_PLAYBACKis The file operation with index 0 is used for playback, that is SNDRV_PCM_STREAM_CAPTURE, the file operation with index 1 is used for recording.

Linux user space has libraries such as alsa-lib and tinyalsa that can be used to interact with the Linux kernel ALSA framework. Libraries such as alsa-lib and tinyalsaioctl interact with the Linux kernel ALSA framework mainly through commands. ioctlThe operation function definition of the PCM device file snd_pcm_ioctl()(located in sound/core/pcm_native.c ) is as follows:

static int snd_pcm_common_ioctl(struct file *file,
				 struct snd_pcm_substream *substream,
				 unsigned int cmd, void __user *arg)
{
	struct snd_pcm_file *pcm_file = file->private_data;
	int res;

	if (PCM_RUNTIME_CHECK(substream))
		return -ENXIO;

	res = snd_power_wait(substream->pcm->card, SNDRV_CTL_POWER_D0);
	if (res < 0)
		return res;

	switch (cmd) {
	case SNDRV_PCM_IOCTL_PVERSION:
		return put_user(SNDRV_PCM_VERSION, (int __user *)arg) ? -EFAULT : 0;
	case SNDRV_PCM_IOCTL_INFO:
		return snd_pcm_info_user(substream, arg);
	case SNDRV_PCM_IOCTL_TSTAMP:	/* just for compatibility */
		return 0;
	case SNDRV_PCM_IOCTL_TTSTAMP:
		return snd_pcm_tstamp(substream, arg);
	case SNDRV_PCM_IOCTL_USER_PVERSION:
		if (get_user(pcm_file->user_pversion,
			     (unsigned int __user *)arg))
			return -EFAULT;
		return 0;
	case SNDRV_PCM_IOCTL_HW_REFINE:
		return snd_pcm_hw_refine_user(substream, arg);
	case SNDRV_PCM_IOCTL_HW_PARAMS:
		return snd_pcm_hw_params_user(substream, arg);
	case SNDRV_PCM_IOCTL_HW_FREE:
		return snd_pcm_hw_free(substream);
	case SNDRV_PCM_IOCTL_SW_PARAMS:
		return snd_pcm_sw_params_user(substream, arg);
	case SNDRV_PCM_IOCTL_STATUS32:
		return snd_pcm_status_user32(substream, arg, false);
	case SNDRV_PCM_IOCTL_STATUS_EXT32:
		return snd_pcm_status_user32(substream, arg, true);
	case SNDRV_PCM_IOCTL_STATUS64:
		return snd_pcm_status_user64(substream, arg, false);
	case SNDRV_PCM_IOCTL_STATUS_EXT64:
		return snd_pcm_status_user64(substream, arg, true);
	case SNDRV_PCM_IOCTL_CHANNEL_INFO:
		return snd_pcm_channel_info_user(substream, arg);
	case SNDRV_PCM_IOCTL_PREPARE:
		return snd_pcm_prepare(substream, file);
	case SNDRV_PCM_IOCTL_RESET:
		return snd_pcm_reset(substream);
	case SNDRV_PCM_IOCTL_START:
		return snd_pcm_start_lock_irq(substream);
	case SNDRV_PCM_IOCTL_LINK:
		return snd_pcm_link(substream, (int)(unsigned long) arg);
	case SNDRV_PCM_IOCTL_UNLINK:
		return snd_pcm_unlink(substream);
	case SNDRV_PCM_IOCTL_RESUME:
		return snd_pcm_resume(substream);
	case SNDRV_PCM_IOCTL_XRUN:
		return snd_pcm_xrun(substream);
	case SNDRV_PCM_IOCTL_HWSYNC:
		return snd_pcm_hwsync(substream);
	case SNDRV_PCM_IOCTL_DELAY:
	{
		snd_pcm_sframes_t delay;
		snd_pcm_sframes_t __user *res = arg;
		int err;

