When the BCM2835 audio output is used, userspace sees a jitter up to 10ms
in the audio position, aka "delay" -- the number of frames that must
be output before a new frame would be played.
Make this a bit nicer for userspace by interpolating the position
using the CPU clock.
The overhead is small -- an extra ktime_get() every time a GPU message
is sent -- and another call and a few calculations whenever the delay
is sought from userland.
At 48,000 frames per second, i.e. approximately 20 microseconds per
frame, it would take a clock inaccuracy of
20 microseconds in 10 milliseconds -- 2,000 parts per million --
to result in an inaccurate estimate, whereas
crystal- or resonator-based clocks typically have an
inaccuracy of 10s to 100s of parts per million.
Signed-off-by: Mike Brady <mikebrady@xxxxxxxxxx>
---
.../vc04_services/bcm2835-audio/bcm2835-pcm.c | 23 ++++++++++++++++++-
.../vc04_services/bcm2835-audio/bcm2835.h | 1 +
2 files changed, 23 insertions(+), 1 deletion(-)
diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
index e66da11af5cf..250adf82385a 100644
--- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
+++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
@@ -12,7 +12,8 @@
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_DRAIN_TRIGGER | SNDRV_PCM_INFO_SYNC_APPLPTR),
+ SNDRV_PCM_INFO_BATCH | SNDRV_PCM_INFO_DRAIN_TRIGGER |
+ SNDRV_PCM_INFO_SYNC_APPLPTR),
.formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,
.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,
.rate_min = 8000,
@@ -74,6 +75,7 @@ void bcm2835_playback_fifo(struct bcm2835_alsa_stream *alsa_stream,
atomic_set(&alsa_stream->pos, pos);
alsa_stream->period_offset += bytes;
+ alsa_stream->interpolate_start = ktime_get();
if (alsa_stream->period_offset >= alsa_stream->period_size) {
alsa_stream->period_offset %= alsa_stream->period_size;
snd_pcm_period_elapsed(substream);
@@ -243,6 +245,7 @@ static int snd_bcm2835_pcm_prepare(struct snd_pcm_substream *substream)
atomic_set(&alsa_stream->pos, 0);
alsa_stream->period_offset = 0;
alsa_stream->draining = false;
+ alsa_stream->interpolate_start = ktime_get();
return 0;
}
@@ -292,6 +295,24 @@ 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;
+ ktime_t now = ktime_get();
+
+ /* Give userspace better delay reporting by interpolating between GPU
+ * notifications, assuming audio speed is close enough to the clock
+ * used for ktime
+ */
+
+ if ((ktime_to_ns(alsa_stream->interpolate_start)) &&
+ (ktime_compare(alsa_stream->interpolate_start, now) < 0)) {
+ u64 interval =
+ (ktime_to_ns(ktime_sub(now,
+ alsa_stream->interpolate_start)));
+ u64 frames_output_in_interval =
+ div_u64((interval * runtime->rate), 1000000000);
+ snd_pcm_sframes_t frames_output_in_interval_sized =
+ -frames_output_in_interval;
+ runtime->delay = frames_output_in_interval_sized;
+ }
return snd_pcm_indirect_playback_pointer(substream,
&alsa_stream->pcm_indirect,
diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
index e13435d1c205..595ad584243f 100644
--- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
+++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835.h
@@ -78,6 +78,7 @@ struct bcm2835_alsa_stream {
unsigned int period_offset;
unsigned int buffer_size;
unsigned int period_size;
+ ktime_t interpolate_start;
struct bcm2835_audio_instance *instance;
int idx;
--
2.17.1
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