commit 3179f6200188 ("ALSA: core: add .get_time_info") had a side effect of changing the behaviour of the PCM runtime tstamp. Prior to this change tstamp was not updated by snd_pcm_update_hw_ptr0() unless the hw_ptr had moved, after this change tstamp was 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 the estimate on devices where the pcm hw does not provide a granular hw_ptr, e.g., devices 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 the driver called snd_pcm_period_elapsed() to notify ALSA core, typically determined by interrupt handling latency. After the change the accuracy of the estimate depended on the latency between the PCM hw completing a period and the application calling snd_pcm_status(), determined by the scheduling of the application process. The maximum error of the estimate is one period length in both cases, but the error average and variance is smaller when it depends on interrupt latency.

Instead of always updating tstamp, update it only if audio_tstamp changed.

Fixes: 3179f6200188 ("ALSA: core: add .get_time_info") Suggested-by: Pierre-Louis Bossart pierre-louis.bossart@linux.intel.com Signed-off-by: Henrik Eriksson henrik.eriksson@axis.com --- Changes since v1: - Use Pierre-Louis Bossart's suggested fix - Clean up commit log

sound/core/pcm_lib.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c index a93a4235a332..10e7ef7a8804 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 (!timespec_equal(&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