		err = snd_pcm_delay(substream, &delay);
		if (err)
			return err;
		if (put_user(delay, res))
			return -EFAULT;
		return 0;
	}
	case __SNDRV_PCM_IOCTL_SYNC_PTR32:
		return snd_pcm_ioctl_sync_ptr_compat(substream, arg);
	case __SNDRV_PCM_IOCTL_SYNC_PTR64:
		return snd_pcm_sync_ptr(substream, arg);
#ifdef CONFIG_SND_SUPPORT_OLD_API
	case SNDRV_PCM_IOCTL_HW_REFINE_OLD:
		return snd_pcm_hw_refine_old_user(substream, arg);
	case SNDRV_PCM_IOCTL_HW_PARAMS_OLD:
		return snd_pcm_hw_params_old_user(substream, arg);
#endif
	case SNDRV_PCM_IOCTL_DRAIN:
		return snd_pcm_drain(substream, file);
	case SNDRV_PCM_IOCTL_DROP:
		return snd_pcm_drop(substream);
	case SNDRV_PCM_IOCTL_PAUSE:
		return snd_pcm_pause_lock_irq(substream, (unsigned long)arg);
	case SNDRV_PCM_IOCTL_WRITEI_FRAMES:
	case SNDRV_PCM_IOCTL_READI_FRAMES:
		return snd_pcm_xferi_frames_ioctl(substream, arg);
	case SNDRV_PCM_IOCTL_WRITEN_FRAMES:
	case SNDRV_PCM_IOCTL_READN_FRAMES:
		return snd_pcm_xfern_frames_ioctl(substream, arg);
	case SNDRV_PCM_IOCTL_REWIND:
		return snd_pcm_rewind_ioctl(substream, arg);
	case SNDRV_PCM_IOCTL_FORWARD:
		return snd_pcm_forward_ioctl(substream, arg);
	}
	pcm_dbg(substream->pcm, "unknown ioctl = 0x%x\n", cmd);
	return -ENOTTY;
}

static long snd_pcm_ioctl(struct file *file, unsigned int cmd,
			  unsigned long arg)
{
	struct snd_pcm_file *pcm_file;

	pcm_file = file->private_data;

	if (((cmd >> 8) & 0xff) != 'A')
		return -ENOTTY;

	return snd_pcm_common_ioctl(file, pcm_file->substream, cmd,
				     (void __user *)arg);
}

Libraries such as alsa-lib and tinyalsa mainly exchange data with the Linux kernel ALSA framework through four commands SNDRV_PCM_IOCTL_WRITEI_FRAMES: , SNDRV_PCM_IOCTL_READI_FRAMES, SNDRV_PCM_IOCTL_WRITEN_FRAMESand . The main difference between these commands is that the format of the transferred data is different. The and command is used to read and write audio data in interleaved format, and the and command is used to read and write audio data in noninterleaved format.SNDRV_PCM_IOCTL_READN_FRAMESioctlioctlSNDRV_PCM_IOCTL_READI_FRAMESSNDRV_PCM_IOCTL_WRITEI_FRAMESSNDRV_PCM_IOCTL_READN_FRAMESSNDRV_PCM_IOCTL_WRITEN_FRAMES

SNDRV_PCM_IOCTL_READI_FRAMESThe and SNDRV_PCM_IOCTL_WRITEI_FRAMEScommands are processed by snd_pcm_xferi_frames_ioctl()the function, SNDRV_PCM_IOCTL_READN_FRAMESand SNDRV_PCM_IOCTL_WRITEN_FRAMESthe and commands are processed by snd_pcm_xfern_frames_ioctl()the function. These two functions are defined (located in sound/core/pcm_native.c ) as follows:

static int snd_pcm_xferi_frames_ioctl(struct snd_pcm_substream *substream,
				      struct snd_xferi __user *_xferi)
{
	struct snd_xferi xferi;
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_sframes_t result;

	if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
		return -EBADFD;
	if (put_user(0, &_xferi->result))
		return -EFAULT;
	if (copy_from_user(&xferi, _xferi, sizeof(xferi)))
		return -EFAULT;
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		result = snd_pcm_lib_write(substream, xferi.buf, xferi.frames);
	else
		result = snd_pcm_lib_read(substream, xferi.buf, xferi.frames);
	if (put_user(result, &_xferi->result))
		return -EFAULT;
	return result < 0 ? result : 0;
}

