linux-kernel - Re: [RFC Patch] gpio: add GPIO hogging mechanism

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Date:	Wed, 29 Oct 2014 11:34:45 -0500
From:	Benoit Parrot <bparrot@...com>
To:	Pantelis Antoniou <pantelis.antoniou@...il.com>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	<linux-gpio@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	<devicetree@...r.kernel.org>
Subject: Re: [RFC Patch] gpio: add GPIO hogging mechanism

Pantelis,

Thanks for the feedback.

Pantelis Antoniou <pantelis.antoniou@...il.com> wrote on Wed [2014-Oct-29 10:53:44 +0200]:
> Hi Benoit,
> 
> > On Oct 21, 2014, at 23:09 , Benoit Parrot <bparrot@...com> wrote:
> > 
> > Based on Boris Brezillion work this is a reworked patch
> > of his initial GPIO hogging mechanism.
> > This patch provides a way to initally configure specific GPIO
> > when the gpio controller is probe.
> > 
> > The actual DT scanning to collect the GPIO specific data is performed
> > as part of the gpiochip_add().
> > 
> > The purpose of this is to allows specific GPIOs to be configured
> > without any driver specific code.
> > This particularly usueful because board design are getting
> > increassingly complex and given SoC pins can now have upward
> > of 10 mux values a lot of connections are now dependent on
> > external IO muxes to switch various modes and combination.
> > 
> > Specific drivers should not necessarily need to be aware of
> > what accounts to a specific board implementation. This board level
> > "description" should be best kept as part of the dts file.
> > 
> 
> This look like it’s going to the right direction. I have a few general
> comments at first.
> 
> 1) It relies on dubious DT binding of having sub-nodes of the
> gpio device implicitly defining hogs.

I think in this instance the nodes are explicitly defining hogs.
Please clarify. What would you like to see here?
> 
> 2) There is no way for having hogs inserted dynamically as far as I can tell, and
> no way to remove a hog either.

The original patch was allowing that but, Linus's review comment suggested this feature be 
part of the gpio-controller's gpiochip_add() hook only.

> 
> 3) I’m not very fond of having this being part of the gpio controller. This
> configuration conceptually has little to do with the gpio controller per se,
> it is more of a board specific thing. Why not come up with a gpio-hog driver that
> references GPIOs? That way with a single gpio-hog driver instance you could setup
> all the GPIO-hogging configuration for all GPIOs on the board, even one that
> lie on different GPIO controllers.

Again this follows Linus's review comment.
I agree that it prevent a centralize spot where all hog would be defined but it has the advantages of not relying on PROBE_DEFER.
I mean since all "gpio-hogs" would be defined in a single board dts file it would not be that hard to figure out the big picture anyways.

> 
> 
> > Signed-off-by: Benoit Parrot <bparrot@...com>
> > ---
> > Documentation/devicetree/bindings/gpio/gpio.txt | 33 +++++++++
> > drivers/gpio/gpiolib-of.c                       | 99 +++++++++++++++++++++++++
> > drivers/gpio/gpiolib.c                          | 81 ++++++++++++++++++++
> > include/linux/of_gpio.h                         | 11 +++
> > 4 files changed, 224 insertions(+)
> > 
> 
> Regards
> 
> — Pantelis
> 

Regards,
Benoit
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