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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Tue, 1 Jun 2021 13:48:43 +0200
Message-ID: <CACRpkdYSMjDaRY2chbFyBTJ4jeAjPtXX3+VW7MHu-wH2QoAfPA@mail.gmail.com>
Subject: Re: [PATCH v3 0/6] RTL8231 GPIO expander support
To:     Michael Walle <michael@walle.cc>
Cc:     Andy Shevchenko <andy.shevchenko@gmail.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Sander Vanheule <sander@svanheule.net>,
        Andrew Lunn <andrew@lunn.ch>, 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>,
        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>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jun 1, 2021 at 1:41 PM Michael Walle <michael@walle.cc> wrote:
> Am 2021-06-01 12:51, schrieb Linus Walleij:
> > On Tue, Jun 1, 2021 at 12:18 PM Michael Walle <michael@walle.cc> wrote:
> >> Am 2021-06-01 11:59, schrieb Linus Walleij:
> >
> >> > Just regarding all registers/memory cells in a register page
> >> > as default volatile (which is what we do a lot of the time)
> >> > has its upsides: bugs like this doesn't happen.
> >>
> >> I don't think this is the bug here. If it is really a write-only
> >> register
> >> the problem is the read in RMW. Because reading the register will
> >> return
> >> the input value instead of the (previously written) output value.
> >
> > True that. Write and read semantics differ on the register.
> >
> > Volatile is used for this and some other things,
> > like for example interrupts being cleared when a register
> > is read so it is strictly read-once.
>
> Isn't that what precious is for?

I never figured that one out. But I assume you are right.

Proper regmap semantics documentation is forthcoming! ;)

Yours,
Linus Walleij