Re: pcm_meter.c issue at s16_update

[Pavel Hofman <pavel.hofman@xxxxxxxxxxx>]

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.

Thanks a lot,

Pavel.