Re: pcm_meter.c issue at s16_update

[Pavel Hofman <pavel.hofman@ivitera.com>]

Dne 03. 08. 20 v 12:48 Pavel Hofman napsal(a):
> 
> 
> Dne 03. 08. 20 v 9:22 Jaroslav Kysela napsal(a):
>> Dne 03. 08. 20 v 8:17 Takashi Iwai napsal(a):
>>> On Sun, 02 Aug 2020 19:50:44 +0200,
>>>>>
>>>>> 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.
>>
>> It would be probably better to check against the boundary / 2 value to
>> check
>> correctly the boundary wrap instead to drop all negative size values:
>>
>>    if (size < 0) {
>>       if (size < -(spcm->boundary / 2))
>>          size += spcm->boundary;
>>       else
>>          size = 0;
>>    }
> 
> Is there a reliable way to detect the boundary wraparound, at best using
> some dedicated API? I could find any, IMO the wraparound does not create
> any notification. The check is OK for a rewind, half of boundary is
> usually a very large number too. I am not sure what would happen at
> reset when application pointer was already past the boundary half - see
> below.
> 
>>
>> The "hidden" pcm restart referred in the comment should not occur,
>> otherwise
>> it's another bug somewhere.
> 
> I do not know the exact moments when plugin API methods are called. The
> fact is Takashi's suggestion to call s16 reset explicitely in
> snd_pcm_meter_reset created this order:
> 
> snd_pcm_meter_reset -> s16->reset
> s16_update: meter->now 22751, s16->old 22751, size 0
> s16_update: meter->now 839, s16->old 22751, size -21912
> 
> I.e. AFTER resetting meter/s16 the variable meter->now was still at the
> original large 22751 (with s16->old equal to its value due to
> s16->reset). The value of meter->now was reset to 839 (= app pointer -
> delay) only in the next call of s16_update (when s16->old was still the
> previous old value => size < 0 => huge size => high CPU load).  From
> this I kind of conclude that the reset is buggy. Maybe the reset code
> should re-calculate meter->now = appl.pointer - delay before aligning
> s16->old = meter->now.
> 
> Nevertheless all this (except for the boundary wraparound) would result
> in the same size = 0, thus skipping samples from the last cycle, just
> like what the proposed patch does.
> 
> 

Please can we reach a decision and close the problem so that affected
use cases do not have to be patched with the next the alsa-lib version?

Thanks a lot in advance,

Pavel.