On 02/09/2018 10:45 AM, Michel Dänzer wrote:
On 2018-02-09 07:24 AM, Mario Kleiner wrote:
On 02/08/2018 03:55 PM, Michel Dänzer wrote:
On 2018-02-08 12:14 PM, Mario Kleiner wrote:
As it turns out, doing so will make any gamma table updates
silently fail, because xf86HandleColorMaps() hooks the
.LoadPalette function to xf86RandR12LoadPalette() if the
.gamma_set function is supported by the ddx, as is in our

Once xf86RandR12LoadPalette() has been called during server
startup, all palette and/or gamma table updates go through
xf86RandR12CrtcComputeGamma() to combine color palette
updates with gamma table updates into a proper hardware lut
for upload into the hw via the .gamma_set function/ioctl,
passing in a palette with palette_red/green/blue_size == the
size given by the visuals red/green/blueMask, which will
be == 1024 for a depth 30 screen.

That in turn is a problem, because the size of the hw lut
crtc->gamma_size is fixed to 256 slots on all kms-drivers
when using the legacy gamma_set ioctl, but
xf86RandR12CrtcComputeGamma() can only handle palettes of a
size <= the hw lut size. As the palette size of 1024 is greater
than the hw lut size of 256, the code silently fails
(gamma_slots == 0 in xf86RandR12CrtcComputeGamma()).

Skipping xf86HandleColormaps() on a depth > 24 screen disables
color palette handling, but keeps at least gamma table updates
via xf86vidmode extension and RandR working.

Sort of... It means xf86VidMode and RandR call directly into the driver
to set their LUTs, with no coordination between them, so whichever calls
into the driver last wins and clobbers the HW LUT.

Wouldn't that be desirable behavior in this case, assuming the client
that calls last is the one that wants something more specific?

If it was that simple... :)

Imagine you use something like Night Light / redshift which uses
xf86VidMode, and some kind of per-monitor gamma calibration tool which
uses RandR. Surely you want to see the combination of the two
transformations, not only one or the other, depending on which one has
ended up calling into the driver last (which could result in
flip-flopping between the two transformations, depending on how often
each tool (re-)sets its LUT).

Ok, thanks for the example. That makes sense. And creates new potential horror-scenario for me, given that my scientific users need very well defined gamma calibrations in many situations. Never thought about redshift or such messing with gamma behind the back of my RandR based code by having combined gamma lut's. The pitfalls never end...

It would be better to fix xf86RandR12CrtcComputeGamma instead.

But how? The current code can upsample an input palette < hw lut by
replicating input values onto multiple hw slots, which makes perfect
sense. But downsampling a bigger input palette, e.g., by just picking
every n'th slot, seems problematic to me. You lose information from the
palette in a way that might not be at all what the application wanted if
it went to the trouble of creating a custom palette, assuming any apps
would do anything meaningful with palettes on a depth 30 screen in the
first place.

You're right that the downsampling could theoretically be a problem. But
in practice, the LUTs should usually correspond to some kind of more or
less smooth curve, and I'm told display hardware tends to interpolate
between the LUT entries.

Depending on hw lut mode.

Unless the application just used the default palette or a
truecolor visual, like probably most do nowadays, which i assume would
be an identity mapping, so it doesn't matter if a palette is applied at

No colormaps means e.g. the gamma sliders in some games don't work.

Ok. I'll try if i can come up with some "take every n'th slot" subsampling in xf86RandR12CrtcComputeGamma, and some switching in modesetting ddx inside xf86HandleColormaps a la

xf86HandleColormaps(pScreen, 1 << sigRgbBits, 10, ...)

to switch maxColors between 256 and 1024 for depth 24 vs. 30.

That creates a bit of a problem for intel-ddx and nouveau-ddx though, as they need the same treatment, but need to work on both older servers and an upcoming v1.20. Or special case handling depending on server version.

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