linux-kernel - Re: [PATCH] gpio: 74x164: Enable output pins after registers are reset

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Date: Tue, 27 Feb 2024 16:53:31 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Arturas Moskvinas <arturas.moskvinas@...il.com>,
	linus.walleij@...aro.org, u.kleine-koenig@...gutronix.de,
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gpio: 74x164: Enable output pins after registers are
 reset

On Tue, Feb 27, 2024 at 02:14:40PM +0100, Bartosz Golaszewski wrote:
> On Tue, Feb 27, 2024 at 7:58 AM Arturas Moskvinas
> <arturas.moskvinas@...il.com> wrote:
> > On 2/26/24 16:01, Andy Shevchenko wrote:
> > > On Mon, Feb 26, 2024 at 03:46:56PM +0200, Arturas Moskvinas wrote:
> > >> Move output enabling after chip registers are cleared.
> > > Does this fix anything? If so, maybe elaborate a bit the potential behavioural
> > > changes on the real lines.
> >
> > Chip outputs are enabled[1] before actual reset is performed[2] which
> > might cause pin output value to flip flop if previous pin value was set
> > to 1 in chip. Change fixes that behavior by making sure chip is fully
> > reset before all outputs are enabled.
> >
> > Flip-flop can be noticed when module is removed and inserted again and
> > one of the pins was changed to 1 before removal. 100 microsecond
> > flipping is noticeable on oscilloscope (100khz SPI bus).
> >
> > For a properly reset chip - output is enabled around 100 microseconds
> > (on 100khz SPI bus) later during probing process hence should be
> > irrelevant behavioral change.
> >
> > [1] -
> > https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L130
> > [2] -
> > https://elixir.bootlin.com/linux/v6.7.4/source/drivers/gpio/gpio-74x164.c#L150
> >
> > Arturas Moskvinas
> 
> And this is precisely the kind of information that needs to go into
> commit messages. I can tell *what* you're doing by looking at the
> code. What I can't tell is *why*.

+1. Please, add this to the commit message of v2, also try to find the commit
that you can mark to be fixed with help of Fixes tag.

-- 
With Best Regards,
Andy Shevchenko