linux-kernel - Re: [PATCH] ASoC: core: restore dpcm flags semantics

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Message-ID: <20200730185229.GH5055@sirena.org.uk>
Date:   Thu, 30 Jul 2020 19:52:29 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     Jerome Brunet <jbrunet@...libre.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        alsa-devel@...a-project.org, Stephan Gerhold <stephan@...hold.net>,
        Kevin Hilman <khilman@...libre.com>,
        linux-kernel@...r.kernel.org, zhangn1985@...look.com,
        linux-amlogic@...ts.infradead.org
Subject: Re: [PATCH] ASoC: core: restore dpcm flags semantics

On Thu, Jul 30, 2020 at 11:06:23AM -0500, Pierre-Louis Bossart wrote:
> On 7/30/20 4:04 AM, Jerome Brunet wrote:
> > On Wed 29 Jul 2020 at 17:56, Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com> wrote:
> > > On 7/29/20 10:46 AM, Jerome Brunet wrote:

> > > > The flag previously allowed card drivers to disable a stream direction on
> > > > a link (whether or not such feature is deemed useful).

Right, and I can see a use case for this if someone has a board that
for some reason didn't physically connect one of the directions for some
reason - perhaps they were running out of pins or something.  It's not
clear if anyone's actually doing that though.

> > > > Forcing the flags to be aligned with DAI caps just make the information
> > > > the flag carry redundant with DAI caps, breaking a few cards along the way.

> > > > This change drops the added error conditions and restore the initial flag
> > > > semantics.

I'm not 100% clear, have we actually found cases where the flags are
used or is this something found through inspection and review?

> >   * It worked for every user of DPCM so a far.

> Not completely true, when Morimoto-san added snd_soc_dai_stream_valid() it
> exposed tons of cases where the information on direction was not provided in
> a reliable at the DAI level. I will assert that we are still finding out
> cases with broken DAI configurations, and as a result we will also find
> broken dailink configurations. Your picture of DPCM as a perfectly
> functional system that I broke is a distortion of reality.

> The reality is that we have to work in steps, first make sure all DAIs are
> properly described, then work on the dailinks and optimize at a later point.
> we will need warnings to find out what the problem cases are, and move
> slowly.

This was all triggered by Morimoto-san's changes like you say.  DPCM has
quite a lot of problems in general, here IIRC the issues were that we
had multiple different ways of doing similar things which it wasn't
quite clear if people were even using.  The intention with the warnings
was to remove them one way or another, they're mainly intended to flush
out actual active usage of the flags as opposed to redundant usage of
them which could be confused/broken.

This could definitely have been clearer in the changelogs though.

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