Future handling of complex RGB devices on Linux v3

[Werner Sembach]

Hi Hans,

Am 18.03.24 um 12:11 schrieb Hans de Goede:
> Hi Werner,
>
> Sorry for the late reply.
np
>
> On 2/22/24 2:14 PM, Werner Sembach wrote:
>> 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.
> Ack this sounds good.
>
>> - 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.
> You don't need a char misc device here, you can just make this sysfs attributes on the LED class device's parent device by using device_driver.dev_groups. Please don't use a char misc device just to attach sysfs attributes to it.
>
> Also I'm a bit unsure about most of these attributes, "use_leds_uapi" was discussed before
> and makes sense. But at least for HID devices the rest of this info is already available
> in sysfs attributes on the HID devices (things like vendor and product id) and since the
> userspace code needs per device code to drive the kbd anyways it can also have device
> specific code to retrieve all this info, so the other sysfs attributes just feel like
> duplicating information. Also there already are a ton of existing hidraw userspace rgbkbd
> drivers which already get this info from other places.
The parent device can be a hid device, a wmi device, a platform device etc. so I 
thought it would be nice to have some unified way for identification.
>
>>      - 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.
> Ack, if this finds it way into some documentation (which it should) please make it
> clear that this is about the "use_leds_uapi" sysfs attribute.
Ack
>
>> - 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.
> IMHO this is the only case where actually using a misc device makes sense, so that
> you have a chardev to do the ioctls on. misc-device-s should really only be used
> when you need a chardev under /dev . Since you don't need the chardev for the e.g.
> hidraw case you should not use a miscdev there IMHO.

My train of thought was that it would be nice to have the use_leds_uapi at a 
somewhat unified location in sysfs. The parent device can be of any kind, like I 
mentioned above, and while for deactivating it can easily be found via 
/sys/class/leds/*/device/. For reactivating, the leds entry doesn't exist.

>
>>      - 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?
> See above, I don't think we want the misc device for the hidraw case, at which
> point I think the helper becomes unnecessary since just a single sysfs write
> callback is necessary.
>
> Also for adding new sysfs attributes it is strongly encouraged to use
> device_driver.dev_groups so that the device core handled registering /
> unregistering the sysfs attributes which fixes a bunch of races; and
> using device_driver.dev_groups does not mix well with a helper as you've
> suggested.
Ok, I will see when I start implementing.
>
>> - 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.
> I think overall the plan sounds good, with my main suggested change
> being to not use an unnecessary misc device for the hid-raw case.
>
> Regards,
>
> Hans
>
>
Thanks for the feedback,

Werner