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From a94c77d797c554678a37fb2b921fbab46f7f205e 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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