[Intel-gfx] [PATCH v2 01/13] drm/i915: Avoid negative shift due to bigjoiner_pipes==0

Ville Syrjala ville.syrjala at linux.intel.com
Wed Feb 23 13:13:03 UTC 2022

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

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 at intel.com>
Fixes: a6e7a006f5d5 ("drm/i915: Change bigjoiner state tracking to use the pipe bitmask")
Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
 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)

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