linux-kernel - Re: [PATCH v2 4/5] hwmon: (gxp_fan

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Date:   Thu, 1 Jun 2023 19:11:10 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "Hawkins, Nick" <nick.hawkins@....com>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        "Verdun, Jean-Marie" <verdun@....com>,
        "brgl@...ev.pl" <brgl@...ev.pl>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "jdelvare@...e.com" <jdelvare@...e.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>
Subject: Re: [PATCH v2 4/5] hwmon: (gxp_fan_ctrl) Provide fan info via gpio

On Thu, Jun 1, 2023 at 5:48 PM Hawkins, Nick <nick.hawkins@....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 could
> apply it to the PSU driver as well. the PSU driver which requires presence
> 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