Re: pcm_meter.c issue at s16_update

[Takashi Iwai <tiwai@suse.de>]

On Sun, 02 Aug 2020 19:50:44 +0200,
Pavel Hofman wrote:
> 
> 
> Dne 28. 07. 20 v 20:54 Pavel Hofman napsal(a):
> > 
> > Dne 28. 07. 20 v 20:04 Pavel Hofman napsal(a):
> >> Dne 28. 07. 20 v 19:04 Takashi Iwai napsal(a):
> >>> Would adding atomic_add(&meter->reset, 1) in snd_pcm_meter_reset()
> >>> help?
> >>>
> >> Unfortunately not.
> >>
> >> s16_reset is called correctly, setting s16->old = meter->now;  But at
> >> that time meter->now is still 22751, setting s16->old to the same value.
> >>
> >> s16_update 1: meter->now 22751, s16->old 22751, size 0
> >>
> >> However, in the next update call meter->now comes from the freshly
> >> started application pointer:
> >>
> >> s16_update 1: meter->now 839, s16->old 22751, size -21912
> >>
> >>
> >> Of course this helps:
> >>
> >> -       if (size < 0)
> >> -               size += spcm->boundary;
> >> +       if (size < 0) {
> >> +               size = meter->now;
> >> +               s16->old = 0;
> >> +       }
> >>
> >> But I understand this is not a solution because:
> >>
> >> * it will not work at reaching spcm->boundary (after thousands of hours?)
> >> * it will cause the same problem when the stream is rewound (which is
> >> the problem now too) - size will equal to large meter->now (length from
> >> the beginning of the stream minus the rewound = large number).
> >>
> > 
> > IMHO there are two cases of the [application pointer + delay] drop
> > compared to the previous run:
> > 
> > * stream start, rewinding => s16->old = meter->now; size =0, i.e.
> > skipping the samples to show
> > * wrapping at spcm->boundary => size += spcm->boundary, i.e. showing the
> > wrapped samples
> > 
> > Optionally the second case could be handled just like the first case by
> > resetting s16->old, assuming the boundary wrap occurs very infrequently.
> 
> The following patch is tested to work OK, no CPU peaks and no meter
> output glitches when the size < 0 condition occurs:
> 
> diff --git a/src/pcm/pcm_meter.c b/src/pcm/pcm_meter.c
> index 20b41876..48df5945 100644
> --- a/src/pcm/pcm_meter.c
> +++ b/src/pcm/pcm_meter.c
> @@ -1098,8 +1098,15 @@ static void s16_update(snd_pcm_scope_t *scope)
>         snd_pcm_sframes_t size;
>         snd_pcm_uframes_t offset;
>         size = meter->now - s16->old;
> -       if (size < 0)
> -               size += spcm->boundary;
> +       if (size < 0) {
> +               /**
> +                * Application pointer adjusted for delay (meter->now)
> has dropped compared
> +                * to the previous update cycle. Either spcm->boundary
> wraparound, pcm rewinding,
> +                * or pcm restart without s16->old properly reset.
> +                * In any case the safest solution is skipping this
> conversion cycle.
> +                */
> +               size = 0;
> +       }
>         offset = s16->old % meter->buf_size;
>         while (size > 0) {
>                 snd_pcm_uframes_t frames = size;
> 
> 
> 
> Please will you accept this (workaround) bugfix? If so, I would send a
> proper patch.

It looks OK, at least this must be safe.
So yes, I'll happily apply if you submit a proper patch.


thanks,

Takashi