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From:   Linus Walleij <linus.walleij@linaro.org>
Date:   Thu, 1 Jun 2023 19:11:10 +0200
Message-ID: <CACRpkdarJCSCif+r1e_jXbbAgv03OgGAO6pkW9x-yiYZJxdGeQ@mail.gmail.com>
Subject: Re: [PATCH v2 4/5] hwmon: (gxp_fan_ctrl) Provide fan info via gpio
To:     "Hawkins, Nick" <nick.hawkins@hpe.com>
Cc:     Guenter Roeck <linux@roeck-us.net>,
        "Verdun, Jean-Marie" <verdun@hpe.com>,
        "brgl@bgdev.pl" <brgl@bgdev.pl>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "krzysztof.kozlowski+dt@linaro.org" 
        <krzysztof.kozlowski+dt@linaro.org>,
        "jdelvare@suse.com" <jdelvare@suse.com>,
        "andy.shevchenko@gmail.com" <andy.shevchenko@gmail.com>,
        "linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hwmon@vger.kernel.org" <linux-hwmon@vger.kernel.org>
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On Thu, Jun 1, 2023 at 5:48=E2=80=AFPM Hawkins, Nick <nick.hawkins@hpe.com>=
 wrote:

> Thank you for your valuable feedback with the solutions you have provided=
.
> Before I proceed though I have a quick query about the fan driver.
> If I were to let the user space "own" gpio pins, would it be permissible =
for
> the userspace to feed a kernel driver data via sysfs?
>
> Ex:
> GPIO Driver -> (OpenBMC) -> Fandriver (sysfs).
>
> Here the GPIO driver would provide fan presence information to OpenBMC
> and then OpenBMC would provide fan presence info to the fan driver.

But why? Don't be so obsessed about userspace doing stuff using
sysfs, usually it is a better idea to let the kernel handle hardware.

I think this is a simple thermal zone you can define in the device
tree as indicated in my previous comment.

> If it were permissible to provide data to the driver via this method I co=
uld
> apply it to the PSU driver as well. the PSU driver which requires presenc=
e
> info to verify a PSU is inserted / removed.

It feels like you are looking for a way for two drivers to communicate
with each other.

This can be done several ways, the most straight-forward is notifiers.
include/linux/notifier.h

Yours,
Linus Walleij