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 hw/xfree86/drivers/modesetting/drmmode_display.c |   21 +++++++++++++++++----
 hw/xfree86/drivers/modesetting/modesetting.man   |    4 ++--
 2 files changed, 19 insertions(+), 6 deletions(-)

New commits:
commit 66e5a5bb12c58413a4d71781c5a7f63f675bfe55
Author: Mario Kleiner <mario.kleiner.de at gmail.com>
Date:   Fri Oct 1 07:42:01 2021 +0200

    modesetting: Enable GAMMA_LUT for lut's with up to 4096 slots.
    
    A lut size of 4096 slots has been verified to work correctly,
    as tested with amdgpu-kms. Intel Tigerlake Gen12 hw has a very
    large GAMMA_LUT size of 262145 slots, but also issues with its
    current GAMMA_LUT implementation, as of Linux 5.14.
    
    Therefore we keep GAMMA_LUT off for large lut's. This currently
    excludes Intel Icelake, Tigerlake and later.
    
    This can be overriden via the "UseGammaLUT" boolean xorg.conf option
    to force use of GAMMA_LUT on or off.
    
    See following link for the Tigerlake situation:
    https://gitlab.freedesktop.org/drm/intel/-/issues/3916#note_1085315
    
    Signed-off-by: Mario Kleiner <mario.kleiner.de at gmail.com>

diff --git a/hw/xfree86/drivers/modesetting/drmmode_display.c b/hw/xfree86/drivers/modesetting/drmmode_display.c
index 1dd6ee9ef..5dfdd30db 100644
--- a/hw/xfree86/drivers/modesetting/drmmode_display.c
+++ b/hw/xfree86/drivers/modesetting/drmmode_display.c
@@ -2404,10 +2404,23 @@ drmmode_crtc_init(ScrnInfoPtr pScrn, drmmode_ptr drmmode, drmModeResPtr mode_res
     xf86DrvMsgVerb(pScrn->scrnIndex, X_INFO, MS_LOGLEVEL_DEBUG,
                    "Allocated crtc nr. %d to this screen.\n", num);
 
-    drmmode_crtc->use_gamma_lut =
-        drmmode_crtc->props[DRMMODE_CRTC_GAMMA_LUT_SIZE].prop_id &&
-        drmmode_crtc->props[DRMMODE_CRTC_GAMMA_LUT_SIZE].value &&
-        xf86ReturnOptValBool(drmmode->Options, OPTION_USE_GAMMA_LUT, TRUE);
+    if (drmmode_crtc->props[DRMMODE_CRTC_GAMMA_LUT_SIZE].prop_id &&
+        drmmode_crtc->props[DRMMODE_CRTC_GAMMA_LUT_SIZE].value) {
+        /*
+         * GAMMA_LUT property supported, and so far tested to be safe to use by
+         * default for lut sizes up to 4096 slots. Intel Tigerlake+ has some
+         * issues, and a large GAMMA_LUT with 262145 slots, so keep GAMMA_LUT
+         * off for large lut sizes by default for now.
+         */
+        drmmode_crtc->use_gamma_lut = drmmode_crtc->props[DRMMODE_CRTC_GAMMA_LUT_SIZE].value <= 4096;
+
+        /* Allow config override. */
+        drmmode_crtc->use_gamma_lut = xf86ReturnOptValBool(drmmode->Options,
+                                                           OPTION_USE_GAMMA_LUT,
+                                                           drmmode_crtc->use_gamma_lut);
+    } else {
+        drmmode_crtc->use_gamma_lut = FALSE;
+    }
 
     if (drmmode_crtc->use_gamma_lut &&
         drmmode_crtc->props[DRMMODE_CRTC_CTM].prop_id) {
diff --git a/hw/xfree86/drivers/modesetting/modesetting.man b/hw/xfree86/drivers/modesetting/modesetting.man
index 0145344d2..bb948380b 100644
--- a/hw/xfree86/drivers/modesetting/modesetting.man
+++ b/hw/xfree86/drivers/modesetting/modesetting.man
@@ -99,8 +99,8 @@ will assign xrandr outputs LVDS and VGA-0 to this instance of the driver.
 .BI "Option \*qUseGammaLUT\*q \*q" boolean \*q
 Enable or disable use of the GAMMA_LUT property, when available.
 When enabled, this option allows the driver to use gamma ramps with more
-entries, if supported by the kernel.
-Default: on.
+entries, if supported by the kernel. By default, GAMMA_LUT will be used for
+kms drivers which are known to be safe for use of GAMMA_LUT.
 .TP
 .SH "SEE ALSO"
 @xservername@(@appmansuffix@), @xconfigfile@(@filemansuffix@), Xserver(@appmansuffix@),