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Date: Tue, 9 Sep 2014 12:46:44 +0300
From: Mika Westerberg <mika.westerberg@linux.intel.com>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        x86@kernel.org, Ning Li <ning.li@intel.com>,
        Alan Cox <alan@linux.intel.com>, Mark Brown <broonie@linaro.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>
Subject: Re: [PATCH 1/2] x86, gpio: Increase ARCH_NR_GPIOs to 512
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On Tue, Sep 09, 2014 at 09:24:59AM +0200, Linus Walleij wrote:
> On Mon, Sep 8, 2014 at 1:47 PM, Mika Westerberg
> <mika.westerberg@linux.intel.com> wrote:
> 
> > Some newer Intel SoCs like Braswell already have more than 256 GPIOs
> > available so the default limit is exceeded. In order to support these add
> > back the custom GPIO header with limit of 512 GPIOs for x86.
> >
> > Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
> 
> Argh! This is the kind of stuff I want to get rid of ....
> 
> Preferably gpio should be a subsystem without a lot of hooks all over
> the place with arch-specific modifications for this and that, including
> the max number of GPIOs.
> 
> I would actually prefer if you bump the value in
> include/asm-generic/gpio.h to 512 over this.

OK, this makes sense and I can do the change for the next revision of
the patch.

> But better still, now that we have descriptors etc would be to define
> some new per-arch selectable config option like
> CONFIG_ONLY_DYNAMIC_GPIO that changes the GPIO
> core to use something like a radix tree to store and retrieve
> descriptors.
> 
> I.e. in drivers/gpio/gpiolib.c get rid of this:
> static struct gpio_desc gpio_desc[ARCH_NR_GPIOS];
> 
> Replace it with a radix tree of descriptors.
> 
> This however makes it *impossible* to use things like desc_to_gpio()
> and/or gpio_to_desc() so the code has to be augmented all over the
> place to avoid any uses of GPIO numbers on that architecture,
> but I am sure it *can* be done on pure ACPI or device tree
> systems, and that's what we should aim for.
> 
> Comments?

That's a rather big rework to the GPIO subsystem and its users. I agree
that it should be the goal eventually. For x86 such conversion is not
that simple because we have systems out there that have neither ACPI nor
DT available.