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.
