linux-kernel - Re: [PATCH v2 1/1] gpio-f7188x: fix base values conflicts with other gpio pins

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Date:   Tue, 30 May 2023 19:53:48 +0200
From:   simon.guinot@...uanux.org
To:     andy.shevchenko@...il.com
Cc:     Linus Walleij <linus.walleij@...aro.org>, xingtong_wu@....com,
        brgl@...ev.pl, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org, henning.schild@...mens.com,
        xingtong.wu@...mens.com
Subject: Re: [PATCH v2 1/1] gpio-f7188x: fix base values conflicts with other
 gpio pins

On Tue, May 30, 2023 at 01:24:30AM +0300, andy.shevchenko@...il.com wrote:
> Mon, May 29, 2023 at 03:54:36PM +0200, simon.guinot@...uanux.org kirjoitti:
> > On Mon, May 29, 2023 at 03:03:28PM +0200, Linus Walleij wrote:
> > > On Mon, May 29, 2023 at 2:27 PM <simon.guinot@...uanux.org> wrote:
> > > 
> > > > It would be nice if a pin number found in the device datasheet could
> > > > still be converted into a Linux GPIO number by adding the base of the
> > > > first bank.
> > > 
> > > We actively discourage this kind of mapping because of reasons stated
> > > in drivers/gpio/TODO: we want dynamic number allocation to be the
> > > norm.
> > 
> > Sure but it would be nice to have a dynamic base applied to a controller
> > (and not to each chip of this controller), and to respect the interval
> > between the chips (as stated in the controllers datasheets).
> 
> What you want is against the architecture. To fix this, you might change
> the architecture of the driver to have one chip for the controller, but
> it's quite questionable change. Also how can you guarantee ordering of
> the enumeration? You probably need to *disable* SMP on the boot time.
> This will still be fragile as long as GPIO chip can be unbound at run
> time. Order can be changed.
> 
> So, the patch is good and the correct way to go.
> 
> P.S. The root cause is that hardware engineers and documentation writers
> do not consider their hardware in the multi-tasking, multi-user general
> purpose operating system, such as Linux. I believe the ideal fix is to fix the
> documentation (datasheet).

Some GPIO controllers (as Super-I/O) are multifunctional devices and
pins are multiplexed. Some can be configured to act as GPIOs and some
cannot. So there are holes. It is an hardware reality and not only an
issue due to poorly written documents (even if there are issues with
them too).

Today we work around these holes by splitting the GPIOs between several
chips. As a consequence "hardware" GPIO numbers don't exist in Linux. It
requires some work from a user to first find the chip a GPIO belongs to
and then compute the number. It is not terrible. But on some machines
with a lot of GPIO controllers and chips it can be quite challenging
(especially when ACPI is involved).

I am only saying it would be nice for Linux users if they could use
hardware GPIO numbers (i.e. as read in hardware documents).

But I understand everything that has been said by everyone and I agree.

Simon

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