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
