Re: [PATCH] drm/i915/dp: Fix potential infinite loop in 128b/132b SST

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

On Tue, Feb 04, 2025 at 05:49:25PM +0200, Jani Nikula wrote:
> Passing 0 as the step only works when there are other reasons to break
> out of the BPP loop in intel_dp_mtp_tu_compute_config(). Otherwise, an
> infinite loop might occur. Fix it by explicitly checking for 0 step.
> 
> Fixes: ef0a0757bbea ("drm/i915/dp: compute config for 128b/132b SST w/o DSC")
> Reported-by: Imre Deak <imre.deak@intel.com>
> Closes: https://lore.kernel.org/r/Z6I0knh2Kt5T0JrT@ideak-desk.fi.intel.com
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

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

> ---
> 
> Note: This needs a backport with (!step) for -rc1.
> ---
>  drivers/gpu/drm/i915/display/intel_dp_mst.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 95abc6ea85eb..1587be5cd0a7 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -353,6 +353,10 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
>  
>  			break;
>  		}
> +
> +		/* Allow using zero step to indicate one try */
> +		if (!bpp_step_x16)
> +			break;
>  	}
>  
>  	if (slots < 0) {
> -- 
> 2.39.5
>