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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 E886DECDFB6 for ; Tue, 1 Oct 2019 06:24:40 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id BD40F21783 for ; Tue, 1 Oct 2019 06:24:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BD40F21783 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:59254 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iFBaR-0002GK-Pa for qemu-devel@archiver.kernel.org; Tue, 01 Oct 2019 02:24:39 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44160) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iFBZR-0001qW-5f for qemu-devel@nongnu.org; Tue, 01 Oct 2019 02:23:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iFBZL-0003Ir-AM for qemu-devel@nongnu.org; Tue, 01 Oct 2019 02:23:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41120) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iFBZL-0003HR-2W for qemu-devel@nongnu.org; Tue, 01 Oct 2019 02:23:31 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id CE781301D671; Tue, 1 Oct 2019 06:23:28 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.36.118.123]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9673B19C58; Tue, 1 Oct 2019 06:23:25 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 27E1F1138648; Tue, 1 Oct 2019 08:23:24 +0200 (CEST) From: Markus Armbruster To: =?utf-8?B?Wm9sdMOhbiBLxZF2w6Fnw7M=?= Subject: Re: [PATCH v4 15/24] audio: add mixing-engine option (documentation) References: <68c4b4bd438e9cb5c97aed32ee31e3dabd96cbf6.1568927990.git.DirtY.iCE.hu@gmail.com> <87sgonma7v.fsf@dusky.pond.sub.org> <4d24c95c-0e98-2646-0b1a-6c7b3afe0e90@gmail.com> <87wodwyabl.fsf@dusky.pond.sub.org> Date: Tue, 01 Oct 2019 08:23:24 +0200 In-Reply-To: (=?utf-8?B?IlpvbHTDoW4JS8WRdsOhZ8OzIidz?= message of "Sun, 29 Sep 2019 21:07:31 +0200") Message-ID: <87d0fh6l2b.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.47]); Tue, 01 Oct 2019 06:23:28 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, Gerd Hoffmann Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" "Zolt=C3=A1n K=C5=91v=C3=A1g=C3=B3" writes: > On 2019-09-25 11:49, Markus Armbruster wrote: >> "Zolt=C3=A1n K=C5=91v=C3=A1g=C3=B3" writes: >> >>> On 2019-09-23 15:08, Markus Armbruster wrote: >>>> "K=C5=91v=C3=A1g=C3=B3, Zolt=C3=A1n" writes: >>>> >>>>> This will allow us to disable mixeng when we use a decent backend. >>>>> >>>>> Disabling mixeng have a few advantages: >>>>> * we no longer convert the audio output from one format to another, w= hen >>>>> the underlying audio system would just convert it to a third form= at. >>>>> We no longer convert, only the underlying system, when needed. >>>>> * the underlying system probably has better resampling and sample for= mat >>>>> converting methods anyway... >>>>> * we may support formats that the mixeng currently does not support (= S24 >>>>> or float samples, more than two channels) >>>>> * when using an audio server (like pulseaudio) different sound card >>>>> outputs will show up as separate streams, even if we use only one >>>>> backend >>>>> >>>>> Disadvantages: >>>>> * audio capturing no longer works (wavcapture, and vnc audio extensio= n) >>>>> * some backends only support a single playback stream or very picky >>>>> about the audio format. In this case we can't disable mixeng. >>>>> >>>>> However mixeng is not removed, only made optional, so this shouldn't = be >>>>> a big concern. >>>>> >>>>> Signed-off-by: K=C5=91v=C3=A1g=C3=B3, Zolt=C3=A1n >>>>> --- >>>>> >>>>> Notes: >>>>> Changes from v1: >>>>> * renamed mixeng to mixing-engine >>>>> >>>>> qapi/audio.json | 5 +++++ >>>>> qemu-options.hx | 6 ++++++ >>>>> 2 files changed, 11 insertions(+) >>>>> >>>>> diff --git a/qapi/audio.json b/qapi/audio.json >>>>> index 9fefdf5186..0535eff794 100644 >>>>> --- a/qapi/audio.json >>>>> +++ b/qapi/audio.json >>>>> @@ -11,6 +11,10 @@ >>>>> # General audio backend options that are used for both playback and >>>>> # recording. >>>>> # >>>>> +# @mixing-engine: use QEMU's mixing engine to mix all streams inside= QEMU. When >>>>> +# set to off, fixed-settings must be also off. Not e= very backend >>>>> +# compatible with the off setting (default on, since= 