On Mon, 09 Sep 2019 21:31:49 +0200, Pavel Hofman wrote:
On Mon, Sep 9, 2019 at 7:44 PM Takashi Iwai tiwai@suse.de 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.