Re: [PATCH v3 1/1] backlight: hid_bl: Add VESA VCP HID backlight driver

[Hans de Goede <hdegoede@redhat.com>]

Hi Julius,

On 9/4/23 21:02, Julius Zint wrote:
> 
> 
> On Mon, 4 Sep 2023, Thomas Weißschuh wrote:
> 
>> +Cc Hans who ins involved with the backlight subsystem
>>
>> Hi Julius,
>>
>> today I stumbled upon a mail from Hans [0], which explains that the
>> backlight subsystem is not actually a good fit (yet?) for external
>> displays.
>>
>> It seems a new API is in the works that would better fit, but I'm not
>> sure about the state of this API. Maybe Hans can clarify.
>>
>> This also ties back to my review question how userspace can figure out
>> to which display a backlight devices applies. So far it can not.
>>
>> [0] https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@redhat.com/
>>
> 
> Hi Thomas,
> 
> thanks for the hint. I will make sure to give this a proper read and
> see, if it fits my use case better then the current backlight subsystem.

Note the actual proposal for the new usespace API for display brightness
control is here:

https://lore.kernel.org/dri-devel/b61d3eeb-6213-afac-2e70-7b9791c86d2e@redhat.com/

I have finished / stabilized the backlight code refactor which I landed
in 6.1, which is a prerequisite for the above proposal. But I have not
been able to make time to actually implement the above proposal; and
I don't know when I will be able to make time for this.

> Especially since I wasnt able to properly address your other review
> comments for now. You are right that the name should align better with
> the kernel module and also, that it is possible for multiple displays to
> be attached.
> 
> In its current state, this would mean that you could only control the
> backlight for the first HID device (enough for me :-).
> 
> The systemd-backlight@.service uses not only the file name, but also the
> full bus path for storing/restoring backlights. I did not yet get around
> to see how the desktops handle brightness control, but since the
> systemd-backlight@.service already uses the name, its important to stay
> the same over multiple boots.
> 
> I would be able to get a handle on the underlying USB device and use the
> serial to uniquely (and persistently) name the backlight. But it does
> feel hacky doing it this way.

So mutter (gnome-shell compositor library) has a similar issue when saving
monitor layouts and I can tell you beforehand that monitor serial numbers
by themselves are not unique enough. Some models just report 123456789
as serial and if you have a dual-monitor setup with 2 such monitors
and name the backlight class device <serial>-vcp-hid or something like that
you will still end up with 2 identical names.

To avoid this when saving monitor layouts mutter saves both the port
to which the monitor is attached (e.g. DP-1 DP-2) and the serialnumber
and on startup / monitor hotplug when it checks to see if it has saved
layout info for the monitor it checks the port+serialnr combination.

So what I think you should do is figure out a way to map which
VCP HID device maps to which drm-connector and then use
the connector-name + serial-nr to generate the backlight device name.

We will need the mapping the a drm-connector object anyway for
the new brightness API proposal from above.

Note this does NOT solve the fact that registering a new backlight
class device for an external monitor on a laptop will hopelessly
confuse userspace, see:

https://lore.kernel.org/lkml/7f2d88de-60c5-e2ff-9b22-acba35cfdfb6@redhat.com/

Regards,

Hans