drm/i915: Make sure wm block/lines are non-decreasing
The watermark algorithm sometimes produces results where higher
watermark levels have smaller blocks/lines watermarks than the lower
levels. That doesn't really make sense as the corresponding latencies
are supposed to be non-decreasing. It's unclear how the hardware
responds to such watermark values, so it seems better to avoid that
case and just make sure the values are always non-decreasing.
Here's an example how things change for such a case on a GLK NUC:
[PLANE:70:cursor A] level wm0, wm1, wm2, wm3, wm4, wm5, wm6, wm7, twm, swm, stwm -> *wm0,*wm1,*wm2,*wm3,*wm4,*wm5,*wm6,*wm7,*twm, swm, stwm
[PLANE:70:cursor A] lines 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 4, 4, 4, 2, 2, 2, 2, 2, 0, 0, 0
[PLANE:70:cursor A] blocks 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 11, 11, 12, 7, 7, 7, 7, 7, 25, 0, 0
[PLANE:70:cursor A] min_ddb 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 12, 12, 13, 8, 8, 8, 8, 8, 26, 0, 0
->
[PLANE:70:cursor A] level wm0, wm1, wm2, wm3, wm4, wm5, wm6, wm7, twm, swm, stwm -> *wm0,*wm1,*wm2,*wm3,*wm4,*wm5,*wm6,*wm7,*twm, swm, stwm
[PLANE:70:cursor A] lines 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 4, 4, 4, 4, 4, 4, 4, 4, 0, 0, 0
[PLANE:70:cursor A] blocks 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 11, 11, 12, 12, 12, 12, 12, 12, 25, 0, 0
[PLANE:70:cursor A] min_ddb 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 -> 12, 12, 13, 13, 13, 13, 13, 13, 26, 0, 0
Whether this actually helps on any display blinking issues is unclear.
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
References: https://gitlab.freedesktop.org/drm/intel/-/issues/8683
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250919193000.17665-14-ville.syrjala@linux.intel.com
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