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

[Imre Deak <imre.deak@intel.com>]

On Thu, Jun 25, 2026 at 05:22:04PM +0300, Jani Nikula wrote:
> 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>

Reviewed-by: Imre Deak <imre.deak@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
>