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 43663C28CF8 for ; Sat, 13 Oct 2018 15:00:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D59AD20877 for ; Sat, 13 Oct 2018 15:00:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D59AD20877 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 S1726771AbeJMWiI convert rfc822-to-8bit (ORCPT ); Sat, 13 Oct 2018 18:38:08 -0400 Received: from vie01a-dmta-pe05-1.mx.upcmail.net ([84.116.36.11]:45230 "EHLO vie01a-dmta-pe05-1.mx.upcmail.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726292AbeJMWiI (ORCPT ); Sat, 13 Oct 2018 18:38:08 -0400 Received: from [172.31.216.235] (helo=vie01a-pemc-psmtp-pe12.mail.upcmail.net) by vie01a-dmta-pe05.mx.upcmail.net with esmtp (Exim 4.88) (envelope-from ) id 1gBLPA-0007OG-Oj for linux-kernel@vger.kernel.org; Sat, 13 Oct 2018 17:00:36 +0200 Received: from helix.aillwee.com ([37.228.204.209]) by vie01a-pemc-psmtp-pe12.mail.upcmail.net with ESMTP id BLP7gHtclkosQBLP7gusC6; Sat, 13 Oct 2018 17:00:36 +0200 X-Env-Mailfrom: mikebrady@eircom.net X-Env-Rcptto: linux-kernel@vger.kernel.org X-SourceIP: 37.228.204.209 X-CNFS-Analysis: v=2.3 cv=NNQEBHyg c=1 sm=1 tr=0 a=/+iDkf0alGTUGXENEoGzTg==:117 a=/+iDkf0alGTUGXENEoGzTg==:17 a=IkcTkHD0fZMA:10 a=smKx5t2vBNcA:10 a=k7f7euTfAAAA:8 a=NEAV23lmAAAA:8 a=9qa3YWOjAAAA:8 a=i44N-QB9AAAA:8 a=5Paetk5zUUA827HfFq0A:9 a=NJsCHVEHyee2Ps1f:21 a=DWSwUDYAIMTd5ek9:21 a=QEXdDO2ut3YA:10 a=2Su-2_OBkbdmYqucsK2F:22 a=z9YhYUifIkxHVZtqNBnJ:22 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 473894E607; Sat, 13 Oct 2018 16:00:33 +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: Sat, 13 Oct 2018 16:00:32 +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: 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> <425F6E5F-782E-4288-ABF0-180504FD7B01@eircom.net> To: Takashi Iwai X-Mailer: Apple Mail (2.3445.100.39) X-CMAE-Envelope: MS4wfBe4Yc9WliXTzHS5xibrujM+3HwYpoqAnO/3jmZGih7Pi1f867LptiXkvFe3/3+yOcDZsxjQ155rnl90cz5ILSHw1G7EbQ4qZpnA4WMfxHMEBM3lKSW8 NL2k/hQjgs09vjUvCf9pv5srrENbH1/O5YrjsIg0jkYPwFwvKxXpQtdkRilMoGR9sNfMvhocyq1QH48XXh4Ni5a1iH/xnQvnpSwsCa6OnzwXwj/iQ9bsyR8O 6boSa0xG95zLs40Q/yhpNW7FksSGZNDOi/JAvfhiFandM9Pdvhd0pSE1TPMLc6+tYf+Jh7sgXE6iM1Tjxth2/tHyC2cBo/SU/Kz1QVwqPCSQj32br194DLBZ uJO9tsDezCmnyUXauqlUktNtvq9raRMhE+t/h6xC9yDKdpqV7Wc= Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 wrote: >> >> 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 >> >