From 5427d3e6684b052562a31709281e54103e329e5a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= <ville.syrjala@linux.intel.com>
Date: Wed, 23 Feb 2022 15:13:03 +0200
Subject: drm/i915: Avoid negative shift due to bigjoiner_pipes==0
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Alt-commit: cccc71b552a1040ad3d738d7ec95570801fb0bf6
Git-commit: 4fe4ed07c815044755075eaad5fe1815436a060f
Patch-mainline: v5.18-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

bigjoiner_pipes==0 leads bigjoiner_master_pipe() to
do BIT(ffs(0)-1) which is undefined behaviour. The code should
actually still work fine since the only place we provoke
that is intel_crtc_bigjoiner_slave_pipes() and it'll bitwise
AND the result with 0, so doesn't really matter what we get
out of bigjoiner_master_pipe(). But best not provoke undefined
behaviour anyway.

Reported-by: kernel test robot <oliver.sang@intel.com>
Fixes: a6e7a006f5d5 ("drm/i915: Change bigjoiner state tracking to use the pipe bitmask")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220223131315.18016-2-ville.syrjala@linux.intel.com
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
(cherry picked from commit cccc71b552a1040ad3d738d7ec95570801fb0bf6)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
 drivers/gpu/drm/i915/display/intel_display.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 80b19c304c43..f3f5f11a5abf 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -346,7 +346,10 @@ static enum pipe bigjoiner_master_pipe(const struct intel_crtc_state *crtc_state
 
 u8 intel_crtc_bigjoiner_slave_pipes(const struct intel_crtc_state *crtc_state)
 {
-	return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
+	if (crtc_state->bigjoiner_pipes)
+		return crtc_state->bigjoiner_pipes & ~BIT(bigjoiner_master_pipe(crtc_state));
+	else
+		return 0;
 }
 
 bool intel_crtc_is_bigjoiner_slave(const struct intel_crtc_state *crtc_state)
-- 
2.38.1