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-rw-r--r--target/linux/brcm2708/patches-4.19/950-0135-bcm2835-interpolate-audio-delay.patch89
1 files changed, 89 insertions, 0 deletions
diff --git a/target/linux/brcm2708/patches-4.19/950-0135-bcm2835-interpolate-audio-delay.patch b/target/linux/brcm2708/patches-4.19/950-0135-bcm2835-interpolate-audio-delay.patch
new file mode 100644
index 0000000000..55cd26b25b
--- /dev/null
+++ b/target/linux/brcm2708/patches-4.19/950-0135-bcm2835-interpolate-audio-delay.patch
@@ -0,0 +1,89 @@
+From 652b0ec2ab2d2e2af7bb4f7d62a4020a20720677 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.
+---
+ .../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;