linux-kernel - Re: [RFC PATCH 2/2] gpio: regmap: Bypass cache for aliased outputs

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Message-ID: <77ae9012af0565fe64e585043541081e48627566.camel@svanheule.net>
Date: Tue, 21 Oct 2025 11:01:47 +0200
From: Sander Vanheule <sander@...nheule.net>
To: Linus Walleij <linus.walleij@...aro.org>, Michael Walle
 <mwalle@...nel.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>, linux-gpio@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] gpio: regmap: Bypass cache for aliased outputs

Hi Linus, Michael,

On Tue, 2025-10-21 at 09:18 +0200, Linus Walleij wrote:
> On Mon, Oct 20, 2025 at 1:56 PM Sander Vanheule <sander@...nheule.net> wrote:
> 
> > GPIO chips often have data input and output registers aliased to the
> > same offset. The output register is non-valitile and could in theory be
> > cached. The input register however is volatile by nature and hence
> > should not be cached, resulting in different requirements for reads and
> > writes.
> > 
> > The generic gpiochip implementation stores a shadow value of the pin
> > output data, which is updated and written to hardware on output data
> > changes. Pin input values are always obtained by reading the aliased
> > data register from hardware.
> > 
> > For gpio-regmap the output data could be in multiple registers, but we
> > can use the regmap cache support to shadow the output values by marking
> > the data registers as non-volatile. By using regmap_read_bypassed() we
> > can still treat the input values as volatile, irrespective of the regmap
> > config. This ensures proper functioning of writing the output register
> > with regmap_write_bits(), which will then use and update the cache only
> > on data writes, gaining some performance from the cached output values.
> > 
> > Signed-off-by: Sander Vanheule <sander@...nheule.net>
> 
> That looks good to me for sure!
> Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

Thanks for the reviews, I'll prepare the full respin for the RTL8231 patches and
send them later today or tomorrow.

Best,
Sander