On Mon, Aug 19, 2019 at 12:16:13PM +0100, Daniel Thompson wrote: > On Mon, Aug 19, 2019 at 12:21:27PM +0200, Uwe Kleine-König wrote: > > > > > In an ideal world the backlight interface would be consistent as you > > > > > suggest, however there are plenty of existing devices which use the > > > > > 'other' scaling (regardless of which is chosen as the 'correct' > > > > > one). Userspace still has to deal with these. And changing previously > > > > > 'logarithmic' drivers to linear (or viceversa) may 'break' userspace, > > > > > when it keeps using its 'old' scaling, which now isn't correct anymore. > > > > > > > > It might be subjective, or maybe I'm just too optimistic, but I think if > > > > there was no policy before about the meaning of > > > > > > > > echo 17 > brightness > > > > > > > > other than "brighter than lower values and darker than higher ones" > > > > introducing (say) the scale is intended to represent a linear brightness > > > > curve is ok. > > > > > > > > Unless userspace jumps through hoops and tries to identify the actual > > > > device it is running on it is wrong on some machines anyhow and we're > > > > only shifting the set of affected machines with a tighter policy (until > > > > that userspace application is fixed). > > > > > > I believe that there are two common approaches by userspace at present: > > > > > > 1. Assume the scale is perceptual and we can directly map a slider > > > to the backlight value. This is common simply because most ACPI > > > backlights are perceptual and therefore when tested in a laptop > > > it works OK. > > > > > > 2. Assume that is max brightness is small (e.g. ACPI) then the > > > scale is perceptual and if the max brightness is large (e.g. > > > a PWM) then the scale is linear and apply a correction > > > function between the slider and the control. > > > > > > That historic baggage makes is diffcult to "just define a standardized > > > scale"... especially given that if we selected a standardized scale we > > > would probably want a perceptual scale with lots of steps (e.g. break > > > the heuristic). > > > > With "perceptual" you mean that logarithmic stuff, right? > > Human perception is fairly complex so it depends how strict you want to > get. At the end of the day what it means is you can map a slider UI > component directly to the backlight range and it will feel right. Thus > a userspace that maps directly to a slider *is* assuming the scale > is perceptual. I have problems to declare something as "the right thing to do" that depends on feeling of users. I much prefer to make a technical device authoritative here (in this case a device that measures emitted light). Other than that I don't have enough experience with backlights to judge the decisions that have to be done and so will stop my participation in this thread now. Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | http://www.pengutronix.de/ | _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
