Date:   Sat, 13 Oct 2018 16:00:32 +0100
From:   Mike Brady <mikebrady@...com.net>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Stefan Wahren <stefan.wahren@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Eric Anholt <eric@...olt.net>,
        linux-rpi-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Phil Elwell <phil@...pberrypi.org>
Subject: Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint
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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.

Apologies
Mike

> On 11 Oct 2018, at 13:53, Mike Brady <mikebrady@...com.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@...com.net> wrote:
>> 
>> Hi Takashi.
>> 
>>> On 9 Oct 2018, at 14:44, Takashi Iwai <tiwai@...e.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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