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@xxxxxxxxxx> wrote: > >>> On Thursday, April 7th, 2022 at 17:38, Hans de Goede <hdegoede@xxxxxxxxxx> 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
