Hello Daniel, On Mon, Aug 19, 2019 at 10:50:37AM +0100, Daniel Thompson wrote: > On Mon, Aug 19, 2019 at 07:46:28AM +0200, Uwe Kleine-König wrote: > > Hello Matthias, > > > > On Fri, Aug 16, 2019 at 02:10:51PM -0700, Matthias Kaehlcke wrote: > > > On Fri, Aug 16, 2019 at 09:47:54PM +0200, Uwe Kleine-König wrote: > > > > On Fri, Aug 16, 2019 at 10:51:57AM -0700, Matthias Kaehlcke wrote: > > > > > Hi Uwe, > > > > > > > > > > On Fri, Aug 16, 2019 at 06:51:48PM +0200, Uwe Kleine-König wrote: > > > > > > On Tue, Jul 09, 2019 at 12:00:05PM -0700, Matthias Kaehlcke wrote: > > > > > > > Backlight brightness curves can have different shapes. The two main > > > > > > > types are linear and non-linear curves. The human eye doesn't > > > > > > > perceive linearly increasing/decreasing brightness as linear (see > > > > > > > also 88ba95bedb79 "backlight: pwm_bl: Compute brightness of LED > > > > > > > linearly to human eye"), hence many backlights use non-linear (often > > > > > > > logarithmic) brightness curves. The type of curve currently is opaque > > > > > > > to userspace, so userspace often uses more or less reliable heuristics > > > > > > > (like the number of brightness levels) to decide whether to treat a > > > > > > > backlight device as linear or non-linear. > > > > > > > > > > > > > > Export the type of the brightness curve via the new sysfs attribute > > > > > > > 'scale'. The value of the attribute can be 'linear', 'non-linear' or > > > > > > > 'unknown'. For devices that don't provide information about the scale > > > > > > > of their brightness curve the value of the 'scale' attribute is 'unknown'. > > > > > > > > > > > > > > Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx> > > > > > > > > > > > > I wonder what kind of problem you are solving here. Can you describe > > > > > > that in a few words? > > > > > > > > > > The human eye perceives brightness in a logarithmic manner. For > > > > > backlights with a linear brightness curve brightness controls like > > > > > sliders need to use a mapping to achieve a behavior that is perceived > > > > > as linear-ish (more details: http://www.pathwaylighting.com/products/downloads/brochure/technical_materials_1466797044_Linear+vs+Logarithmic+Dimming+White+Paper.pdf) > > > > > > > > > > As of now userspace doesn't have information about the type of the > > > > > brightness curve, and often uses heuristics to make a guess, which may > > > > > be right most of the time, but not always. The new attribute eliminates > > > > > the need to guess. > > > > > > > > This is about backlights right? So the kernel provides to userspace an > > > > interval [0, x] for some x and depending on the physics of the the > > > > backlight configuring x/2 (probably?) either means 50% measured light or > > > > 50% perceived light, right? > > > > > > correct > > > > > > > I wonder if it would be possible instead of giving different backlight > > > > implementations the freedom to use either linear or logarithmic (or > > > > quadratic?) scaling and tell userspace which of the options were picked > > > > require the drivers to provide a (say) linear scaling and then userspace > > > > wouldn't need to care about the exact physics. > > > > > > 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? I would tend to go for linear because this is easily measureable and also is straight forward to implement in the usual cases (attention: I assume that "usual" means something like PWM and I don't know much about the physics of backlights but just assume that a PWM will create a linear mapping). > > And the big upside is that in the end (i.e. when all kernel drivers and > > userspace applications are adapted to provide/consume the "correct" > > curve) the result is simpler. > > My view is that this convergence will eventually be achieved but it will > happen through the obsolescence of the backlight sysfs interface. The > sysfs interface has other flaws, in particular no integration with the > DRM connector API. > > Thus I would expect an alternative interface to emerge, most likely as > part of the DRM connector API. I'd expect such a new API to a > perceptual scale and to have a fixed max brightness with enough > steps to support animated backlight effects (IIRC 0..100 has been > proposed in the past) Then work on the new stuff instead of making the old stuff (that is intended to die) harder to use correctly? > In the mean time getting the existing collection of backlight drivers > marked up as linear/logarithmic/etc will ease the introduction of that > API because, within the kernel, we might have gathered enough knowledge > to have some hope of correctly mapping each backlight onto a > standardized scale. It would be enough to do this in a code comment then. That would come without the need to adapt the old userspace API. Also when the old solution works at 95% instead of 90% before, it will resist harder to dying. 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
