drm/i915/mst: fix INT_MAX to .4 fixed point conversion mistake

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")
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13624
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204153717.2996923-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
index b8f4618d59839ba36a6a04d9048a20693b31714c..ff3ae5db4b54d843c7058a183bbd30ad5cb95f5d 100644 (file)
--- 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:
@@ -270,7 +270,7 @@ 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;