Dne 09. 08. 20 v 22:29 Jaroslav Kysela napsal(a):
Dne 09. 08. 20 v 9:05 Pavel Hofman napsal(a):
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.
Yes, it's a good argument. In this case, the s16->old value is not properly
synced during the reset operation, otherwise the boundary / 2 threshold
(change limit) is sufficient to detect the boundary wrap.
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?
I think that this problem should be fixed for reset and rewind separately. The
meter->reset should be set in snd_pcm_meter_reset() inside the running_mutex
lock to serialize correctly the update operations in the
snd_pcm_meter_thread(). And perhaps, we can follow this logic for the rewind.
I mean, we should ensure to call the s16->reset at the proper time to avoid
broken old/now combinations inside the scope "clients".
Your proposed solution is just a workaround.
I am well aware of that. The main cause of the problem is that the
existing code assumes that a drop in the meter->now value is caused by
the pcm->boundary wraparound. Only for that particular case the existing
size += spcm->boundary code is correct, for all the other cases it is
grossly wrong, locking the thread for many tens of seconds and jamming
CPU. If there was a callback or some other way to signal the boundary
wraparound that "dangerous" code would be called only for that special
case (which is extremely rare in usual setups).
I do not know all cases when the meter->now can drop. Reset, rewind, any
other (xrun)? If a single case is omitted, the same problem prevails.
No problem with resetting where appropriate, but still I would suggest
to not keep size += spcm->boundary in the s16_update as is now.
Regards,
Pavel.