static int snd_pcm_xfern_frames_ioctl(struct snd_pcm_substream *substream,
				      struct snd_xfern __user *_xfern)
{
	struct snd_xfern xfern;
	struct snd_pcm_runtime *runtime = substream->runtime;
	void *bufs;
	snd_pcm_sframes_t result;

	if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
		return -EBADFD;
	if (runtime->channels > 128)
		return -EINVAL;
	if (put_user(0, &_xfern->result))
		return -EFAULT;
	if (copy_from_user(&xfern, _xfern, sizeof(xfern)))
		return -EFAULT;

	bufs = memdup_user(xfern.bufs, sizeof(void *) * runtime->channels);
	if (IS_ERR(bufs))
		return PTR_ERR(bufs);
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		result = snd_pcm_lib_writev(substream, bufs, xfern.frames);
	else
		result = snd_pcm_lib_readv(substream, bufs, xfern.frames);
	kfree(bufs);
	if (put_user(result, &_xfern->result))
		return -EFAULT;
	return result < 0 ? result : 0;
}

snd_pcm_xferi_frames_ioctl()The and snd_pcm_xfern_frames_ioctl()function copies the parameters to the kernel space stack, and performs read and write operations through snd_pcm_lib_write()the , snd_pcm_lib_read(), snd_pcm_lib_writev()and four functions. snd_pcm_lib_readv()The definitions of these four functions (located in include/sound/pcm.h ) are as follows:

static inline snd_pcm_sframes_t
snd_pcm_lib_write(struct snd_pcm_substream *substream,
		  const void __user *buf, snd_pcm_uframes_t frames)
{
	return __snd_pcm_lib_xfer(substream, (void __force *)buf, true, frames, false);
}

static inline snd_pcm_sframes_t
snd_pcm_lib_read(struct snd_pcm_substream *substream,
		 void __user *buf, snd_pcm_uframes_t frames)
{
	return __snd_pcm_lib_xfer(substream, (void __force *)buf, true, frames, false);
}

static inline snd_pcm_sframes_t
snd_pcm_lib_writev(struct snd_pcm_substream *substream,
		   void __user **bufs, snd_pcm_uframes_t frames)
{
	return __snd_pcm_lib_xfer(substream, (void *)bufs, false, frames, false);
}

static inline snd_pcm_sframes_t
snd_pcm_lib_readv(struct snd_pcm_substream *substream,
		  void __user **bufs, snd_pcm_uframes_t frames)
{
	return __snd_pcm_lib_xfer(substream, (void *)bufs, false, frames, false);
}

All audio data transfer operations are ultimately __snd_pcm_lib_xfer()completed by the function. __snd_pcm_lib_xfer()The function definition (located in sound/core/pcm_lib.c ) is as follows:

typedef int (*pcm_transfer_f)(struct snd_pcm_substream *substream,
			      int channel, unsigned long hwoff,
			      void *buf, unsigned long bytes);

typedef int (*pcm_copy_f)(struct snd_pcm_substream *, snd_pcm_uframes_t, void *,
			  snd_pcm_uframes_t, snd_pcm_uframes_t, pcm_transfer_f);
 . . . . . .
/* sanity-check for read/write methods */
static int pcm_sanity_check(struct snd_pcm_substream *substream)
{
	struct snd_pcm_runtime *runtime;
	if (PCM_RUNTIME_CHECK(substream))
		return -ENXIO;
	runtime = substream->runtime;
	if (snd_BUG_ON(!substream->ops->copy_user && !runtime->dma_area))
		return -EINVAL;
	if (runtime->status->state == SNDRV_PCM_STATE_OPEN)
		return -EBADFD;
	return 0;
}

static int pcm_accessible_state(struct snd_pcm_runtime *runtime)
{
	switch (runtime->status->state) {
	case SNDRV_PCM_STATE_PREPARED:
	case SNDRV_PCM_STATE_RUNNING:
	case SNDRV_PCM_STATE_PAUSED:
		return 0;
	case SNDRV_PCM_STATE_XRUN:
		return -EPIPE;
	case SNDRV_PCM_STATE_SUSPENDED:
		return -ESTRPIPE;
	default:
		return -EBADFD;
	}
}

