On 29.03.2016 21:15, Georg Chini wrote:
On 29.03.2016 20:59, Raymond Yau wrote:
>>
>> The USB driver will submit N silence URBs on startup in the
prepare and you will have to wait for those URBs to retire before the
samples are queued. There is very little 'USB processing'. If you
want to reduce this delay you have to use smaller periods, it'll
decrease the size of the URBs. I guess it could be possible to change
the URB size after the start but that's not implemented atm.
>>
For loopback, the source capture the same amount of data while you
wait for the retitement of those urbs
Yes, that is exactly the point.
Well, it wasn't the point - see my reply to Tanu's last mail on that topic.
> I don't want to shorten the latency. I only want the latency
reported correctly. To me it still
> looks like the real latency of the driver is not what it reports,
because the time that the
> audio spends in the URB's is not taken into account. What I am
seeing is, that the real
> latency is around 10ms longer than expected.
The total number of URBs for the endpoint is not allowed to exceed
MAX_URBS (which the patch increases from 8 to 12).
Do this match with your measurement
How much audio does one URB hold? The time I measure is between 8 and
9 ms and does not
depend much on the configured sink latency as far as I can tell. (I
tried latencies between
around 10ms and 2s). I did however not check the dependency in detail,
most observations
are with sink latencies in the range of 10 - 20ms.
OK, I did a few more measurements and the numbers I have given above are
not correct.
The actual difference in overall latency is 12ms.
When I run module-loopback with 40ms configured latency, I will see
about 42ms with my
code that accounts for the delay and 54ms with the old code.
So if an URB holds 1ms of audio, this could match.
I think the remaining 2ms are hardware delays, they are slightly
different for different
combinations of source/sink and by setting small latency offsets (HDA
source: 0ms
HDA sink: 2.8ms, USB source: 1.0ms and USB sink: 1.8ms) I am at 40ms
+/-0.5ms
for all combinations.
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