Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint [Resend in plain text...]

Takashi Iwai
2018-10-09T13:44:32Z

On Tue, 09 Oct 2018 15:18:15 +0200,
Mike Brady wrote:
>
> >> @Mike: Do you want to write a patch series which upstream "interpolate
> >> audio delay" and addresses Takashi's comments?
> >>
> >> I would help you, in case you have questions about setup a Raspberry Pi
> >> with Mainline kernel or patch submission.
> >
> > Well, the question is who really wants this. The value given by that
> > patch is nothing but some estimation and might be even incorrect.
> >
> > PulseAudio won't need it any longer when you set the BATCH flag.
> > Then it'll switch from tsched mode to the old mode, and the delay
> > value would be almost irrelevant.
>
> Well, two answers. First, Shairport Sync
> (https://github.com/mikebrady/shairport-sync) needs it — whenever a
> packet of audio frames is about to be added to the output queue (at
> approximately 7.9 millisecond intervals), the delay is checked to
> try to maintain sync to within a few milliseconds. The BCM2835 audio
> device is the only one I have yet come across with so much
> jitter. Whatever other drivers do, the delay they report doesn’t
> suffer from anything like this level of jitter.

OK, if there is another application using that delay value, it's worth
to consider providing a fine-grained value.

> The second answer is that the veracity of the ALSA documentation
> depends on it — any application using the ALSA system for
> synchronisation will rely on this being an accurate reflection of
> the situation. AFAIK there is really no workaround it if the
> application is confined to “safe” ALSA
> (http://0pointer.de/blog/projects/guide-to-sound-apis).

> On LMKL.org, Takashi wrote:
>
> > Date Wed, 19 Sep 2018 11:52:33 +0200
> > From Takashi Iwai <>
> > Subject Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint
>
> > [snip]
>
> > That's OK, as long as the computation is accurate enough (at least not
> > exceed the actual position) and is light-weight.
>
> > [snip]
>
> The overhead is small -- an extra ktime_get() every time a GPU message
> is sent -- and another call and a few calculations whenever the delay
> is sought from userland.
>
> At 48,000 frames per second, i.e. approximately 20 microseconds per
> frame, it would take a clock inaccuracy of roughly
> 20 microseconds in 10 milliseconds -- 2,000 parts per million — to
> result in an inaccurate estimate.
> Crystal or resonator-based clocks typically have an inaccuracy of
> 10s to 100s of parts per million.
>
> Finally, to see the effect of the absence and presence of this
> interpolation, please have a look at this:
> https://github.com/raspberrypi/firmware/issues/1026#issuecomment-415746016,
> where a downstream version of this fix was being discussed.

I'm not opposing to the usage of delay value. The attribute is
provided exactly for such a purpose. It's a good thing (tm).

The potential problem is, however, rather the implementation: it's
using a system timer for interpolation, which is known to drift from
the actual clocks. Though, one may say that in such a use case, we
may ignore the drift since the interpolation is so narrow.

But another question is whether it should be implemented in each
driver level. The time-stamping is basically a PCM core
functionality, and nothing specific to the hardware, especially when
it's referring to the system timer.

e.g. you can think in a different way, too: we may put a timestamp at
each hwptr update, and pass it as-is, instead of updating the
timestamp at each position query. This will effectively gives the
accurate position-timestamp pair, and user-space may interpolate as it
likes, too.

In anyway, if *this* kind of feature needs to be merged, it's
definitely to be discussed with the upstream. So, if you're going to
merge that sort of path, please keep Cc to alsa-devel ML.

thanks,

Takashi