From fc1ad3891e4489881d285826dbb3ad996d2d727d Mon Sep 17 00:00:00 2001 From: wm4 Date: Wed, 13 Jan 2016 19:44:47 +0100 Subject: [PATCH 140/782] 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. --- .../vc04_services/bcm2835-audio/bcm2835-pcm.c | 12 +++++++++++- .../staging/vc04_services/bcm2835-audio/bcm2835.h | 1 + 2 files changed, 12 insertions(+), 1 deletion(-) --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c @@ -11,7 +11,7 @@ /* 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, @@ -81,6 +81,8 @@ void bcm2835_playback_fifo(struct bcm283 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); @@ -306,6 +308,7 @@ 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, @@ -397,12 +400,19 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s { 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); --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h @@ -133,6 +133,7 @@ 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;