linux-kernel - Re: [RFC 0/1] hwmon: (hp-wmi-sensors) Support autoloading

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Message-ID: <Zfkm71dmnRsdmYJz@equiv.tech>
Date: Mon, 18 Mar 2024 22:47:27 -0700
From: James Seo <james@...iv.tech>
To: Armin Wolf <W_Armin@....de>
Cc: jdelvare@...e.com, linux@...ck-us.net, linux-hwmon@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 0/1] hwmon: (hp-wmi-sensors) Support autoloading

On Mon, Mar 18, 2024 at 10:57:31PM +0100, Armin Wolf wrote:
> Currently, the hp-wmi-sensors driver needs to be loaded manually
> on supported machines. This however is unnecessary since the WMI
> id table can be used to support autoloading.
> 
> However the driver might conflict with the hp-wmi driver since both
> seem to use the same WMI GUID for registering notify handler.
> 
> I am thus submitting this patch as an RFC for now.
> 
> Armin Wolf (1):
>   hwmon: (hp-wmi-sensors) Add missing MODULE_DEVICE_TABLE()
> 
>  drivers/hwmon/hp-wmi-sensors.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> --
> 2.39.2
>

Autoloading was deliberately left out for now because of the GUID
conflict with hp-wmi's WMI notify handler.

HP's GUID reuse across product lines for different types of WMI
objects with different names and shapes means that with a patch like
this, many systems that should only load hp-wmi-sensors but not
hp-wmi will try to autoload both. (Perhaps all of them; I want to say
that the GUID 5FB7F034-2C63-45e9-BE91-3D44E2C707E4, which is the
second of the two GUIDs that hp-wmi uses to autoload, exists on every
HP system I've examined.)

Meanwhile, hp-wmi does various other platform things, and there's so
much hardware out there that who knows, maybe there are some systems
that really should load both. I don't think so but I can't rule it
out.

Unlike hp-wmi-sensors, hp-wmi doesn't survive failure to install its
notify handler, which sets up a potential race condition depending on
when hp-wmi and hp-wmi-sensors loads on a given system.

Therefore, I intended to add autoloading at the same time as
converting hp-wmi-sensors to use the bus-based WMI interface once
aggregate WMI devices are better supported.

As you mentioned [1], I ran into issues when I tried to do the
conversion by simply adding the GUID to struct wmi_driver.id_table.
That resulted in two separate independent instances of hp_wmi_sensors
being loaded, which isn't what I wanted.

[1] https://lore.kernel.org/linux-hwmon/cd81a7d6-4b81-f074-1f28-6d1b5300b937@gmx.de/