Hi all,
I am trying to understand the precision of htimestamp for samples captured with
a kernel real-time, and I am not sure if I understood well, because I have
found old informations togheter with new informations and I've the doubt that
something changed. I am interested in this because of a project for my degree
thesis.
snd_pcm_hw_params_supports_audio_wallclock_ts returns 0 on capture, so my
hardware doesn't support wallclock timestamps on capture.
I've understood that SND_PCM_TSTAMP_ENABLE is equivalent to
SND_PCM_TSTAMP_MMAP, so it updates timestamp at every position update on DMA
from the driver. I've read elsewhere that this timestamp is updated every
period-size ( equal interrupt), is it correct?
I've digged in the code to understand better inner working of htimestamp, but
I've found that snd_pcm_htimestamp in pcm.c calls:
err = pcm->fast_ops->htimestamp(pcm->fast_op_arg, avail, tstamp);
And I have not found where fast_ops is initialized, maybe in the driver? Anyway
I thougth that maybe in my case the called function is snd_pcm_hw_htimestamp in
pcm_hw.c :
" while (1) {
avail1 = snd_pcm_hw_avail_update(pcm);
if (avail1 < 0)
return avail1;
if (ok && (snd_pcm_uframes_t)avail1 == *avail)
break;
*avail = avail1;
*tstamp = snd_pcm_hw_fast_tstamp(pcm);
ok = 1;
}"
that calls snd_pcm_hw_fast_tstamp, that calls finally:
"#define FAST_PCM_TSTAMP(hw) \
((hw)->mmap_status->tstamp)
"
And finally is this the timestamp that is updated every period-size? Am I
correct that in the case that hardware doesn't support wallclock timestamp is
snd_pcm_hw_htimestamp in pcm_hw.c the called function?
Thanks in advance
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