Future handling of complex RGB devices on Linux v3

[Werner Sembach]

Am 20.03.24 um 12:16 schrieb Werner Sembach:
> 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.
Is this what I have been searching for? https://docs.kernel.org/usb/gadget_hid.html
>
> If a virtual HID device is possible and the WMI interface can reasonably be 
> mapped to the LampArray API this might be the best starting point:
>
> - Implement a Virtual HID device with LampArray
>
> - Implement LampArray in OpenRGB
>
> - (Optional) Implement a generic LampArray leds subsystem driver that maps to 
> the single zone control and ads the use_leds_uapi sysfs switch to the virtual 
> HID device
>
> - (Optional) Implement vendor specific controls for 
> AutonomousMode/built-in-firmware-effects via custom HID commands
>
> - (Optional) Implement Virtual HID devices for actual HID devices that don't 
> support LampArray in firmware (Open question: How to prevent userspace/OpenRGB 
> from interacting with original HID when the virtual HID device is not in 
> AutonomousMode? How to associate the original and virtual HID device to each 
> other that userspace can easily recognize this relation? Or is it possible to 
> add virtual HID commands on top of a real HID device, making it look exactly 
> like the pure virtual devices for userspace?)
>
> The LampArray API hereby is made with the intention to be used for multi leds 
> devices, like per-key-backlight keyboards, unlike the leds UAPI. And it is 
> coming anyway with new RGB devices soon. So it would not conflict with a 
> "don't introduce unnecessary UAPI interfaces" principle. Are there any plans 
> already of Wrapping LampArray in some kind ioctl/sysfs API? Or just have it 
> used via hidraw? Or was there no discussion about it till now?
>
> Regards,
>
> Werner
>
>>
>>     - The actual logic interacting with this low level UAPI is implemented by 
>> a userspace driver
>>
>> Implementation wise: For the creation of the misc device with the 
>> use_leds_uapi switch a helper function/macro might be useful? Wonder if it 
>> should go into leds.h, led-class-multicolor.h, or a new header file?
>>
>> - Out of my head it would look something like this:
>>
>> led_classdev_add_optional_misc_control(
>>     struct led_classdev *led_cdev,
>>     char* name,
>>     char* device_type,
>>     char* firmware_version_string,
>>     char* serial_number,
>>     void (*deregister_led)(struct led_classdev *led_cdev),
>>     void (*reregister_led)(struct led_classdev *led_cdev))
>>
>> Let me know your thoughts and hopefully I can start implementing it soon for 
>> one of our devices.
>>
>> Kind regards,
>>
>> Werner Sembach
>>