KMS backlight ABI proposition

[Hans de Goede]

Hi,

On 24-02-17 10:34, Jani Nikula wrote:
> On Fri, 24 Feb 2017, Hans de Goede <hdegoede at redhat.com> wrote:
>> On 24-02-17 09:43, Jani Nikula wrote:
>>> I don't think we have any good ideas how to solve the property max
>>> changing on the fly. But based on the discussion so far, it's starting
>>> to look like we'll need to study that more thoroughly.
>>
>> As I mentioned in another part of the thread, I think we can just return
>> -EBUSY if udev tries to change the binding when a drm-node is open
>> *and* the backlight/brightness property has been accessed (even if
>> read only) through that drm-node. We can reset the busy flag when
>> the (last open) drm-node gets closed.
>
> That's certainly easier than coming up with ways to notify the userspace
> about property range changes. The obvious downside is that you have to
> kill X to do the re-association. That's fine for when everything just
> works (i.e. everything happens before the drm node is opened), but for
> debugging it's a bit painful. Can we live with that?

Sure, debugging udev rules is somewhat tricky in general (requires
manually triggering udev). When asking users to test udev rule / hwdb
patches I usually just ask them to reboot.

>> I'm adding the *and* the backlight/brightness property has been accessed
>> condition so that udev can still do its thing while a boot splash
>> screen is active. This assumes that boot splash screens do not
>> touch the brightness.
>
> I presume udev will have to do its job before the drm node is opened, I
> think relying on the userspace not touching the brightness property is
> going to be racy/flaky.

Making sure that udev does its job before we show the splashscreen
is tricky, note the idea is that the splashscreen *never* touches
the brightness property and by the time regular X / wayland launches
udev should have had plenty of time to complete its job.

I do not know how hard it will be to add the code to detect triggering
the property being touched, but that is the best I can come up with
to still allow the bootsplash (e.g. plymouth) to use the drm-node.

Regards,

Hans