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

[Julius Zint <julius@zint.sh>]

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On Wed, 6 Sep 2023, Hans de Goede wrote:

> 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
> 

Thank you for all this additional information. I have watched the talks
and read up upon the mail threads you`ve linked.

I will see if I can make the mapping to the DRM connector and plan to
update this patchset.

Thanks,

Julius