From a14147d0a900ea72bdf56b1f758d471a51937bfb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Thu, 14 Oct 2021 21:18:56 +0300
Subject: drm/i915: Disable DSB usage for now
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Git-commit: 99510e1afb4863a225207146bd988064c5fd0629
Patch-mainline: v5.17-rc1
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225
Turns out the DSB has trouble correctly loading the gamma LUT.
From a cursory look maybe like some entries do not load
properly, or they get loaded with some gibberish. Unfortunately
our current kms_color/etc. tests do not seem to catch this.
I had a brief look at the generated DSB batch and it looked
correct. Tried a few quick tricks like writing the index
register twice/etc. but didn't see any improvement.
Also tried switching to the 10bit gamma mode in case
there is yet another issue with the multi-segment mode, but
even the 10bit mode was showing issues.
Switching to mmio fixes all of it. I suppose one theory is that
maybe the DSB bangs on the LUT too quickly and it can't keep up
and instead some data either gets dropped or corrupted. To confirm
that someone should try to slow down the DSB's progress a bit.
Another thought was that maybe the LUT has crappy dual porting
and you get contention if you try to load it during active
scanout. But why then would the mmio path work, unless it's
just sufficiently slow?
Whatever the case, this is currently busted so let's disable
it until we get to the root of the problem.
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/3916
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014181856.17581-2-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
drivers/gpu/drm/i915/i915_pci.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
index b8586a555244..52749b8f4f6b 100644
--- a/drivers/gpu/drm/i915/i915_pci.c
+++ b/drivers/gpu/drm/i915/i915_pci.c
@@ -873,7 +873,7 @@ static const struct intel_device_info jsl_info = {
TGL_CURSOR_OFFSETS, \
.has_global_mocs = 1, \
.has_pxp = 1, \
- .display.has_dsb = 1
+ .display.has_dsb = 0 /* FIXME: LUT load is broken with DSB */
static const struct intel_device_info tgl_info = {
GEN12_FEATURES,
--
2.38.1