Commit f6da082a authored by wm4's avatar wm4 Committed by popcornmix
Browse files

bcm2835: interpolate audio delay 

It appears the GPU only sends us a message all 10ms to update
the playback progress. Other than this, the playback position
(what SNDRV_PCM_IOCTL_DELAY will return) is not updated at all.
Userspace will see jitter up to 10ms in the audio position.

Make this a bit nicer for userspace by interpolating the
position using the CPU clock.

I'm not sure if setting snd_pcm_runtime.delay is the right
approach for this. Or if there is maybe an already existing
mechanism for position interpolation in the ALSA core.

I only set SNDRV_PCM_INFO_BATCH because this appears to remove
at least one situation snd_pcm_runtime.delay is used, so I have
to worry less in which place I have to update this field, or
how it interacts with the rest of ALSA.

In the future, it might be nice to use VC_AUDIO_MSG_TYPE_LATENCY.
One problem is that it requires sending a videocore message, and
waiting for a reply, which could make the implementation much
harder due to locking and synchronization requirements.
parent c90694c0

drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c

+11 −2
Original line number Diff line number Diff line
@@ -22,7 +22,7 @@ 
/* hardware definition */

static const struct snd_pcm_hardware snd_bcm2835_playback_hw = {

	.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |

	SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID),

	SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID | SNDRV_PCM_INFO_BATCH),

	.formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,

	.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,

	.rate_min = 8000,
@@ -93,6 +93,8 @@ void bcm2835_playback_fifo(struct bcm2835_alsa_stream *alsa_stream) 
		alsa_stream->pos %= alsa_stream->buffer_size;

	}



	alsa_stream->interpolate_start = ktime_get_ns();



	if (alsa_stream->substream) {

		if (new_period)

			snd_pcm_period_elapsed(alsa_stream->substream);
@@ -323,6 +325,7 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream) 
	alsa_stream->buffer_size = snd_pcm_lib_buffer_bytes(substream);

	alsa_stream->period_size = snd_pcm_lib_period_bytes(substream);

	alsa_stream->pos = 0;

	alsa_stream->interpolate_start = ktime_get_ns();



	audio_debug("buffer_size=%d, period_size=%d pos=%d frame_bits=%d\n",

		alsa_stream->buffer_size, alsa_stream->period_size,
@@ -415,13 +418,19 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_substream *substream) 
{

	struct snd_pcm_runtime *runtime = substream->runtime;

	struct bcm2835_alsa_stream *alsa_stream = runtime->private_data;



	u64 now = ktime_get_ns();



	audio_debug("pcm_pointer... (%d) hwptr=%d appl=%d pos=%d\n", 0,

		frames_to_bytes(runtime, runtime->status->hw_ptr),

		frames_to_bytes(runtime, runtime->control->appl_ptr),

		alsa_stream->pos);



	/* Give userspace better delay reporting by interpolating between GPU

	 * notifications, assuming audio speed is close enough to the clock

	 * used for ktime */

	if (alsa_stream->interpolate_start && alsa_stream->interpolate_start < now)

		runtime->delay = -(int)div_u64((now - alsa_stream->interpolate_start) * runtime->rate,  1000000000);



	return snd_pcm_indirect_playback_pointer(substream,

		&alsa_stream->pcm_indirect,

		alsa_stream->pos);

drivers/staging/vc04_services/bcm2835-audio/bcm2835.h

+1 −0
Original line number Diff line number Diff line
@@ -137,6 +137,7 @@ struct bcm2835_alsa_stream { 
	unsigned int pos;

	unsigned int buffer_size;

	unsigned int period_size;

	u64 interpolate_start;



	atomic_t retrieved;

	struct bcm2835_audio_instance *instance;