linux-kernel - In kernel virtual HID devices (was Future handling of complex RGB devices on Linux v3)

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Date: Mon, 25 Mar 2024 15:25:19 +0100
From: Hans de Goede <hdegoede@...hat.com>
To: Werner Sembach <wse@...edocomputers.com>
Cc: Lee Jones <lee@...nel.org>, jikos@...nel.org,
 linux-kernel@...r.kernel.org, Jelle van der Waa <jelle@...aa.nl>,
 Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 linux-input@...r.kernel.org, ojeda@...nel.org, linux-leds@...r.kernel.org,
 Pavel Machek <pavel@....cz>, Gregor Riepl <onitake@...il.com>,
 Benjamin Tissoires <bentiss@...nel.org>, Jiri Kosina <jikos@...nel.org>
Subject: In kernel virtual HID devices (was Future handling of complex RGB
 devices on Linux v3)

+Cc: Bentiss, Jiri

Hi Werner,

On 3/20/24 12:16 PM, Werner Sembach wrote:
> Hi Hans and the others,
> 
> Am 22.02.24 um 14:14 schrieb Werner Sembach:
>> Hi,
>>
>> Thanks everyone for the exhaustive feedback. And at least this thread is a good comprehesive reference for the future ^^.
>>
>> To recap the hopefully final UAPI for complex RGB lighting devices:
>>
>> - By default there is a singular /sys/class/leds/* entry that treats the device as if it was a single zone RGB keyboard backlight with no special effects.
>>
>> - There is an accompanying misc device with the sysfs attributes "name", "device_type",  "firmware_version_string", "serial_number" for device identification and "use_leds_uapi" that defaults to 1.
>>
>>     - If set to 0 the /sys/class/leds/* entry disappears. The driver should keep the last state the backlight was in active if possible.
>>
>>     - If set 1 it appears again. The driver should bring it back to a static 1 zone setting while avoiding flicker if possible.
>>
>> - If the device is not controllable by for example hidraw, the misc device might also implement additional ioctls or sysfs attributes to allow a more complex low level control for the keyboard backlight. This is will be a highly vendor specific UAPI.
> So in the OpenRGB issue thread https://learn.microsoft.com/en-us/windows-hardware/design/component-guidelines/dynamic-lighting-devices aka HID LampArray was mentioned. I did dismiss it because I thought that is only relevant for firmware, but I now stumbled upon the Virtual HID Framework (VHF) https://learn.microsoft.com/en-us/windows-hardware/drivers/hid/virtual-hid-framework--vhf- and now I wonder if an equivalent exists for Linux? A quick search did not yield any results for me.

Oh, interesting. I did not know about the HID LampArray API.

About your question about virtual HID devices, there is uHID,
but as the name suggests this allows userspace to emulate a HID
device.

In your case you want to do the emulation in kernel so that you
can translate the proprietary WMI calls to something HID LampArray
compatible.

I guess you could do this by defining your own HID transport driver,
like how e.g. the i2c-hid code defines 1 i2c-hid parent + 1 HID
"client" for each device which talks HID over i2c in the machine.

Bentiss, Jiri, do you have any input on this. Would something like
that be acceptable to you (just based on the concept at least) ?

Regards,

Hans