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Date: Mon, 25 Mar 2024 18:01:02 +0100
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Subject: Re: Future handling of complex RGB devices on Linux v3
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To: Hans de Goede <hdegoede@redhat.com>
Cc: Lee Jones <lee@kernel.org>, jikos@kernel.org,
 linux-kernel@vger.kernel.org, Jelle van der Waa <jelle@vdwaa.nl>,
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 linux-input@vger.kernel.org, ojeda@kernel.org, linux-leds@vger.kernel.org,
 Pavel Machek <pavel@ucw.cz>, Gregor Riepl <onitake@gmail.com>
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From: Werner Sembach <wse@tuxedocomputers.com>
In-Reply-To: <8264bce7-eda0-4e1c-8c72-a6a78ee3a7bd@redhat.com>
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Hi Hans,

Am 25.03.24 um 15:18 schrieb Hans de Goede:
> Hi Werner,
>
> On 3/19/24 4:18 PM, Werner Sembach wrote:
>> 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.
> I think that some shared ioctl for identifying devices which need a misc-device
> makes sense. But for existing hidraw supported keyboards there already is existing
> userspace support, which already retreives this in a hidraw specific manner.
>
> And I doubt that the existing userspace projects will add support for a new method
> which is only available on new kernels, while they will also need to keep the old
> method around to keep things working with older kernels.
>
> So at least for the hidraw case I see little value in exporting the same info
> in another way.

Ack on the no new device, but since we are adding use_leds_uapi to the parent 
device anyway, having the identification on non-HID devices wie sysfs entries 
would help even more in not creating a new device just to handle an ioctl.

Just to note here. When we go the LampArray route, everything is a HID device 
anyway.

>
>
>>>>       - 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.
> That is an interesting point. But I would expect any process/daemon which wants to
> reactivate the in kernel LED class support to be the same process as which
> deactivated it. So that daemon can simply canonicalize the /sys/class/leds/*/device
> symlink or canonicalize the entire /sys/class/leds/*/device/use_leds_uapi path
> and store the canonicalized version for when the daemon wants to reactivate things.

Ack

Regards,

Werner

>
> So I still think that having the miscdevice just for enumeration and
> use_leds_uapi purposes is overkill.
>
> Regards,
>
> Hans
>
>