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Subject: Re: [alsa-devel] [PATCH v2] staging: bcm2835-audio: interpolate audio
 delay
From:   Mike Brady <mikebrady@eircom.net>
In-Reply-To: <a703f8a2-e85a-bc5b-750a-cc43ef252ba5@gmail.com>
Date:   Wed, 24 Oct 2018 09:20:09 +0100
Cc:     stefan.wahren@i2se.com, devel@driverdev.osuosl.org,
        alsa-devel@alsa-project.org, f.fainelli@gmail.com,
        julia.lawall@lip6.fr, tiwai@suse.de, gregkh@linuxfoundation.org,
        linux-kernel@vger.kernel.org, eric@anholt.net,
        linux-rpi-kernel@lists.infradead.org,
        nishka.dasgupta_ug18@ashoka.edu.in,
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References: <20181022191708.GA4659@ubuntu>
 <a703f8a2-e85a-bc5b-750a-cc43ef252ba5@gmail.com>
To:     Kirill Marinushkin <k.marinushkin@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
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Hi Kirill. Thanks for your comments.

> On 22 Oct 2018, at 23:25, Kirill Marinushkin <k.marinushkin@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”).

> 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, it must be wrapped, but AFAIK we are unable to increment a buffer index used in the snd_pcm_delay calculation. Hence the calculation of the actual position would be wrong. This is why the snd_pcm_runtime delay field is used. On reflection, BTW, the patch assumes that the field's original value was zero — that can be rectified.

> 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).

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@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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