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

[Hans de Goede]

Hi Simon,

On 4/8/22 10:22, Simon Ser wrote:
> Hi Hans,
> 
> Thanks for your details replies!
> 
> On Thursday, April 7th, 2022 at 19:43, Hans de Goede <hdegoede at redhat.com> wrote:
> 
>>> 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.
> 
> Cool, makes sense to me!
> 
>>>> 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.
> 
> Since this is new uAPI there's no concern about backwards compat here. So it's
> pretty much a matter of how we want the uAPI to look like. I was suggesting
> using a range because it's self-describing, but maybe it's an abuse.
> 
> Daniel Vetter, what do you think? If a property's range is going to be updated
> on the fly, should we go for it, or should we use a separate prop to describe
> the max value?

Daniel, as explained in my replies to you, I do believe that dynamically
updating the range is unavoidable. Especially once we also add support
for setting a monitor's brightness over DDC/CI.

Since external monitors (with/without DDC/CI support) can come and go and
since properties cannot be added/removed after connector registration, we
need a way to let userspace know if brightness control is actually available
or not and what the range is. We can use a max value of 0 for not available
and other values for the actual range, which I believe is a sane API.

But the question from Simon then still remains, do we update the range
of the property on the fly, followed by a connector hotplug uevent; or
do we use a separate brightness_max property for this?

Note that as Rasterman indicated that with DDC/CI support we could also
offer other properties (for which I see no reason atm) and if we do
say also add a contrast property over DDC/CI then if we go the
separate brightness_max route that would mean adding 2 props for
each setting which we want to support.

Regards,

Hans