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From: Maxime Ripard <maxime.ripard@bootlin.com>
To: Harald Geyer <harald@ccbib.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, info@olimex.com,
	Mark Brown <broonie@kernel.org>, Chen-Yu Tsai <wens@csie.org>,
	Rob Herring <robh+dt@kernel.org>,
	ibu@radempa.de, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC] arm64: dts: allwinner: a64: teres-i: Enable audio
Date: Tue, 12 Feb 2019 11:09:29 +0100
Message-ID: <20190212100929.iqsxu443qrkl6myf@flea> (raw)
In-Reply-To: <E1gtUZn-0000NW-MA@stardust.g4.wien.funkfeuer.at>

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On Tue, Feb 12, 2019 at 10:42:03AM +0100, Harald Geyer wrote:
> Maxime Ripard writes:
> > On Mon, Feb 11, 2019 at 08:32:35PM +0100, Harald Geyer wrote:
> > > > We want to model this properly. I guess using a pinctrl driver
> > > > controlled through GPIO (similar to what regulator-gpio is) would be a
> > > > good first step.
> > > 
> > > I considered this too, but didn't like it:
> > > 
> > > 1) Seems like a bit of overkill.
> > >
> > > 2) The HW at hand is a rather different kind of multiplexer than
> > > what pinctrl assumes. We don't want two mutually exclusive devices,
> > > (Ie don't make the kernel unbind /dev/console for the sake of audio.)
> > > but we want switch the jack between two devices, that might both be
> > > active at the same time. This looks more like the channel multiplexers
> > > used with many ADCs and such. I guess, I could start a new subsystem
> > > around this. Seems like even more overkill.
> > 
> > I'm not quite sure about how that's different from what pinctrl
> > assumes. pinctrl assumes to handle devices that have multiple signals
> > as input, and one as output. Isn't that exactly what you have?
> 
> I think the pinctrl way would be to have the audio card device
> request the HP jack and the uart node request the HP jack and only
> once device could probe successfully. Ie it is about ressource
> allocation, not true multiplexing where both devices can use the
> ressource at the same time. Am I wrong?

By default, it's what happens yes, but you can definitely have more
complex behaviour that would support changing dynamically back and
forth that muxing.

> Or course we don't actually want true multiplexing for audio quality
> reasons, but I don't see how we could use pinctrl without doing nasty
> things to /dev/console ...

You can't have multiplexing, because that device doesn't allow you to:
it allows only a single signal out at a given time. If you're using
audio, you will not be able to get your logs out of the UART.

And /dev/console is always there as far as I'm aware.

> > And pinctrl can be used dynamically as well if you need to
> 
> Can you explain or point me to the relevant explanation in the docs?
> I don't seem to know about it.

pinctrl_select_state is what you would want to call, you would have a
good example of that runtime change in
drivers/i2c/muxes/i2c-mux-pinctrl.c.

> > > Instead I just got the original patch working, by implementing
> > > "output-high" DT property in sunxi-pinctrl. I'll send a patch for
> > > review soon.
> > 
> > What do you want to do with output-high exactly?
> 
> Exactly what I do in the patch that started this thread.
> (I'll resend when wens' cpvdd patch is available for me to rebase onto.)

There's a few issues with that approach as well:

  - We're actively trying to remove the pinctrl nodes for the GPIOs

  - It's completely static: if one only wants to use the UART all the
    time, they would have to change the DTB, which might or might not
    be possible. Note that this could be trickier: why would you
    prevent the console from working for example if the audio support
    isn't built in? or as a module that might or might not be loaded?

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190211111245.GA18147@lst.de>
2019-02-11 13:36 ` Harald Geyer
2019-02-11 15:39   ` Maxime Ripard
2019-02-11 19:32     ` Harald Geyer
2019-02-12  8:38       ` Maxime Ripard
2019-02-12  9:42         ` Harald Geyer
2019-02-12 10:09           ` Maxime Ripard [this message]
2019-02-12 19:37             ` Harald Geyer
2019-02-13  9:44               ` Maxime Ripard
2019-02-13 11:43                 ` Harald Geyer
2019-02-13 15:53                   ` Maxime Ripard
2019-02-14  0:12                     ` Harald Geyer
2019-02-15 14:20                       ` Torsten Duwe
2019-02-16 20:47                         ` Harald Geyer
2019-02-17 11:30                           ` Torsten Duwe
2019-02-18 10:24                           ` Maxime Ripard
2019-04-30 13:32                             ` Torsten Duwe
2019-05-02  7:46                               ` Maxime Ripard
2019-05-02 14:48                                 ` Harald Geyer
2019-02-01 11:37 Harald Geyer
2019-02-12  8:34 ` Chen-Yu Tsai

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