linux-kernel - Re: linux-6.4 alsa sound broken

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Message-ID: <ce9116be-748c-9f8b-390a-a20661914f14@perex.cz>
Date:   Wed, 3 May 2023 22:17:53 +0200
From:   Jaroslav Kysela <perex@...ex.cz>
To:     Takashi Iwai <tiwai@...e.de>,
        Jeff Chua <jeff.chua.linux@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        ALSA development <alsa-devel@...a-project.org>
Subject: Re: linux-6.4 alsa sound broken

On 03. 05. 23 22:00, Oswald Buddenhagen wrote:
> On Wed, May 03, 2023 at 09:32:02PM +0200, Jaroslav Kysela wrote:
>> On 03. 05. 23 18:10, Takashi Iwai wrote:
>>> The dmix uses the silence_size=boundary as a fill-all operation, and
>>> it's a free-wheel mode, so supposedly something was overlooked in your
>>> code refactoring.
>>>
>>> Could you check it and address quickly?  I'd like to fix it before
>>> 6.4-rc1 release, so if no fix comes up in a couple of days, I'll have
>>> to revert the change for 6.4-rc1.
>>
>> I would revert this patch.
> 
>> It seems that this "do silence right after the playback is finished"
>> mechanism is not handled in the updated code (and I overlooked that,
>> too):
>>
> no, there is nothing wrong with the code _per se_.
> 
> what's happening is that the dmix plugin doesn't update the application
> pointer, and somehow gets away with it.

Dmix uses the free mode, because multiple applications can write to the 
buffer. We cannot do application pointer updates in the shared resource.

> anyway, this means that we need to revert the code path for top-up mode,
> which means reverting most of the patch's "meat".
> i think i can do better than your proposal, but not today anymore.

Ok, let's see. I tried to be minimalistic to fix bugs and then we can talk 
about the improvements.

> fwiw, the echo results from the plugin apparently summing up the samples
> in the buffer without clearing it first, that is, it relies on the
> auto-silencing doing the clearing, which the patch broke under the given
> circumstances. rather obvious in retrospect.

Dmix does not know which samples were updated by other applications. The 
application tracks only own samples.

						Jaroslav

-- 
Jaroslav Kysela <perex@...ex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.