[alsa-devel] [PATCH v2] staging: bcm2835-audio: interpolate audio delay

Wed Oct 24 20:06:18 CEST 2018

Hello Mike,

On 10/24/18 10:20, Mike Brady wrote:
> Hi Kirill. Thanks for your comments.
> 
>> On 22 Oct 2018, at 23:25, Kirill Marinushkin <k.marinushkin at gmail.com> wrote:
>>
>> AFAIU, this patch is wrong. Please correct me, maybe I misunderstand something.
>>
>>> The problem that this patch seeks to resolve is that when userland asks for
>>> the delay
>>
>> The userspace asks not for delay, but for the pointer.
> 
> The call in question is snd_pcm_delay. I presume this delay is calculated from knowledge of the stream position “pos", the period (buffer?) number (and period/buffer size) and the snd_pcm_runtime structure’s “delay" field (“runtime->delay”).
> 

In kernel, this delay is calculated in `snd_pcm_calc_delay()`, defined at
`sound/core/pcm_native.c:889`. If you analyze how this calculation is done, you
will see that the returned value is a summary of the following fields:

* runtime->status->hw_ptr
* runtime->buffer_size
* runtime->control->appl_ptr
* runtime->boundary
* runtime->delay

>> You modify the function, which is called `snd_bcm2835_pcm_pointer`. Here you are
>> supposed to increase `alsa_stream->pos` with the proper offset. Instead, you
>> imitate a delay, but in fact the delay is not increased.
>>
>> So, the proper solution should be to fix the reported pointer.
> 
> I think there is a difficulty with this. The “pos” pointer looks to have to be modulo the buffer size. This causes a problem, as I see it, in that if the calculated (pos + interpolated delay in bytes) is longer than the buffer size

There is no "interpolated delay". The concept of "interpolated delay" is
incorrect. When you play sound - the pointer increments. But in this commit you
increment the delay, as if sound doesn't play.

>> As a result,
>> userspace will recieve the correct delay, instead of these crazy 10 ms.
> 
> Just to point out that with the proposed patch, it appears that the correct delay is being reported, (apart, possibly, from any delay originally set in the snd_pcm_delay field, as mentioned above).

Then I would like to point out the alsa-lib function `snd_pcm_avail()` - it will
return the wrong value.

> 
> All the best,
> Mike
> 
>> FYI, there is
>>> a discussion of the effects of a downstream equivalent of this suggested patch
>>> at:
>>> https://github.com/raspberrypi/firmware/issues/1026#issuecomment-415746016.
>>
>> Thank you for the link, it clarified for me what you try to achieve.
>>
>> On 10/22/18 21:17, Mike Brady wrote:
>>> 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 at eircom.net>
>>> ---
>>> Changes in v2 -- remove inappropriate addition of SNDRV_PCM_INFO_BATCH flag
>>>
>>> .../vc04_services/bcm2835-audio/bcm2835-pcm.c | 20 +++++++++++++++++++
>>> .../vc04_services/bcm2835-audio/bcm2835.h     |  1 +
>>> 2 files changed, 21 insertions(+)
>>>
>>> diff --git a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
>>> index e66da11af5cf..9053b996cada 100644
>>> --- a/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
>>> +++ b/drivers/staging/vc04_services/bcm2835-audio/bcm2835-pcm.c
>>> @@ -74,6 +74,7 @@ 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 +244,7 @@ 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 +294,24 @@ 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;
>>>
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> 