[PATCH] drm/i915/mst: limit DP MST ESI service loop

[Jani Nikula <jani.nikula@intel.com>]

The loop in intel_dp_check_mst_status() keeps servicing interrupts
originating from the sink without bound. Add an upper bound to the new
interrupts occurring during interrupt processing to not get stuck on
potentially stuck sink devices. Use arbitrary 32 tries to clear incoming
interrupts in one go.

Discovered using AI-assisted static analysis confirmed by Intel Product
Security.

Note: The condition likely pre-dates the commit in the Fixes: tag, but
this is about as far back as a backport has any chance of
succeeding. Before that, the retry had a goto.

Reported-by: Martin Hodo <martin.hodo@intel.com>
Fixes: 3c0ec2c2d594 ("drm/i915: Flatten intel_dp_check_mst_status() a bit")
Cc: <stable@vger.kernel.org> # v5.8+
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dp.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 6e3fa6662cbe..ade7e51e7590 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -5590,8 +5590,9 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
 	struct intel_display *display = to_intel_display(intel_dp);
 	bool force_retrain = intel_dp_link_training_get_force_retrain(intel_dp->link.training);
 	bool reprobe_needed = false;
+	int tries = 33;
 
-	for (;;) {
+	while (--tries) {
 		u8 esi[4] = {};
 		u8 ack[4] = {};
 		bool new_irqs;
@@ -5634,6 +5635,11 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
 			break;
 	}
 
+	if (!tries) {
+		drm_dbg_kms(display->drm, "DPRX ESI not clearing, device may be stuck\n");
+		reprobe_needed = true;
+	}
+
 	return !reprobe_needed;
 }
 
-- 
2.47.3