Re: [PATCH] drm/i915/mst: fix INT_MAX to .4 fixed point conversion mistake

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

On Tue, Feb 04, 2025 at 05:37:17PM +0200, Jani Nikula wrote:
> intel_dp_mtp_tu_compute_config() conversion to use .4 fixed point didn't
> take into account that intel_dp_mst_max_dpt_bpp() may return INT_MAX
> when the transport limitation is not relevant. Converting INT_MAX to .4
> fixed point results in -1.0, which then gets used as if it were a real
> max BPP value:
> 
> i915 0000:00:02.0: [drm:intel_dp_mtp_tu_compute_config [i915]] Limiting bpp to max DPT bpp (24.0000 -> -1.0000)
> i915 0000:00:02.0: [drm:intel_dp_mtp_tu_compute_config [i915]] Looking for slots in range min bpp 18.0000 max bpp -1.0000
> 
> Just return 0 for "no max DPT BPP", and handle it explicitly.
> 
> Fixes: 67782bf6e8a6 ("drm/i915/mst: Convert intel_dp_mtp_tu_compute_config() to .4 format")
> 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_mst.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index b729e27cdde2..95abc6ea85eb 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -111,7 +111,7 @@ static int intel_dp_mst_max_dpt_bpp(const struct intel_crtc_state *crtc_state,
>  		&crtc_state->hw.adjusted_mode;
>  
>  	if (!intel_dp_is_uhbr(crtc_state) || DISPLAY_VER(display) >= 20 || !dsc)
> -		return INT_MAX;
> +		return 0;
>  
>  	/*
>  	 * DSC->DPT interface width:
> @@ -248,7 +248,7 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
>  	}
>  
>  	max_dpt_bpp_x16 = fxp_q4_from_int(intel_dp_mst_max_dpt_bpp(crtc_state, dsc));
> -	if (max_bpp_x16 > max_dpt_bpp_x16) {
> +	if (max_dpt_bpp_x16 && max_bpp_x16 > max_dpt_bpp_x16) {
>  		drm_dbg_kms(display->drm, "Limiting bpp to max DPT bpp (" FXP_Q4_FMT " -> " FXP_Q4_FMT ")\n",
>  			    FXP_Q4_ARGS(max_bpp_x16), FXP_Q4_ARGS(max_dpt_bpp_x16));
>  		max_bpp_x16 = max_dpt_bpp_x16;
> -- 
> 2.39.5
>