On Tue, Jun 11, 2019 at 03:30:19PM -0700, Matthias Kaehlcke wrote: > On Tue, Jun 11, 2019 at 09:55:30AM -0700, Brian Norris wrote: > > On Tue, Jun 11, 2019 at 3:49 AM Daniel Thompson > > <daniel.thompson@xxxxxxxxxx> wrote: > > > This is a long standing flaw in the backlight interfaces. AFAIK generic > > > userspaces end up with a (flawed) heuristic. > > > > Bingo! Would be nice if we could start to fix this long-standing flaw. > > Agreed! > > How could a fix look like, a sysfs attribute? Would a boolean value > like 'logarithmic_scale' or 'linear_scale' be enough or could more > granularity be needed? Certainly "linear" (this device will work more or less correctly if the userspace applies perceptual curves). Not sure about logarithmic since what is actually useful is something that is "perceptually linear" (logarithmic is merely a way to approximate that). I do wonder about a compatible string like most-detailed to least-detailed description. This for a PWM with the auto-generated tables we'd see something like: cie-1991,perceptual,non-linear For something that is non-linear but we are not sure what its tables are we can offer just "non-linear". > > The new attribute could be optional (it only exists if explicitly > specified by the driver) or be set to a default based on a heuristic > if not specified and be 'fixed' on a case by case basis. The latter > might violate "don't break userspace" though, so I'm not sure it's a > good idea. I think we should avoid any heuristic! There are several drivers and we may not be able to work through all of them and make the correct decision. Instead one valid value for the sysfs should be "unknown" and this be the default for drivers we have not analysed (this also makes it easy to introduce change here). We should only set the property to something else for drivers that have been reviewed. There could be a special case for pwm_bl.c in that I'm prepared to assume that the hardware components downstream of the PWM have a roughly linear response and that if the user provided tables that their function is to provide a perceptually comfortable response. Daniel.
