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- From 965303a0745da159e5eaa72694421ce95ccf8421 Mon Sep 17 00:00:00 2001
- From: wm4 <wm4@nowhere>
- Date: Wed, 13 Jan 2016 19:44:47 +0100
- Subject: [PATCH] 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.
- ---
- sound/arm/bcm2835-pcm.c | 12 +++++++++++-
- sound/arm/bcm2835.h | 1 +
- 2 files changed, 12 insertions(+), 1 deletion(-)
- --- a/sound/arm/bcm2835-pcm.c
- +++ b/sound/arm/bcm2835-pcm.c
- @@ -25,7 +25,7 @@
- /* hardware definition */
- static 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,
- @@ -99,6 +99,8 @@ static irqreturn_t bcm2835_playback_fifo
- 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);
- @@ -399,6 +401,7 @@ static int snd_bcm2835_pcm_prepare(struc
- 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,
- @@ -495,6 +498,7 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
- {
- struct snd_pcm_runtime *runtime = substream->runtime;
- bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
- + u64 now = ktime_get_ns();
-
- audio_info(" .. IN\n");
-
- @@ -503,6 +507,12 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
- 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);
- +
- audio_info(" .. OUT\n");
- return snd_pcm_indirect_playback_pointer(substream,
- &alsa_stream->pcm_indirect,
- --- a/sound/arm/bcm2835.h
- +++ b/sound/arm/bcm2835.h
- @@ -137,6 +137,7 @@ typedef struct bcm2835_alsa_stream {
- unsigned int pos;
- unsigned int buffer_size;
- unsigned int period_size;
- + u64 interpolate_start;
-
- uint32_t enable_fifo_irq;
- irq_handler_t fifo_irq_handler;
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