[RFC] drm/kms: control display brightness through drm_connector properties

Fri Apr 8 14:55:51 UTC 2022

Hi,

On 4/8/22 16:08, Alex Deucher wrote:
> On Fri, Apr 8, 2022 at 4:07 AM Daniel Vetter <daniel at ffwll.ch> wrote:
>>
>> On Thu, Apr 07, 2022 at 05:05:52PM -0400, Alex Deucher wrote:
>>> On Thu, Apr 7, 2022 at 1:43 PM Hans de Goede <hdegoede at redhat.com> wrote:
>>>>
>>>> Hi Simon,
>>>>
>>>> On 4/7/22 18:51, Simon Ser wrote:
>>>>> Very nice plan! Big +1 for the overall approach.
>>>>
>>>> Thanks.
>>>>
>>>>> On Thursday, April 7th, 2022 at 17:38, Hans de Goede <hdegoede at redhat.com> wrote:
>>>>>
>>>>>> The drm_connector brightness properties
>>>>>> =======================================
>>>>>>
>>>>>> bl_brightness: rw 0-int32_max property controlling the brightness setting
>>>>>> of the connected display. The actual maximum of this will be less then
>>>>>> int32_max and is given in bl_brightness_max.
>>>>>
>>>>> Do we need to split this up into two props for sw/hw state? The privacy screen
>>>>> stuff needed this, but you're pretty familiar with that. :)
>>>>
>>>> Luckily that won't be necessary, since the privacy-screen is a security
>>>> feature the firmware/embedded-controller may refuse our requests
>>>> (may temporarily lock-out changes) and/or may make changes without
>>>> us requesting them itself. Neither is really the case with the
>>>> brightness setting of displays.
>>>>
>>>>>> bl_brightness_max: ro 0-int32_max property giving the actual maximum
>>>>>> of the display's brightness setting. This will report 0 when brightness
>>>>>> control is not available (yet).
>>>>>
>>>>> I don't think we actually need that one. Integer KMS props all have a
>>>>> range which can be fetched via drmModeGetProperty. The max can be
>>>>> exposed via this range. Example with the existing alpha prop:
>>>>>
>>>>>     "alpha": range [0, UINT16_MAX] = 65535
>>>>
>>>> Right, I already knew that, which is why I explicitly added a range
>>>> to the props already. The problem is that the range must be set
>>>> before registering the connector and when the backlight driver
>>>> only shows up (much) later during boot then we don't know the
>>>> range when registering the connector. I guess we could "patch-up"
>>>> the range later. But AFAIK that would be a bit of abuse of the
>>>> property API as the range is intended to never change, not
>>>> even after hotplug uevents. At least atm there is no infra
>>>> in the kernel to change the range later.
>>>>
>>>> Which is why I added an explicit bl_brightness_max property
>>>> of which the value gives the actual effective maximum of the
>>>> brightness.
>>
>> Uh ... I'm not a huge fan tbh. The thing is, if we allow hotplugging
>> brightness control later on then we just perpetuate the nonsense we have
>> right now, forever.
>>
>> Imo we should support two kinds of drivers:
>>
>> - drivers which are non-crap, and make sure their backlight driver is
>>   loaded before they register the drm_device (or at least the
>>   drm_connector). For those we want the drm_connector->backlight pointer
>>   to bit static over the lifetime of the connector, and then we can also
>>   set up the brightness range correctly.
>>
>> - funny drivers which implement the glorious fallback dance which
>>   libbacklight implements currently in userspace. Imo for these drivers we
>>   should have a libbacklight_heuristics_backlight, which normalizes or
>>   whatever, and is also ways there. And then internally handles the
>>   fallback mess to the "right" backlight driver.
>>
>> We might have some gaps on acpi systems to make sure the drm driver can
>> wait for the backlight driver to show up, but that's about it.
>>
>> Hotplugging random pieces later on is really not how drivers work nowadays
>> with deferred probe and component framework and all that.
>>
>>>> I did consider using the range for this and updating it
>>>> on the fly I think nothing is really preventing us from
>>>> doing so, but it very much feels like abusing the generic
>>>> properties API.
>>>>
>>>>>> bl_brightness_0_is_min_brightness: ro, boolean
>>>>>> When this is set to true then it is safe to set brightness to 0
>>>>>> without worrying that this completely turns the backlight off causing
>>>>>> the screen to become unreadable. When this is false setting brightness
>>>>>> to 0 may turn the backlight off, but this is not guaranteed.
>>>>>> This will e.g. be true when directly driving a PWM and the video-BIOS
>>>>>> has provided a minimum (non 0) duty-cycle below which the driver will
>>>>>> never go.
>>>>>
>>>>> Hm. It's quite unfortunate that it's impossible to have strong guarantees
>>>>> here.
>>>>>
>>>>> Is there any way we can avoid this prop?
>>>>
>>>> Not really, the problem is that we really don't know if 0 is off
>>>> or min-brightness. In the given example where we actually never go
>>>> down to a duty-cycle of 0% because the video BIOS tables tell us
>>>> not to, we can be certain that setting the brightness prop to 0
>>>> will not turn of the backlight, since we then set the duty-cycle
>>>> to the VBT provided minimum. Note the intend here is to only set
>>>> the boolean to true if the VBT provided minimum is _not_ 0, 0
>>>> just means the vendor did not bother to provide a minimum.
>>>>
>>>> Currently e.g. GNOME never goes lower then something like 5%
>>>> of brightness_max to avoid accidentally turning the screen off.
>>>>
>>>> Turning the screen off is quite bad to do on e.g. tablets where
>>>> the GUI is the only way to undo the brightness change and now
>>>> the user can no longer see the GUI.
>>>>
>>>> The idea behind this boolean is to give e.g. GNOME a way to
>>>> know that it is safe to go down to 0% and for it to use
>>>> the entire range.
>>>
>>> Why not just make it policy that 0 is defined as minimum brightness,
>>> not off, and have all drivers conform to that?
>>
>> Because the backlight subsystem isn't as consistent on this, and it's been
>> an epic source of confusion since forever.
>>
>> What's worse, there's both userspace out there which assumes brightness =
>> 0 is a really fast dpms off _and_ userspace that assumes that brightness =
>> 0 is the lowest setting. Of course on different sets of machines.
>>
>> So yeah we're screwed. I have no idea how to get out of this.
> 
> Yes, but this is a new API.  So can't we do better?  Sure the old
> backlight interface is broken, but why carry around clunky workarounds
> for new interfaces?

Right we certainly need to define the behavior of the new API
clearly, so that userspace does not misuse / misinterpret it.

The intend is for brightness=0 to mean minimum brightness
to still be able to see what is on the screen. But the problem
is that in many cases the GPU driver directly controls a PWM
output, no minimum PWM value is given in the video BIOS tables
and actually setting the PWM to 0% dutycycle turns off the
screen.

So we can only promise a best-effort to make brightness=0
mean minimum brightness, combined with documenting that it
may turn off the backlight and that userspace _must_ never
depend on it turning off the backlight.

Also note that setting a direct PWM output to duty-cycle 0%
does not guarantee that the backlight goes off, this may be
an input for a special backlight LED driver IC like the
TI LP855x series which can have an internal lookup
table causing it to actually output a minimum brightness
when its PWM input is at 0% dutycycle.  So this is a case
where we just don't get enough info from the fw/hw to be able
to offer the guarantees which we would like to guarantee.

Regards,

Hans