xfree86: Let xf86RandR12CrtcComputeGamma() deal with non-power-of-2 sizes.

The assumption in the upsampling code was that the crtc->gamma_size
size of the crtc's gamma table is a power of two. This is true for
almost all current driver + gpu combos at least on Linux, with typical
sizes of 256, 512, 1024 or 4096 slots.

However, Intel Gen-11 Icelake and later are outliers, as their gamma
table has 2^18 + 1 slots, very big and not a power of two!

Try to make upsampling behave at least reasonable: Replicate the
last gamma value to fill up remaining crtc->gamma_red/green/blue
slots, which would normally stay uninitialized. This is important,
because while the intel display driver does not actually use all
2^18+1 values passed as part of a GAMMA_LUT, it does need the
very last slot, which would not get initialized by the old code.

This should hopefully create reasonable behaviour with Icelake+
but is untested on the actual Intel hw due to lack of suitable
hw.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>

hw/xfree86/modes/xf86RandR12.c

@@ -1255,8 +1255,8 @@ xf86RandR12CrtcComputeGamma(xf86CrtcPtr crtc, LOCO *palette,
 {
     int gamma_slots;
     unsigned shift;
-    CARD32 value;
     int i, j;
+    CARD32 value = 0;
 
     for (shift = 0; (gamma_size << shift) < (1 << 16); shift++);
 
@@ -1273,6 +1273,10 @@ xf86RandR12CrtcComputeGamma(xf86CrtcPtr crtc, LOCO *palette,
             for (j = 0; j < gamma_slots; j++)
                 crtc->gamma_red[i * gamma_slots + j] = value;
         }
+
+        /* Replicate last value until end of crtc for gamma_size not a power of 2 */
+        for (j = i * gamma_slots; j < crtc->gamma_size; j++)
+                crtc->gamma_red[j] = value;
     } else {
         /* Downsampling of larger palette to smaller hw lut size */
         for (i = 0; i < crtc->gamma_size; i++) {
@@ -1299,6 +1303,10 @@ xf86RandR12CrtcComputeGamma(xf86CrtcPtr crtc, LOCO *palette,
             for (j = 0; j < gamma_slots; j++)
                 crtc->gamma_green[i * gamma_slots + j] = value;
         }
+
+        /* Replicate last value until end of crtc for gamma_size not a power of 2 */
+        for (j = i * gamma_slots; j < crtc->gamma_size; j++)
+            crtc->gamma_green[j] = value;
     } else {
         /* Downsampling of larger palette to smaller hw lut size */
         for (i = 0; i < crtc->gamma_size; i++) {
@@ -1325,6 +1333,10 @@ xf86RandR12CrtcComputeGamma(xf86CrtcPtr crtc, LOCO *palette,
             for (j = 0; j < gamma_slots; j++)
                 crtc->gamma_blue[i * gamma_slots + j] = value;
         }
+
+        /* Replicate last value until end of crtc for gamma_size not a power of 2 */
+        for (j = i * gamma_slots; j < crtc->gamma_size; j++)
+            crtc->gamma_blue[j] = value;
     } else {
         /* Downsampling of larger palette to smaller hw lut size */
         for (i = 0; i < crtc->gamma_size; i++) {