Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint [Resend in plain text...]
From: Mike Brady
Date: Thu Oct 11 2018 - 08:53:58 EST
Hi Takashi. Just testing out the updated bcm2835 audio driver â 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 â I havenât tried lower than about 1700.
Is this change meant to happen?
> On 9 Oct 2018, at 16:28, Mike Brady <mikebrady@xxxxxxxxxx> wrote:
> Hi Takashi.
>> On 9 Oct 2018, at 14:44, Takashi Iwai <tiwai@xxxxxxx> 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 â 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 doesnât
>>> 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 â 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 âsafeâ ALSA
>>> 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
>>>> That's OK, as long as the computation is accurate enough (at least not
>>>> exceed the actual position) and is light-weight.
>>> 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 â 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:
>>> 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.
> Thatâs a fair point. I donât 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. Iâm afraid I donât 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.
> Thatâs 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 readyâ.)