On Mon, 09 Sep 2019 21:31:49 +0200, Pavel Hofman wrote: > > On Mon, Sep 9, 2019 at 7:44 PM Takashi Iwai <tiwai@xxxxxxx> wrote: > > > The mmap fluctuations seem to be fixed by using increases in hardware > > pointer pcm->hw.ptr values instead of frames variable. Please do these > > values wrap up or do they grow to max long? Still contiguous after 10 > > minutes of running. > > Could you elaborate the requirement? The description isn't clear to > me. > > I need to slave EP OUT of asynchronous USB-audio gadget (i.e. device) to > another (output) soundcard in the system so that adaptive resampling in the > chain "USB gadget -> processing -> output soundcard" is avoided. For that I > need to measure average throughput to/from alsa hw:X devices, to gather data > for the feedback endpoint implemented by the gadget driver. I do not have any > control over configuration of the devices, the feedback must run independently > of the rest of the chain, of the applications used. > > In snd_pcm_hw_mmap_commit I subtract the advancement of pcm->hw.ptr from value > approx. 10 seconds ago, divided by passed nanosecs measured by clock_gettime > (CLOCK_MONOTONIC). For that I need the pcm->hw.ptr either to be monotonic, or > I just reset the averaging if a non-monotony is detected (hw pointer in > previous (n-1) call of snd_pcm_hw_mmap_commit was higher than current value). > > For better precision, would it make sense to update the pcm->hw.ptr with > snd_pcm_avail call in that method? It may not be even possible in that method > due to some locking, I really do not know. But my precision requirements are > not very strict since I can average over long time (minutes) for the feedback > (the clock difference is usually very small). The value read from snd_pcm_status() should give the accurate pair of the position and the timestamp. You need to check the delay count as well, which compensates the in-flight samples. Takashi > > You should be able to skip XRUN check by changing the stop_threshold. > Setting it to the boundary size essentially means skipping XRUN > checks. > > Actually if xruns could be detected by discontinuity of the hw pointer, I > would not need any other method. > > > In anyway, I put Pierre to Cc, as he's done some works in the past for > the accurate position reporting on HD-audio and USB-audio devices. > He can give a better hint in this regard. > > Thanks a lot, I very much appreciate your help. I do not know these low-level > alsa details. But I will gladly learn. > > Pavel. > > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx https://mailman.alsa-project.org/mailman/listinfo/alsa-devel
