pcm_meter.c issue at s16_update

[Pavel Hofman]

Dne 28. 07. 20 v 19:04 Takashi Iwai napsal(a):
> On Tue, 28 Jul 2020 18:46:00 +0200,
> Pavel Hofman wrote:
>>
>>
>>>
>>> The debug around the event looks like this, suddenly meter->now drops down:
>>>
>>> s16_update 1: meter->now 2567498, s16->old 2566593, size 905
>>> s16_update 1: meter->now 2568401, s16->old 2567498, size 903
>>> s16_update 1: meter->now 20786, s16->old 20786, size 0
>>> s16_update 1: meter->now 1065, s16->old 20786, size -19721
>>> s16_update 1: meter->now 24839, s16->old 24838, size 1
>>> s16_update 1: meter->now 701, s16->old 24839, size -24138
>>> s16_update 1: meter->now 1253162, s16->old 701, size 1252461
>>> s16_update 1: meter->now 1255148, s16->old 1253162, size 1986
>>>
>>> ..........
>>>
>>> s16_update 1: meter->now 11136, s16->old 10261, size 875
>>> s16_update 1: meter->now 22525, s16->old 22524, size 1
>>> s16_update 1: meter->now 963, s16->old 22525, size -21562
>>> s16_update 1: meter->now 1270914, s16->old 963, size 1269951
>>> s16_update 1: meter->now 1272917, s16->old 1270914, size 2003
>>>
>>
>> I think the problem is that s16->old is not reset when status.appl_ptr
>> is zeroed and starts running again. There is a call
>>
>> static void s16_reset(snd_pcm_scope_t *scope)
>> {
>> 	snd_pcm_scope_s16_t *s16 = scope->private_data;
>> 	snd_pcm_meter_t *meter = s16->pcm->private_data;
>> 	s16->old = meter->now;
>> }
>>
>> but I do not know when this method is called and whether the meter->now
>> is already assigned to the newly zeroed status.appl_ptr.
> 
> 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).


BTW, please why is snd_pcm_meter_update_scope called only in capture stream?

Thanks,

Pavel.