From: Pavel Hofman <pavhofman@gmail.com>
To: Takashi Iwai <tiwai@suse.de>
Cc: Alsa-devel@alsa-project.org,
Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Subject: Re: [alsa-devel] Measuring Device Throughput - MMAP vs. RW
Date: Mon, 9 Sep 2019 21:31:49 +0200 [thread overview]
Message-ID: <CA+SWvNw_=Fj_3M3sqVP767OOR+YDY=6-rPtPgmGXg+BcvuG3oA@mail.gmail.com> (raw)
In-Reply-To: <s5hzhjd9xg4.wl-tiwai@suse.de>
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).
>
> 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.
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next prev parent reply other threads:[~2019-09-09 19:33 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-08 15:23 [alsa-devel] Measuring Device Throughput - MMAP vs. RW Pavel Hofman
2019-09-09 16:30 ` Pavel Hofman
2019-09-09 17:44 ` Takashi Iwai
2019-09-09 19:31 ` Pavel Hofman [this message]
2019-09-10 7:18 ` Takashi Iwai
2019-09-10 7:48 ` Pavel Hofman
2019-09-10 7:59 ` Pavel Hofman
2019-09-10 19:27 ` Pavel Hofman
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