On Tue, Jan 13, 2026 at 05:00:31PM -0700, Nathan Chancellor wrote:
> 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().
>
> Transform the if statements into a switch statement with a default case
> with the existing error print and an early return to avoid the invalid
> division. Add a comment to note this helps the compiler, even though the
> case is known to be unreachable. With this, pixel_dev's default zero
> initialization can be dropped, as it is dead with this change.
>
> Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> Link:
> https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448
> [1]
> Reported-by: kernel test robot <[email protected]>
> Closes:
> https://lore.kernel.org/oe-kbuild-all/[email protected]/
> Suggested-by: Konrad Dybcio <[email protected]>
> Signed-off-by: Nathan Chancellor <[email protected]>
> ---
> Changes in v2:
> - Transform existing if statement structure into a switch case with a
> default case that early returns, avoiding the invalid division
> (Konrad, Dmitry). Add a commment about the reachability of the default
> case.
> - Drop the default zero initialization of pixel_div, as it is no longer
> used due to the new switch statement.
> - Link to v1:
> https://patch.msgid.link/20260108-drm-msm-dp_ctrl-avoid-zero-div-v1-1-6a8debcb3...@kernel.org
> ---
> drivers/gpu/drm/msm/dp/dp_ctrl.c | 24 ++++++++++++++++++------
> 1 file changed, 18 insertions(+), 6 deletions(-)
>
Reviewed-by: Dmitry Baryshkov <[email protected]>
--
With best wishes
Dmitry
