linux-kernel - Re: [PATCH 01/14] mfd: arizona: Add jack pointer to struct arizona

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Message-ID: <20201229150807.GF4786@sirena.org.uk>
Date:   Tue, 29 Dec 2020 15:08:07 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Lee Jones <lee.jones@...aro.org>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Cezary Rojewski <cezary.rojewski@...el.com>,
        Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        Liam Girdwood <liam.r.girdwood@...ux.intel.com>,
        Jie Yang <yang.jie@...ux.intel.com>,
        patches@...nsource.cirrus.com, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org
Subject: Re: [PATCH 01/14] mfd: arizona: Add jack pointer to struct arizona

On Tue, Dec 29, 2020 at 02:57:38PM +0100, Hans de Goede wrote:
> On 12/29/20 2:06 PM, Charles Keepax wrote:

> > I would agree with Mark though that if extcon exists for external
> > connectors it seems odd that audio jacks would have their own
> > special way rather than just using the connector stuff.

> Well as I said above in me experience the extcon code is (was) mostly
> meant for kernel internal use. The sysfs API is more of a debugging
> tool then anything else (IMHO).

No, that's not the case.  extcon is a port of an Android custom API that
looks very similar to what ended up in mainline, it was also a
combination of sysfs and uevents but a bit less generic.  It mainly
existed to provide information to userspace about what was plugged into
the various ports on devices, things like headphone jacks and the
pre-USB C multifunction connectors you used to get on phones.  In kernel
use wasn't really a thing for that as far as I can remember.  It's
become a bit less of a pressing concern for Android with the move to USB
C and the deprecation of headphone jacks in favour of a combination of
USB C and Bluetooth but the use case is still there and it's clear that
none of the audio stuff is currently exactly designed.

The issues I'm seeing are more to do with nobody working on things, I
guess mainly due to the change in priorities for Android systems and in
my case a job change.

> Also the kernel has support for a lot of sound devices, including
> many with jack-detection support. Yet a grep for EXTCON_JACK_HEADPHONE
> over the entire mainline kernel tree shows that only extcon-arizona.c
> is using it. So given that we have dozens of drivers providing jack
> functionality through the sound/core/jack.c core and only 1 driver
> using the extcon interface I believe that the ship on how to export
> this to userspace has long sailed, since most userspace code will
> clearly expect the sound/core/jack.c way of doing things to be used.

The whole purpose of creating sound/core/jack.c is to abstract away the
userspace interfaces from the drivers, most audio devices shouldn't be
working with extcon directly just as they shouldn't be creating input
devices or ALSA controls directly.  The missing bit there is to add
extcon into jack.c.

BTW note that the existing arizona extcon driver does provide an input
device so I'm guesing that whatever userspace you're concerned about is
one that uses only the ALSA controls for jack detection.

> Arguably we should/could maybe even drop the extcon part of extcon-arizona.c
> but I did not do that as I did not want to regress existing userspace
> code which may depend on this (on specific embedded/android devices).

I do think pushing things over to extcon is a useful goal, the existing
interfaces have fairly clear issues that it does actually address.

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