Hi Takashi. > On 9 Oct 2018, at 14:44, Takashi Iwai <ti...@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
