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

[Hans de Goede]

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.

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.

> For instance if we can guarantee that the min level won't turn the screen
> completely off we could make the range start from 1 instead of 0.
> Or allow -1 to mean "minimum value, maybe completely off".

Right, the problem is we really don't know and when the range is
e.g. 0-65535 then something like 1 will almost always still just
turn the screen completely off. There will be a value of say like
150 or some such which is then the actual minimum value to still
get the backlight to light up at all. The problem is we have
no clue what the actual minimum is. And if the PWM output does
not directly drive the LEDs but is used as an input for some
LED backlight driver chip, that chip itself may have a lookup
table (which may also take care of doing perceived brightness
mapping) and may guarantee a minimum backlight even when given
a 0% duty cycle PWM signal...

This prop is sort of orthogonal to the generic change to
drm_connector props, so we could also do this later as a follow up
change. At a minimum when I code this up this should be in its
own commit(s) I believe.

But I do think having this will be useful for the above
GNOME example.

>> bl_brightness_control_method: ro, enum, possible values:
>> none: The GPU driver expects brightness control to be provided by another
>> driver and that driver has not loaded yet.
>> unknown: The underlying control mechanism is unknown.
>> pwm: The brightness property directly controls the duty-cycle of a PWM
>> output.
>> firmware: The brightness is controlled through firmware calls.
>> DDC/CI: The brightness is controlled through the DDC/CI protocol.
>> gmux: The brightness is controlled by the GMUX.
>> Note this enum may be extended in the future, so other values may
>> be read, these should be treated as "unknown".
>>
>> When brightness control becomes available after being reported
>> as not available before (bl_brightness_control_method=="none")
>> a uevent with CONNECTOR=<connector-id> and
>>
>> PROPERTY=<bl_brightness_control_method-id> will be generated
>>
>> at this point all the properties must be re-read.
>>
>> When/once brightness control is available then all the read-only
>> properties are fixed and will never change.
>>
>> Besides the "none" value for no driver having loaded yet,
>> the different bl_brightness_control_method values are intended for
>> (userspace) heuristics for such things as the brightness setting
>> linearly controlling electrical power or setting perceived brightness.
> 
> Can you elaborate? I don't know enough about brightness control to
> understand all of the implications here.

So after sending this email I was already thinking myself that this
one might not be the best idea. Another shortcoming of the current
backlight API is that it does not let userspace know if the
number is a linear control of the time the LEDs are on vs off
(assuming a LED backlight) or if some component already uses a
lookup table to make 0-100% be more of a linear scale in the
human perception, which is very much non linear. See e.g.:

https://www.sciencedirect.com/topics/computer-science/perceived-brightness

"refers to the perceived amount of light coming from self-luminous sources"
"Perceived brightness is a very nonlinear function of the amount of light emitted by a lamp."

The problem is that at the kernel level we have no idea if
we are controlling "the amount of light emitted" or
perceived brightness and it would be sorta nice for userspace
to know. So the idea here is/was to allow userspace to make some
educated guess here. E.g. a bl_brightness_control_method of "PWM"
hints strongly at "the amount of light emitted" (but this is
not true 100% of the time).  ATM userspace does not do any
"perceived brightness" curve correction so for the first
implementation of moving brightness control to drm properties
I believe it might be better to just park the whole
bl_brightness_control_method propery idea.

Which would leave the problem of communicating the control_method=="none"
case but we can just use bl_brightness_max == 0 for that.

Regards,

Hans