by Takashi Iwai

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

On Sat, 13 Oct 2018 17:00:32 +0200,
Mike Brady wrote:
>
> Hi Takashi. My apologies — t turns out I was wrong. My measurements were systematically wrong due to integer truncation going from 64 bit to 32 bit representation.

That relieved me ;) I thought of starting checking in the next week,
as nothing obvious came to my mind.

In anyways thank you for your testing!

Takashi

>
> Apologies
> Mike
>
> > On 11 Oct 2018, at 13:53, Mike Brady <[email protected]> wrote:
> >
> > Hi Takashi. Just testing out the updated bcm2835 audio driver — it seems that it will underflow at somewhere above about 4410 and below 5120 frames, whereas the present driver is happy down to at least 2000 frames — I haven’t tried lower than about 1700.
> >
> > Is this change meant to happen?
> >
> > Regards
> > Mike
> >
> >
> >> On 9 Oct 2018, at 16:28, Mike Brady <[email protected]> wrote:
> >>
> >> Hi Takashi.
> >>
> >>> On 9 Oct 2018, at 14:44, Takashi Iwai <[email protected]> wrote:
> >>>
> >>> 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.
> >>
> >> Yes, that was my thought. I guess another thing in its favour is that this audio device will always
> >> be in partnership with a processor as part of an SoC, so it will always be likely to have a reasonably
> >> accurate clock.
> >>
> >>> 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.
> >>
> >> That’s a fair point. I don’t know what is done in other drivers, but can only report that with one possible exception,
> >> the DACs used with Shairport Sync by many end users report well-behaved delay figures, certainly to within two microseconds. I’m afraid I don’t know how they do it.
> >>
> >>> 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.
> >>
> >> That’s not a bad idea, and I might take it up on the alsa-devel mailing list, as you suggest.
> >>
> >>> 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.
> >>
> >> In the meantime, would you think that the balance of convenience lies with this interpolation scheme? (Finally, I have a patch ready….)
> >> Regards
> >> Mike
> >>
> >>>
> >>> thanks,
> >>>
> >>> Takashi
> >>
> >
>