From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.5 required=3.0 tests=FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 36972C64EAD for ; Tue, 9 Oct 2018 15:28:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EF5FE214C4 for ; Tue, 9 Oct 2018 15:28:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org EF5FE214C4 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=eircom.net Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726764AbeJIWqP convert rfc822-to-8bit (ORCPT ); Tue, 9 Oct 2018 18:46:15 -0400 Received: from vie01a-dmta-pe08-1.mx.upcmail.net ([84.116.36.20]:47518 "EHLO vie01a-dmta-pe08-1.mx.upcmail.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726393AbeJIWqP (ORCPT ); Tue, 9 Oct 2018 18:46:15 -0400 Received: from [172.31.216.44] (helo=vie01a-pemc-psmtp-pe02) by vie01a-dmta-pe08.mx.upcmail.net with esmtp (Exim 4.88) (envelope-from ) id 1g9twD-0007LO-N3 for linux-kernel@vger.kernel.org; Tue, 09 Oct 2018 17:28:45 +0200 Received: from helix.aillwee.com ([37.228.204.209]) by vie01a-pemc-psmtp-pe02 with SMTP @ mailcloud.upcmail.net id lTUd1y01r4XbgXZ01TUfie; Tue, 09 Oct 2018 17:28:40 +0200 X-SourceIP: 37.228.204.209 Received: from [192.168.50.181] (apu.aillwee.com [192.168.50.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by helix.aillwee.com (Postfix) with ESMTPSA id 3F22C4E607; Tue, 9 Oct 2018 16:28:37 +0100 (IST) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 12.0 \(3445.100.39\)) Subject: Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint [Resend in plain text...] From: Mike Brady In-Reply-To: Date: Tue, 9 Oct 2018 16:28:36 +0100 Cc: Stefan Wahren , Greg Kroah-Hartman , Eric Anholt , linux-rpi-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Phil Elwell Content-Transfer-Encoding: 8BIT Message-Id: <425F6E5F-782E-4288-ABF0-180504FD7B01@eircom.net> References: <20180904155858.8001-1-tiwai@suse.de> <20180904155858.8001-18-tiwai@suse.de> <4c5f9aed-8fbe-fe22-0c8d-097d8915805c@i2se.com> <8866e22a-6cd7-d32d-92e5-9a4e60206d2f@i2se.com> <828AF61F-4F6F-44C3-B463-7FE4EB8974F1@eircom.net> To: Takashi Iwai X-Mailer: Apple Mail (2.3445.100.39) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Takashi. > On 9 Oct 2018, at 14:44, Takashi Iwai 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