[alsa-devel] [RFC PATCH] ALSA: core: do not update tstamp when hw ptr has not moved

[Pierre-Louis Bossart]

On 11/16/17 8:48 AM, Takashi Iwai wrote:
> On Thu, 16 Nov 2017 13:43:57 +0100,
> Henrik Eriksson wrote:
>>
>> Hello
>>
>> This reverts a change that was part of a larger commit.  That change
>> broke timestamps in some application code for us as outlined in the log,
>> and reverting it obviously fixes this.  Since the original change did
>> not explain why the behaviour was modified I can't speak for the effects
>> on that use case.
>>
>> Regards,
>> /henrik
>>> 8------------------------------------------------------8<
>> Commit 3179f62001880e588e229db3006a59ad87b7792a ("ALSA: core: add
>> .get_time_info") made an undocumented change to the behaviour of the
>> PCM runtime tstamp.  Prior to this change the tstamp was not updated
>> by snd_pcm_update_hw_ptr0() unless the hw ptr had moved, after this
>> change the tstamp is always updated.
>>
>> For an application, using alsa-lib, doing snd_pcm_readi() followed by
>> snd_pcm_status() to estimate the age of the read samples by subtracting
>> status->avail * [sample rate] from status->tstamp this change degraded
>> the accuracy of that estimate on devices where the pcm hw does not
>> provide a granular hw pointer.  On a device using
>> soc-generic-dmaengine-pcm.c and a dma-engine with residue_granularity
>> DMA_RESIDUE_GRANULARITY_DESCRIPTOR the accuracy of the estimate depended
>> on the latency between the PCM hw completing a period and when the
>> driver called snd_pcm_period_elapsed() to notify ALSA core, typically
>> determined by interrupt handling latency.  After this change the
>> accuracy of the estimate is determined by the latency between the PCM hw
>> completing a period and the application calling snd_pcm_status(),
>> determined by the scheduling of the application process.
>>
>> Revert the part of commit 3179f62001880e588e229db3006a59ad87b7792a
>> ("ALSA: core: add .get_time_info") that changed be behaviour of
>> snd_pcm_update_hw_ptr0() when the PCM hw pointer has not moved.
>>
>> Fixes: 3179f6200188 ("ALSA: core: add .get_time_info")
>> Signed-off-by: Henrik Eriksson <henrik.eriksson at axis.com>
> 
> So this is a regression and we need to address it.
> 
> I *guess* that the reason to update tstamp event at the same hwptr is
> that now we can get a more fine-grained tstamp.  OTOH, we have to
> avoid the regression by that.
> 
> What about the (untested) patch like below?  It skips the tstamp
> update as default but follows the new standard when a better tstamp
> mode is deployed.

update_audio_tstamp() only updates the audio timestamp when the type is 
DEFAULT already, but indeed update runtime->status->tstamp is updated 
unconditionally.

I have a bit of heartburn with the suggested solutions because the delay 
can change even with the same hw_ptr. Not updating the timestamps would 
be going back to the days where timing updates was dependent on DMA 
granularity.

Maybe we could make the tstamp update dependent on a change in 
audio_tstamp, e.g.


diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index a93a4235a332..b606f3ea7a17 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -248,8 +248,10 @@ static void update_audio_tstamp(struct 
snd_pcm_substream *substream,
                                 runtime->rate);
                 *audio_tstamp = ns_to_timespec(audio_nsecs);
         }
-       runtime->status->audio_tstamp = *audio_tstamp;
-       runtime->status->tstamp = *curr_tstamp;
+       if (runtime->status->audio_tstamp != *audio_tstamp) {
+               runtime->status->audio_tstamp = *audio_tstamp;
+               runtime->status->tstamp = *curr_tstamp;
+       }

         /*
          * re-take a driver timestamp to let apps detect if the 
reference tstamp



> 
> 
> thanks,
> 
> Takashi
> 
> ---
> diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
> index a93a4235a332..2ae4a2539e6e 100644
> --- a/sound/core/pcm_lib.c
> +++ b/sound/core/pcm_lib.c
> @@ -438,7 +438,15 @@ static int snd_pcm_update_hw_ptr0(struct snd_pcm_substream *substream,
>   
>    no_delta_check:
>   	if (runtime->status->hw_ptr == new_hw_ptr) {
> -		update_audio_tstamp(substream, &curr_tstamp, &audio_tstamp);
> +		/*
> +		 * update audio tstamp only when a finer tstamp mode is used;
> +		 * the default skips it, otherwise it may lead to a regression
> +		 * when the system relies on tstamp update period
> +		 */
> +		if (substream->ops->get_time_info &&
> +		    runtime->audio_tstamp_report.actual_type !=
> +		    SNDRV_PCM_AUDIO_TSTAMP_TYPE_DEFAULT)
> +			update_audio_tstamp(substream, &curr_tstamp, &audio_tstamp);
>   		return 0;
>   	}
>   
>