4.2) >>>>> +# >>>> >>>> Last sentence no verb. >>>> >>>> Which backends are compatible? >>> >>> Actually that's a simplification, it depends on a few things. When >>> mixeng is off, qemu will try to use the same format as the emulated >>> sound card, and if the backend doesn't support that format, it won't >>> work (no audio). Also attaching multiple sound cards to the same >>> audiodev might not work, if the backend doesn't support multiple >>> playback streams. If you use pulseaudio, it'll work without problems, >>> if you use alsa, it depends on your device. If you use a hw: device >>> directly, you'll likely only be able to use one emulated sound card >>> with a few selected audio formats. If you use dmix: (and plug), alsa >>> will handle the conversion and mixing, so it will work no matter what >>> format the emulated sound card uses. With OSS the situation is >>> probably similar, it depends on the kernel/hw what works and what not. >>> wav and spice certainly doesn't support multiple streams. I'm not >>> completely sure about the other backends right now, but I think dsound >>> and coreaudio can handle the necessary sample format conversions and >>> mixing. >>> >>>> What happens when you try the off setting with incompatible backends? >>> See above. >> >> What happens *exactly*? >> >> I'm asking because I'm concerned about the user experience. When a user >> asks for a combination of things QEMU can't provide, such as mixeng off >> with an incompatible backend, QEMU should fail with a suitable error >> message. Does it? > > Error handling is not the best in the audio subsystem, if something > fails it generally just prints a warning to the console and continues, > and something will happen... For example, this is what happens when I > try to open one hw device twice. I ran qemu with: > > -audiodev > alsa,id=3Dfoo,in.dev=3Dhw:1,,0,out.mixing-engine=3Doff,out.dev=3Dhw:1,,0 > -device piix4-usb-uhci -device usb-audio,audiodev=3Dfoo -device > AC97,audiodev=3Dfoo > > When the guest tried to initialize the AC97 card, I got an error: > > alsa: Could not initialize DAC > alsa: Failed to open `hw:1,0': > alsa: Reason: Device or resource busy > > And it just continued. And the sound worked, but with wrong sample > rate (AC97 wants 44100 Hz, but USB audio previously opened the alsa > device with 48000 Hz). I'll fix this bug in the next revision, > audio_pcm_hw_add_* shouldn't fall back to other HWs without mixeng. > But even with that, the result will be that one emulated sound card > will work and the other won't. > > It's not ideal, but fixing it would require a lot of effort. Right > now, if you specify an invalid audiodev for alsa (even with mixeng), > it'll just print an error to the console and continue without audio. Should we document this general error handling deficiency somehow? >> Sometimes rejecting non-working configurations is impractical. Is it >> here? > > I think it is. It depends on the backend, its settings, the frontend > (emulated sound card), and how the guest uses it. We currently don't > know what formats does a backend support, what formats can a frontend > produce, and even if we would know that, just because a frontend can > produce a format that the backend doesn't understand doesn't mean that > it will actually do it. For example, right now with this patch series > applied, usb-audio can produce 7.1 audio. If we want to be strict, it > means we can only use it with backends that support at least 8 > channels, even if the user only wants to use stereo audio. > >> If yes, we should call out the problematic configurations in >> documentation. > > I think we should rather list known working configurations, and leave > the others as "try at your own risk" because there's too many things > that can go wrong. (pulseaudio will work, alsa with dmix too. Need > to check coreaudio, dsound and oss. spice and wavcapture won't work.) Far from ideal, but better than nothing. Possibly naive idea: what about automatically falling back to mixeng on when mixeng off doesn't work? Requires detecting "doesn't work", which I understand just isn't there. Any other reasons why this couldn't be done? Way out of scope for this series, of course.