pcm_meter.c issue at s16_update

[Pavel Hofman]

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,
>> 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.
> 
> 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.


Pavel.