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=-0.8 required=3.0 tests=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 32622C64EB8 for ; Tue, 9 Oct 2018 13:44:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 003B821502 for ; Tue, 9 Oct 2018 13:44:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 003B821502 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de 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 S1726846AbeJIVBa (ORCPT ); Tue, 9 Oct 2018 17:01:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:50118 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726540AbeJIVBa (ORCPT ); Tue, 9 Oct 2018 17:01:30 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 31296B012; Tue, 9 Oct 2018 13:44:28 +0000 (UTC) Date: Tue, 09 Oct 2018 15:44:27 +0200 Message-ID: From: Takashi Iwai To: Mike Brady Cc: Stefan Wahren , Greg Kroah-Hartman , Eric Anholt , linux-rpi-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Phil Elwell Subject: Re: [PATCH 17/29] staging: bcm2835-audio: Add 10ms period constraint [Resend in plain text...] In-Reply-To: <828AF61F-4F6F-44C3-B463-7FE4EB8974F1@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> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI/1.14.6 (Maruoka) FLIM/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL/10.8 Emacs/26 (x86_64-suse-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI 1.14.6 - "Maruoka") Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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