linux-kernel - Re: [RFC PATCH] gpio: support for GPIO forwarding

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Message-ID: <CAAVeFu+JHMf__s0hfCJq31kZP-mp02bbzVywq+eZ9JVyP9boSw@mail.gmail.com>
Date:	Tue, 10 Feb 2015 18:44:34 +0900
From:	Alexandre Courbot <gnurou@...il.com>
To:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [RFC PATCH] gpio: support for GPIO forwarding

On Wed, Feb 4, 2015 at 11:11 PM, Heikki Krogerus
<heikki.krogerus@...ux.intel.com> wrote:
> On Wed, Feb 04, 2015 at 10:51:27AM +0100, Linus Walleij wrote:
>> On Fri, Jan 30, 2015 at 5:17 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>> > On Friday, January 30, 2015 03:48:30 PM Linus Walleij wrote:
>>
>> >> So you could detect one by making a checksum of the binary or something.
>> >>
>> >> And then you'd know that the table with this checksum needs patching?
>> >
>> > At a single table level it is generally difficult to say whether or not
>> > things are going to work.
>> >
>> > What needs to work is the namespace which is built from all of the tables
>> > provided combined.  So the namespace needs to be populated first and then
>> > fixes applied on top of that (presumably by deleting, adding or replacing
>> > objects).
>> >
>> > Now, in theory, you *may* be able to figure out that combination of tables
>> > A produces namespace B which then will require fix X if the system is Y,
>> > but quite frankly I wouldn't count on that.
>> >
>> > Moreover, fixups (or "patches" as I called them, but that wasn't exactly
>> > correct) need to be provided in the form of AML definition blocks to apply on
>> > top of an already populated namespace and if you want to use a binary kernel image,
>> > you can't really afford putting all that stuff for all systems it can possibly
>> > run on into it.  This means that distros need to be able to combine a fixup for
>> > the ACPI tables with the binary kernel and install the result into the system's
>> > boot medium (whatever it is).  Also it should be possible to update the fixup
>> > and the kernel image separately if necessary.
>> >
>> > Now from the kernel's perspective that raises the question: "What if the
>> > ACPI tables fixup provided by the distro is not sufficient?"
>> >
>> > That needs to be addressed somehow in the code.
>>
>> Yeah I guess I'm convinced that we need to handle this particular
>> weirdness in the gpio-acpi code... if it can be contained there as
>> expressed by Alexandre.
>
> I'm still fine if we want to confine this "gpio forwarding" to acpi
> if you guys want it, but I was looking at the Sound SoC drivers and I
> realised that we do have places which pass gpio descriptors to other
> devices in platform data. And these of course aren't always used on
> acpi platforms. By greping gpio_desc I saw at least these files are
> passing it in platform data structures:
>
> include/sound/soc.h
> include/linux/leds.h
> include/linux/usb/usb_phy_generic.h
>
> There are probable others but I checked those. And of course now there
> is nothing preventing people from adding more of them.

For sound/soc.h, the member is indeed public but I don't see it being
used to pass descriptors around between drivers.

For linux/leds.h, I think this is the reason why we introduced
devm_get_gpiod_from_child()

These looks more like a bad usage of GPIO descriptors, but AFAICT they
can be fixed by fixing the drivers and, by all means, this is where we
should do it.

This is a different situation from yours where we are trying to deal
with broken firmware and need to resort to tricks at one point or the
other.

Or am I missing your point?
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