Reading out the DP encoders' DPCD during booting or resume is only required for enabled encoders: such encoders may be modesetted during the initial commit and the link training this involves depends on an initialized DPCD. For DDI encoders reading out the DPCD is skipped, do the same on pre-DDI platforms.

Atm, the first DPCD readout without a sink connected - which is a likely scneario if the encoder is disabled - leaves intel_dp->num_common_rates at 0, which resulted in

intel_dp_sync_state()->intel_dp_max_common_rate()

in a

intel_dp->common_rates[-1]

access. This by definition results in an undefined behaviour, though to my best knowledge in all HW/compiler configurations it actually results in accessing the array item type value preceding the array. In this case the preceding value happens to be intel_dp->num_common_rates, which is 0, so this issue - by luck - didn't cause a user visible problem.

Nevertheless it's still an undefined behaviour and in CONFIG_UBSAN builds leads to a kernel BUG() (which revealed this problem for us), hence CC:stable.

A related problem in case the encoder is enabled but the sink is not connected or the DPCD readout fails is fixed by the next patch.

v2: Amend the commit message describing the root cause of the CONFIG_UBSAN BUG().

Fixes: a532cde31de3 ("drm/i915/tc: Fix TypeC port init/resume time sanitization") References: https://gitlab.freedesktop.org/drm/intel/-/issues/4297 Reported-and-tested-by: Mat Jonczyk mat.jonczyk@o2.pl Cc: Mat Jonczyk mat.jonczyk@o2.pl Cc: José Roberto de Souza jose.souza@intel.com Cc: Jani Nikula jani.nikula@intel.com Cc: Ville Syrjälä ville.syrjala@linux.intel.com Cc: stable@vger.kernel.org Signed-off-by: Imre Deak imre.deak@intel.com --- drivers/gpu/drm/i915/display/intel_dp.c | 3 +++ 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c index 9d8132dd4cc5a..23de500d56b52 100644 --- a/drivers/gpu/drm/i915/display/intel_dp.c +++ b/drivers/gpu/drm/i915/display/intel_dp.c @@ -2007,6 +2007,9 @@ void intel_dp_sync_state(struct intel_encoder *encoder, { struct intel_dp *intel_dp = enc_to_intel_dp(encoder);

+ if (!crtc_state) + return; + /* * Don't clobber DPCD if it's been already read out during output * setup (eDP) or detect.