linux-kernel - Re: [PATCH 2/8] gpio: tqmx86: introduce shadow register for GPIO output value

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Date: Wed, 29 May 2024 14:02:55 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: Linus Walleij <linus.walleij@...aro.org>,
	Bartosz Golaszewski <brgl@...ev.pl>, linux-gpio@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Gregor Herburger <gregor.herburger@...group.com>,
	linux@...tq-group.com
Subject: Re: [PATCH 2/8] gpio: tqmx86: introduce shadow register for GPIO
 output value

On Wed, May 29, 2024 at 09:45:14AM +0200, Matthias Schiffer wrote:
> The TQMx86 GPIO controller uses the same register address for input and
> output data. Reading the register will always return current inputs
> rather than the previously set outputs (regardless of the current
> direction setting). Therefore, using a RMW pattern does not make sense
> when setting output values. Instead, the previously set output register
> value needs to be stored as a shadow register.
> 
> As there is no reliable way to get the current output values from the
> hardware, also initialize all channels to 0, to ensure that stored and
> actual output values match. This should usually not have any effect in
> practise, as the TQMx86 UEFI sets all outputs to 0 during boot.
> 
> Also prepare for extension of the driver to more than 8 GPIOs by using
> DECLARE_BITMAP.
> 
> Fixes: b868db94a6a7 ("gpio: tqmx86: Add GPIO from for this IO controller")
> Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>

Reviewed-by: Andrew Lunn <andrew@...n.ch>

    Andrew