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

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Message-ID: <ZfowhGaCWffQ2N1K@equiv.tech>
Date: Tue, 19 Mar 2024 17:40:36 -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 Tue, Mar 19, 2024 at 02:00:06PM +0100, Armin Wolf wrote:
> Am 19.03.24 um 06:47 schrieb James Seo:
> 
>> 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.
> 
> After taking a look at a ACPI table dump of a HP machine, i noticed that
> HPBIOS_BIOSEvent has the GUID 2B814318-4BE8-4707-9D84-A190A859B5D0, which is
> different than the event GUID used by hp-wmi.
> 
> According your comment in hp_wmi_notify(), i assume that some machines have
> mixed-up event GUIDs.

I investigated further. Every HP machine in the Linux Hardware Database that
has \\.\root\WMI\hpqBEvnt at 95F24279-4D7B-4334-9387-ACCDC67EF61C also has
\\.\root\WMI\HPBIOS_BIOSEvent at 2B814318-4BE8-4707-9D84-A190A859B5D0.

> I thing it would be best to create a separate WMI driver for the event and
> use a notifier chain (see include/linux/notifier.h) to distribute the event data.
> 
> In case of event GUID 95F24279-4D7B-4334-9387-ACCDC67EF61C, both hp-wmi and
> hp-wmi-sensors can subscribe on this notifier and receive event data without
> stepping on each other's toes.
> 
> The same can be done for the event GUID 2B814318-4BE8-4707-9D84-A190A859B5D0,
> with a separate notifier chain.
> 
> This would decouple the event handling from the event data consumers, allowing
> both hp-wmi and hp-wmi-sensors to coexist.

No objections from me for this specific use case to work around the GUID conflict.
hp-wmi-sensors should indeed subscribe on 2B814318-4BE8-4707-9D84-A190A859B5D0
for some of those machines.

Any ideas for getting rid of wmi_query_block() for fetching
\\.\root\HP\InstrumentedBIOS\HPBIOS_PlatformEvents? I know other drivers are
also using it for getting blocks other than their "main" GUID.

> I can provide a prototype implementation, but unfortunately i have no HP machine
> myself for testing. But i might be able to find one to test my changes.

Happy to test. (Also happy to try it myself, but I'd need some help.)

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