KMS backlight ABI proposition

[Daniel Thompson]

On 24/02/17 08:43, Jani Nikula wrote:
 >> - "Some PWM based backlight allow adjusting the PWM modulation
 >> frequency." you don't need a motivation for *why* I would want to
 >> change the mod freq on the fly, actually in my experience you
 >> shouldn't since this can lead to flickery backlights.
 >
 > The modulation frequency is usually an OEM design choice. The higher
 > the
 > frequency, less flicker, but also fewer user distinguishable levels of
 > backlight (signal rise/fall times come into play). Occasionally there
 > have been requests to be able to adjust the frequency. We can of course
 > decide that's an implementation detail and not let userspace change it.

In principle the backlight max level could also change during monitor 
hotplug; recent versions DDC/CI include a backlight properties. We have 
no driver for this right now but conversation is starting (and proper 
backlight support in DRM connectors will probably help it happen faster).

I'm afraid I have no idea if this is "real" or now. Right now I have no 
clue how many monitors implement it. For regular desktop monitors which 
mirror old school brightness/contrast in their UI then 0 wouldn't 
surprise me much ;-)


Daniel.