/* update to the given appl_ptr and call ack callback if needed;
 * when an error is returned, take back to the original value
 */
int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
			   snd_pcm_uframes_t appl_ptr)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_uframes_t old_appl_ptr = runtime->control->appl_ptr;
	int ret;

	if (old_appl_ptr == appl_ptr)
		return 0;

	runtime->control->appl_ptr = appl_ptr;
	if (substream->ops->ack) {
		ret = substream->ops->ack(substream);
		if (ret < 0) {
			runtime->control->appl_ptr = old_appl_ptr;
			return ret;
		}
	}

	trace_applptr(substream, old_appl_ptr, appl_ptr);

	return 0;
}

/* the common loop for read/write data */
snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
				     void *data, bool interleaved,
				     snd_pcm_uframes_t size, bool in_kernel)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_uframes_t xfer = 0;
	snd_pcm_uframes_t offset = 0;
	snd_pcm_uframes_t avail;
	pcm_copy_f writer;
	pcm_transfer_f transfer;
	bool nonblock;
	bool is_playback;
	int err;

	err = pcm_sanity_check(substream);
	if (err < 0)
		return err;

	is_playback = substream->stream == SNDRV_PCM_STREAM_PLAYBACK;
	if (interleaved) {
		if (runtime->access != SNDRV_PCM_ACCESS_RW_INTERLEAVED &&
		    runtime->channels > 1)
			return -EINVAL;
		writer = interleaved_copy;
	} else {
		if (runtime->access != SNDRV_PCM_ACCESS_RW_NONINTERLEAVED)
			return -EINVAL;
		writer = noninterleaved_copy;
	}

	if (!data) {
		if (is_playback)
			transfer = fill_silence;
		else
			return -EINVAL;
	} else if (in_kernel) {
		if (substream->ops->copy_kernel)
			transfer = substream->ops->copy_kernel;
		else
			transfer = is_playback ?
				default_write_copy_kernel : default_read_copy_kernel;
	} else {
		if (substream->ops->copy_user)
			transfer = (pcm_transfer_f)substream->ops->copy_user;
		else
			transfer = is_playback ?
				default_write_copy : default_read_copy;
	}

	if (size == 0)
		return 0;

	nonblock = !!(substream->f_flags & O_NONBLOCK);

	snd_pcm_stream_lock_irq(substream);
	err = pcm_accessible_state(runtime);
	if (err < 0)
		goto _end_unlock;

	runtime->twake = runtime->control->avail_min ? : 1;
	if (runtime->status->state == SNDRV_PCM_STATE_RUNNING)
		snd_pcm_update_hw_ptr(substream);

	/*
	 * If size < start_threshold, wait indefinitely. Another
	 * thread may start capture
	 */
	if (!is_playback &&
	    runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
	    size >= runtime->start_threshold) {
		err = snd_pcm_start(substream);
		if (err < 0)
			goto _end_unlock;
	}

