Hi Daniel, On Wed, Jun 12, 2019 at 12:03:25PM +0100, Daniel Thompson wrote: > 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". Thanks for the feedback! It seems clear that we want a string for the added flexibility. I can work on a patch with the compatible string like description you suggested and we can discuss in the review if we want to go with that or prefer something else. > > 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. Agreed > 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). An "unknown" value sounds good, it allows userspace to just do what it did/would hace done before this attribute existed. > 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. Unfortunately this isn't universally true :( At least several Chrome OS devices use a linear brightness scale and userspace does the transformation in the animated slider. A quick 'git grep -A10 brightness-levels arch' suggests that there are multiple other devices/platforms using a linear scale. We could treat devices with a predefined brightness table as "unknown", unless there is a (new optional) DT property that indicates the type of the scale. Cheers Matthias
