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91 lines
3.5 KiB
Diff
91 lines
3.5 KiB
Diff
From 641d7675c007bdf97f10b320fa38d1f089d43d5d Mon Sep 17 00:00:00 2001
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From: wm4 <wm4@nowhere>
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Date: Wed, 13 Jan 2016 19:44:47 +0100
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Subject: [PATCH 133/304] bcm2835: interpolate audio delay
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It appears the GPU only sends us a message all 10ms to update
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the playback progress. Other than this, the playback position
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(what SNDRV_PCM_IOCTL_DELAY will return) is not updated at all.
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Userspace will see jitter up to 10ms in the audio position.
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Make this a bit nicer for userspace by interpolating the
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position using the CPU clock.
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I'm not sure if setting snd_pcm_runtime.delay is the right
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approach for this. Or if there is maybe an already existing
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mechanism for position interpolation in the ALSA core.
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I only set SNDRV_PCM_INFO_BATCH because this appears to remove
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at least one situation snd_pcm_runtime.delay is used, so I have
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to worry less in which place I have to update this field, or
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how it interacts with the rest of ALSA.
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In the future, it might be nice to use VC_AUDIO_MSG_TYPE_LATENCY.
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One problem is that it requires sending a videocore message, and
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waiting for a reply, which could make the implementation much
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harder due to locking and synchronization requirements.
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---
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sound/arm/bcm2835-pcm.c | 12 +++++++++++-
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sound/arm/bcm2835.h | 1 +
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2 files changed, 12 insertions(+), 1 deletion(-)
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--- a/sound/arm/bcm2835-pcm.c
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+++ b/sound/arm/bcm2835-pcm.c
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@@ -25,7 +25,7 @@
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/* hardware definition */
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static struct snd_pcm_hardware snd_bcm2835_playback_hw = {
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.info = (SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_BLOCK_TRANSFER |
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- SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID),
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+ SNDRV_PCM_INFO_MMAP | SNDRV_PCM_INFO_MMAP_VALID | SNDRV_PCM_INFO_BATCH),
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.formats = SNDRV_PCM_FMTBIT_U8 | SNDRV_PCM_FMTBIT_S16_LE,
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.rates = SNDRV_PCM_RATE_CONTINUOUS | SNDRV_PCM_RATE_8000_48000,
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.rate_min = 8000,
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@@ -99,6 +99,8 @@ static irqreturn_t bcm2835_playback_fifo
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alsa_stream->pos %= alsa_stream->buffer_size;
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}
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+ alsa_stream->interpolate_start = ktime_get_ns();
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+
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if (alsa_stream->substream) {
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if (new_period)
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snd_pcm_period_elapsed(alsa_stream->substream);
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@@ -399,6 +401,7 @@ static int snd_bcm2835_pcm_prepare(struc
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alsa_stream->buffer_size = snd_pcm_lib_buffer_bytes(substream);
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alsa_stream->period_size = snd_pcm_lib_period_bytes(substream);
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alsa_stream->pos = 0;
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+ alsa_stream->interpolate_start = ktime_get_ns();
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audio_debug("buffer_size=%d, period_size=%d pos=%d frame_bits=%d\n",
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alsa_stream->buffer_size, alsa_stream->period_size,
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@@ -495,6 +498,7 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
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{
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struct snd_pcm_runtime *runtime = substream->runtime;
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bcm2835_alsa_stream_t *alsa_stream = runtime->private_data;
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+ u64 now = ktime_get_ns();
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audio_info(" .. IN\n");
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@@ -503,6 +507,12 @@ snd_bcm2835_pcm_pointer(struct snd_pcm_s
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frames_to_bytes(runtime, runtime->control->appl_ptr),
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alsa_stream->pos);
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+ /* Give userspace better delay reporting by interpolating between GPU
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+ * notifications, assuming audio speed is close enough to the clock
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+ * used for ktime */
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+ if (alsa_stream->interpolate_start && alsa_stream->interpolate_start < now)
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+ runtime->delay = -(int)div_u64((now - alsa_stream->interpolate_start) * runtime->rate, 1000000000);
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+
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audio_info(" .. OUT\n");
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return snd_pcm_indirect_playback_pointer(substream,
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&alsa_stream->pcm_indirect,
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--- a/sound/arm/bcm2835.h
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+++ b/sound/arm/bcm2835.h
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@@ -137,6 +137,7 @@ typedef struct bcm2835_alsa_stream {
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unsigned int pos;
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unsigned int buffer_size;
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unsigned int period_size;
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+ u64 interpolate_start;
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uint32_t enable_fifo_irq;
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irq_handler_t fifo_irq_handler;
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