	avail = snd_pcm_avail(substream);
	while (size > 0) {
		snd_pcm_uframes_t frames, appl_ptr, appl_ofs;
		snd_pcm_uframes_t cont;
		if (!avail) {
			if (!is_playback &&
			    runtime->status->state == SNDRV_PCM_STATE_DRAINING) {
				snd_pcm_stop(substream, SNDRV_PCM_STATE_SETUP);
				goto _end_unlock;
			}
			if (nonblock) {
				err = -EAGAIN;
				goto _end_unlock;
			}
			runtime->twake = min_t(snd_pcm_uframes_t, size,
					runtime->control->avail_min ? : 1);
			err = wait_for_avail(substream, &avail);
			if (err < 0)
				goto _end_unlock;
			if (!avail)
				continue; /* draining */
		}
		frames = size > avail ? avail : size;
		appl_ptr = READ_ONCE(runtime->control->appl_ptr);
		appl_ofs = appl_ptr % runtime->buffer_size;
		cont = runtime->buffer_size - appl_ofs;
		if (frames > cont)
			frames = cont;
		if (snd_BUG_ON(!frames)) {
			err = -EINVAL;
			goto _end_unlock;
		}
		if (!atomic_inc_unless_negative(&runtime->buffer_accessing)) {
			err = -EBUSY;
			goto _end_unlock;
		}
		snd_pcm_stream_unlock_irq(substream);
		err = writer(substream, appl_ofs, data, offset, frames,
			     transfer);
		snd_pcm_stream_lock_irq(substream);
		atomic_dec(&runtime->buffer_accessing);
		if (err < 0)
			goto _end_unlock;
		err = pcm_accessible_state(runtime);
		if (err < 0)
			goto _end_unlock;
		appl_ptr += frames;
		if (appl_ptr >= runtime->boundary)
			appl_ptr -= runtime->boundary;
		err = pcm_lib_apply_appl_ptr(substream, appl_ptr);
		if (err < 0)
			goto _end_unlock;

		offset += frames;
		size -= frames;
		xfer += frames;
		avail -= frames;
		if (is_playback &&
		    runtime->status->state == SNDRV_PCM_STATE_PREPARED &&
		    snd_pcm_playback_hw_avail(runtime) >= (snd_pcm_sframes_t)runtime->start_threshold) {
			err = snd_pcm_start(substream);
			if (err < 0)
				goto _end_unlock;
		}
	}
 _end_unlock:
	runtime->twake = 0;
	if (xfer > 0 && err >= 0)
		snd_pcm_update_state(substream, runtime);
	snd_pcm_stream_unlock_irq(substream);
	return xfer > 0 ? (snd_pcm_sframes_t)xfer : err;
}
EXPORT_SYMBOL(__snd_pcm_lib_xfer);

__snd_pcm_lib_xfer()The execution process of the function is as follows:

1. Call pcm_sanity_check()the function to perform a plausibility check, and pcm_sanity_check()the function checks the DMA buffer and status. The audio device driver needs to ensure that __snd_pcm_lib_xfer()the DMA buffer is ready when the function is executed.

2. According to the parameters passed in, that is, whether the data format is interleaved or noninterleaved , playing or recording, whether the data buffer is empty, whether the operation is initiated by kernel space code or user space code, etc., select writerand transferoperations. writerThe and transferoperations are mainly used to transfer data between the incoming data buffer and DMA buffer.

3. Check runtimethe status of , and when the status is not SNDRV_PCM_STATE_PREPARED, SNDRV_PCM_STATE_RUNNINGand SNDRV_PCM_STATE_PAUSED, report an error and return.

4. When runtimethe status is SNDRV_PCM_STATE_RUNNING, call snd_pcm_update_hw_ptr(substream)the function to update hw_ptr.

5. For audio recording, if runtimethe status is SNDRV_PCM_STATE_PREPAREDand the number of frames requested to be recorded exceeds the initiation threshold, the snd_pcm_start(substream)function is called to trigger recording.

6. Call snd_pcm_avail(substream)the function to obtain the available space in the DMA buffer in frames.
   Access to the DMA buffer by user space applications and Linux kernel device drivers is a typical reader-writer model. For playback, the user space application writes data to the DMA buffer, and the Linux kernel device driver reads the data from the DMA buffer and sends it to the hardware device. For recording, the Linux kernel device driver obtains data from the hardware device and writes it to the DMA buffer, and the user space application reads the data from the DMA buffer and performs further processing.
   The available space here is from the perspective of the user space application. That is, for playback, the available space refers to the amount of data that can be written to the DMA buffer. For recording, it refers to the amount of data that can be written from the DMA buffer. The amount of data read.

