linux-kernel - Re: [PATCH v3 0/6] RTL8231 GPIO expander support

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Message-ID: <401805ef27bb273d7aca4f3377b53b07@walle.cc>
Date:   Tue, 01 Jun 2021 13:41:57 +0200
From:   Michael Walle <michael@...le.cc>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Andrew Lunn <andrew@...n.ch>, Pavel Machek <pavel@....cz>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 0/6] RTL8231 GPIO expander support

Am 2021-06-01 12:51, schrieb Linus Walleij:
> On Tue, Jun 1, 2021 at 12:18 PM Michael Walle <michael@...le.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?

> So the regmap config is really important to get right.
> 
> IIUC one of the ambitions around Rust is to encode this
> in how memory is specified in the language. (I am still
> thinking about whether that is really a good idea or not.)

-- 
-michael