[PATCH] drm/msm/dp: Avoid division by zero in msm_dp_ctrl_config_msa()

[Nathan Chancellor <nathan@kernel.org>]

An (admittedly problematic) optimization change in LLVM 20 [1] turns
known division by zero into the equivalent of __builtin_unreachable(),
which invokes undefined behavior if it is encountered in a control flow
graph, destroying code generation. When compile testing for x86_64,
objtool flags an instance of this optimization triggering in
msm_dp_ctrl_config_msa(), inlined into msm_dp_ctrl_on_stream():

  drivers/gpu/drm/msm/msm.o: warning: objtool: msm_dp_ctrl_on_stream(): unexpected end of section .text.msm_dp_ctrl_on_stream

The zero division happens if the else branch in the first if statement
in msm_dp_ctrl_config_msa() is taken because pixel_div is initialized to
zero and it is not possible for LLVM to eliminate the else branch since
rate is still not known after inlining into msm_dp_ctrl_on_stream().
Change pixel_div to one to make the division well defined in the
presence of an unsupported rate, relying on the DRM_ERROR print to
indicate the error to the user.

Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
Link: https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448 [1]
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202601081959.9UVJEOfP-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/gpu/drm/msm/dp/dp_ctrl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/dp/dp_ctrl.c b/drivers/gpu/drm/msm/dp/dp_ctrl.c
index cbcc7c2f0ffc..e4731c059ed8 100644
--- a/drivers/gpu/drm/msm/dp/dp_ctrl.c
+++ b/drivers/gpu/drm/msm/dp/dp_ctrl.c
@@ -2395,7 +2395,7 @@ static void msm_dp_ctrl_config_msa(struct msm_dp_ctrl_private *ctrl,
 			       bool is_ycbcr_420)
 {
 	u32 pixel_m, pixel_n;
-	u32 mvid, nvid, pixel_div = 0, dispcc_input_rate;
+	u32 mvid, nvid, pixel_div = 1, dispcc_input_rate;
 	u32 const nvid_fixed = DP_LINK_CONSTANT_N_VALUE;
 	u32 const link_rate_hbr2 = 540000;
 	u32 const link_rate_hbr3 = 810000;

---
base-commit: 66691e272e40c91305f1704695e0cb340cd162ff
change-id: 20260108-drm-msm-dp_ctrl-avoid-zero-div-be5dc40cbc18

Best regards,
--  
Nathan Chancellor <nathan@kernel.org>