7. All data transfer requests are processed through a loop.
   (1). When the available space of DMA buffer is 0, wait until the available space is greater than 0.
   (2). Calculate the amount of copied data, the location of the copied destination memory in the DMA buffer, etc.
   (3). Calling writerand transferoperations transfer data between the DMA buffer and the incoming buffer, and are passed to the operation appl_ofsas hwoffparameters writer.
   (4). Check runtimethe status of .
   (5). Update and call pcm_lib_apply_appl_ptr()the function to apply the new appl_ptr.
   (6). Based on the number of frames transferred, update the offset, the number of frames to be transferred, the number of frames that have been transferred and the space available in the DMA buffer.
   (7). For audio playback, if runtimethe status is SNDRV_PCM_STATE_PREPAREDand the amount of data that can be played in the DMA buffer exceeds the initiation threshold, the snd_pcm_start(substream)function is called to trigger playback.

writerUnder different circumstances, the and transferoperation definitions (located in sound/core/pcm_lib.c ) for transferring data between the incoming data buffer and DMA buffer are as follows:

/* calculate the target DMA-buffer position to be written/read */
static void *get_dma_ptr(struct snd_pcm_runtime *runtime,
			   int channel, unsigned long hwoff)
{
	return runtime->dma_area + hwoff +
		channel * (runtime->dma_bytes / runtime->channels);
}

/* default copy_user ops for write; used for both interleaved and non- modes */
static int default_write_copy(struct snd_pcm_substream *substream,
			      int channel, unsigned long hwoff,
			      void *buf, unsigned long bytes)
{
	if (copy_from_user(get_dma_ptr(substream->runtime, channel, hwoff),
			   (void __user *)buf, bytes))
		return -EFAULT;
	return 0;
}

/* default copy_kernel ops for write */
static int default_write_copy_kernel(struct snd_pcm_substream *substream,
				     int channel, unsigned long hwoff,
				     void *buf, unsigned long bytes)
{
	memcpy(get_dma_ptr(substream->runtime, channel, hwoff), buf, bytes);
	return 0;
}

/* fill silence instead of copy data; called as a transfer helper
 * from __snd_pcm_lib_write() or directly from noninterleaved_copy() when
 * a NULL buffer is passed
 */
static int fill_silence(struct snd_pcm_substream *substream, int channel,
			unsigned long hwoff, void *buf, unsigned long bytes)
{
	struct snd_pcm_runtime *runtime = substream->runtime;

	if (substream->stream != SNDRV_PCM_STREAM_PLAYBACK)
		return 0;
	if (substream->ops->fill_silence)
		return substream->ops->fill_silence(substream, channel,
						    hwoff, bytes);

	snd_pcm_format_set_silence(runtime->format,
				   get_dma_ptr(runtime, channel, hwoff),
				   bytes_to_samples(runtime, bytes));
	return 0;
}

/* default copy_user ops for read; used for both interleaved and non- modes */
static int default_read_copy(struct snd_pcm_substream *substream,
			     int channel, unsigned long hwoff,
			     void *buf, unsigned long bytes)
{
	if (copy_to_user((void __user *)buf,
			 get_dma_ptr(substream->runtime, channel, hwoff),
			 bytes))
		return -EFAULT;
	return 0;
}

/* default copy_kernel ops for read */
static int default_read_copy_kernel(struct snd_pcm_substream *substream,
				    int channel, unsigned long hwoff,
				    void *buf, unsigned long bytes)
{
	memcpy(buf, get_dma_ptr(substream->runtime, channel, hwoff), bytes);
	return 0;
}

/* call transfer function with the converted pointers and sizes;
 * for interleaved mode, it's one shot for all samples
 */
static int interleaved_copy(struct snd_pcm_substream *substream,
			    snd_pcm_uframes_t hwoff, void *data,
			    snd_pcm_uframes_t off,
			    snd_pcm_uframes_t frames,
			    pcm_transfer_f transfer)
{
	struct snd_pcm_runtime *runtime = substream->runtime;

	/* convert to bytes */
	hwoff = frames_to_bytes(runtime, hwoff);
	off = frames_to_bytes(runtime, off);
	frames = frames_to_bytes(runtime, frames);
	return transfer(substream, 0, hwoff, data + off, frames);
}

