Messages in this thread | | | Date | Sat, 13 Oct 2018 17:45:22 +0200 | From | Takashi Iwai <> | Subject | Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint [Resend in plain text...] |
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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 <mikebrady@eircom.net> 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 <mikebrady@eircom.net> wrote: > >> > >> Hi Takashi. > >> > >>> On 9 Oct 2018, at 14:44, Takashi Iwai <tiwai@suse.de> 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 > >> > > >
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