From 7756cf7615123045c60b9f9e311d2491a5e23424 Mon Sep 17 00:00:00 2001 From: wm4 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;