/* call transfer function with the converted pointers and sizes for each
 * non-interleaved channel; when buffer is NULL, silencing instead of copying
 */
static int noninterleaved_copy(struct snd_pcm_substream *substream,
			       snd_pcm_uframes_t hwoff, void *data,
			       snd_pcm_uframes_t off,
			       snd_pcm_uframes_t frames,
			       pcm_transfer_f transfer)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	int channels = runtime->channels;
	void **bufs = data;
	int c, err;

	/* convert to bytes; note that it's not frames_to_bytes() here.
	 * in non-interleaved mode, we copy for each channel, thus
	 * each copy is n_samples bytes x channels = whole frames.
	 */
	off = samples_to_bytes(runtime, off);
	frames = samples_to_bytes(runtime, frames);
	hwoff = samples_to_bytes(runtime, hwoff);
	for (c = 0; c < channels; ++c, ++bufs) {
		if (!data || !*bufs)
			err = fill_silence(substream, c, hwoff, NULL, frames);
		else
			err = transfer(substream, c, hwoff, *bufs + off,
				       frames);
		if (err < 0)
			return err;
	}
	return 0;
}

transferOperation is used to perform a data copy. Depending on whether the source of initiating data transfer is the kernel or user space application, and whether it is playback or recording, transferthere are four default implementations of the operation, namely default_write_copy_kernel(), default_read_copy_kernel(), default_write_copy(), and default_read_copy(). These functions obtain the DMA pointer according to channeland hwoffthrough get_dma_ptr()the function, and pass the DMA pointer provided by the kernel. copy_from_user(), memcpy()and copy_to_user()functions transfer data between the incoming data buffer and DMA buffer.

From get_dma_ptr()the function, we can see that when the data format is noninterleaved, the data layout of each channel in the DMA buffer is.

writerThe operation is used to prepare for data copying. It converts data copy parameters such as offset and size in frame units into bytes, and calls the operation to transferperform data copying. writerThere are two default implementations of operations, namely interleaved_copy()and noninterleaved_copy(). When the data format is noninterleaved, writerthe operation copies the data of each channel separately. The incoming audio data is saved in multiple different buffers, one for each channel. The audio data is passed through a pointer array.

writerThe offset pointing transferto the DMA buffer used in the and operations is calculated by. In the reader-writer model of DMA buffer access, for playback, it is the write pointer, for recording, it is the read pointer.hwoffruntime->control->appl_ptrruntime->control->appl_ptr

snd_pcm_avail()The function is used to obtain the available space in the DMA buffer from the perspective of the user space application in frames. The Linux kernel also provides another function to obtain the available space in frames from the perspective of the audio hardware snd_pcm_hw_avail()device driver. Size, these two functions are defined (located in sound/core/pcm_local.h ) as follows:

static inline snd_pcm_uframes_t
snd_pcm_avail(struct snd_pcm_substream *substream)
{
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		return snd_pcm_playback_avail(substream->runtime);
	else
		return snd_pcm_capture_avail(substream->runtime);
}

static inline snd_pcm_uframes_t
snd_pcm_hw_avail(struct snd_pcm_substream *substream)
{
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
		return snd_pcm_playback_hw_avail(substream->runtime);
	else
		return snd_pcm_capture_hw_avail(substream->runtime);
}

These two functions dispatch operations to four other functions snd_pcm_playback_avail(), , snd_pcm_capture_avail(), snd_pcm_playback_hw_avail()and , depending on whether the type of stream is playback or recording snd_pcm_capture_hw_avail(). snd_pcm_playback_avail()The function definitions (located in include/sound/pcm.h ) are as follows:

/**
 * snd_pcm_playback_avail - Get the available (writable) space for playback
 * @runtime: PCM runtime instance
 *
 * Result is between 0 ... (boundary - 1)
 */
static inline snd_pcm_uframes_t snd_pcm_playback_avail(struct snd_pcm_runtime *runtime)
{
	snd_pcm_sframes_t avail = runtime->status->hw_ptr + runtime->buffer_size - runtime->control->appl_ptr;
	if (avail < 0)
		avail += runtime->boundary;
	else if ((snd_pcm_uframes_t) avail >= runtime->boundary)
		avail -= runtime->boundary;
	return avail;
}

