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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Sun, 13 Jun 2021 00:29:49 +0300
Message-ID: <CAHp75VeOMb2xUJ+g2UQJnBybmehmYr0dGPEzDZObUGr=Q95+wA@mail.gmail.com>
Subject: Re: [PATCH v5 2/8] gpio: regmap: Add quirk for aliased data registers
To:     Sander Vanheule <sander@svanheule.net>
Cc:     Pavel Machek <pavel@ucw.cz>, Rob Herring <robh+dt@kernel.org>,
        Lee Jones <lee.jones@linaro.org>,
        Mark Brown <broonie@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Michael Walle <michael@walle.cc>,
        Linus Walleij <linus.walleij@linaro.org>,
        Bartosz Golaszewski <bgolaszewski@baylibre.com>,
        Linux LED Subsystem <linux-leds@vger.kernel.org>,
        devicetree <devicetree@vger.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
        Andrew Lunn <andrew@lunn.ch>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sun, Jun 13, 2021 at 12:13 AM Sander Vanheule <sander@svanheule.net> wrote:
>
> Some chips have the read-only input and write-only output data registers
> aliased to the same offset. As a result it is not possible to perform
> read-modify-writes on the output values, when a line is still configured
> as input.
>
> Add a quirk for aliased data registers, and document how the regmap
> should be set up for correct operation.

I still believe that there is no issue with gpio-regmap and we don't
need any quirk there.

The issue is in the regmap APIs (implementation) itself. Hardware with
the concept of reading X / writing Y at the same offset is okay per
se. regmaps doesn't support it properly and should be fixed (amended)
in a way that you provide this kind of register description thru
regmap configuration or so.

I expressed the idea of trying to implement regmap-8250 as an example
of the support for such hardware. And OTOH that this kind of hardware
is not unusual.

-- 
With Best Regards,
Andy Shevchenko