/**
 * snd_pcm_capture_avail - Get the available (readable) space for capture
 * @runtime: PCM runtime instance
 *
 * Result is between 0 ... (boundary - 1)
 */
static inline snd_pcm_uframes_t snd_pcm_capture_avail(struct snd_pcm_runtime *runtime)
{
	snd_pcm_sframes_t avail = runtime->status->hw_ptr - runtime->control->appl_ptr;
	if (avail < 0)
		avail += runtime->boundary;
	return avail;
}

/**
 * snd_pcm_playback_hw_avail - Get the queued space for playback
 * @runtime: PCM runtime instance
 */
static inline snd_pcm_sframes_t snd_pcm_playback_hw_avail(struct snd_pcm_runtime *runtime)
{
	return runtime->buffer_size - snd_pcm_playback_avail(runtime);
}

/**
 * snd_pcm_capture_hw_avail - Get the free space for capture
 * @runtime: PCM runtime instance
 */
static inline snd_pcm_sframes_t snd_pcm_capture_hw_avail(struct snd_pcm_runtime *runtime)
{
	return runtime->buffer_size - snd_pcm_capture_avail(runtime);
}

Regarding the reader-writer model of DMA buffer access, for playback, runtime->status->hw_ptrit is the read pointer, ( runtime->control->appl_ptr- runtime->status->hw_ptr) is the amount of data that has been written but not yet played, and for recording, runtime->status->hw_ptrit is the write pointer.

__snd_pcm_lib_xfer()Function call snd_pcm_start()The function triggers data transfer to start execution. This function definition (located in sound/core/pcm_native.c ) is as follows:

/*
 * start callbacks
 */
static int snd_pcm_pre_start(struct snd_pcm_substream *substream,
			     snd_pcm_state_t state)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	if (runtime->status->state != SNDRV_PCM_STATE_PREPARED)
		return -EBADFD;
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
	    !snd_pcm_playback_data(substream))
		return -EPIPE;
	runtime->trigger_tstamp_latched = false;
	runtime->trigger_master = substream;
	return 0;
}

static int snd_pcm_do_start(struct snd_pcm_substream *substream,
			    snd_pcm_state_t state)
{
	if (substream->runtime->trigger_master != substream)
		return 0;
	return substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_START);
}

static void snd_pcm_undo_start(struct snd_pcm_substream *substream,
			       snd_pcm_state_t state)
{
	if (substream->runtime->trigger_master == substream)
		substream->ops->trigger(substream, SNDRV_PCM_TRIGGER_STOP);
}

static void snd_pcm_post_start(struct snd_pcm_substream *substream,
			       snd_pcm_state_t state)
{
	struct snd_pcm_runtime *runtime = substream->runtime;
	snd_pcm_trigger_tstamp(substream);
	runtime->hw_ptr_jiffies = jiffies;
	runtime->hw_ptr_buffer_jiffies = (runtime->buffer_size * HZ) / 
							    runtime->rate;
	runtime->status->state = state;
	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
	    runtime->silence_size > 0)
		snd_pcm_playback_silence(substream, ULONG_MAX);
	snd_pcm_timer_notify(substream, SNDRV_TIMER_EVENT_MSTART);
}

static const struct action_ops snd_pcm_action_start = {
	.pre_action = snd_pcm_pre_start,
	.do_action = snd_pcm_do_start,
	.undo_action = snd_pcm_undo_start,
	.post_action = snd_pcm_post_start
};

/**
 * snd_pcm_start - start all linked streams
 * @substream: the PCM substream instance
 *
 * Return: Zero if successful, or a negative error code.
 * The stream lock must be acquired before calling this function.
 */
int snd_pcm_start(struct snd_pcm_substream *substream)
{
	return snd_pcm_action(&snd_pcm_action_start, substream,
			      SNDRV_PCM_STATE_RUNNING);
}

Here, the operation of the audio hardware device driver triggeris called to trigger the initiation of hardware data transmission.