From fd13841d8b2e8b619f542ed75950c3b206605340 Mon Sep 17 00:00:00 2001 From: Imre Deak Date: Wed, 17 Apr 2024 01:10:03 +0300 Subject: [PATCH] drm/i915/dp_mst: Account for channel coding efficiency in the DSC DPT bpp limit The DSC DPT interface BW limit check should take into account the link clock's (aka DDI clock in bspec) channel coding efficiency overhead. Bspec suggests that the FEC overhead needs to be applied, however HW people claim this isn't the case, nor is any overhead applicable. However based on testing various 5k/6k modes both on the DELL U3224KBA monitor and the Unigraf UCD-500 CTS test device, both the channel coding efficiency (which includes the FEC overhead) and an additional 3% overhead must be accounted for to get these modes working. Bspec: 49259 v2: - Apply an additional 3% overhead, add a commit log and code comment about these overheads and the relation to the Bspec BW limit formula. Reviewed-by: Ankit Nautiyal Signed-off-by: Imre Deak Link: https://patchwork.freedesktop.org/patch/msgid/20240416221010.376865-5-imre.deak@intel.com --- drivers/gpu/drm/i915/display/intel_dp_mst.c | 23 +++++++++++++++++++-- 1 file changed, 21 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 58eb6bf33c92..350532c011ac 100644 --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c @@ -59,11 +59,30 @@ static int intel_dp_mst_check_constraints(struct drm_i915_private *i915, int bpp if (intel_dp_is_uhbr(crtc_state) && DISPLAY_VER(i915) < 14 && dsc) { int output_bpp = bpp; int symbol_clock = intel_dp_link_symbol_clock(crtc_state->port_clock); + /* + * Bspec/49259 suggests that the FEC overhead needs to be + * applied here, though HW people claim that neither this FEC + * or any other overhead is applicable here (that is the actual + * available_bw is just symbol_clock * 72). However based on + * testing on MTL-P the + * - DELL U3224KBA display + * - Unigraf UCD-500 CTS test sink + * devices the + * - 5120x2880/995.59Mhz + * - 6016x3384/1357.23Mhz + * - 6144x3456/1413.39Mhz + * modes (all the ones having a DPT limit on the above devices), + * both the channel coding efficiency and an additional 3% + * overhead needs to be accounted for. + */ + int available_bw = mul_u32_u32(symbol_clock * 72, + drm_dp_bw_channel_coding_efficiency(true)) / + 1030000; if (output_bpp * adjusted_mode->crtc_clock > - symbol_clock * 72) { + available_bw) { drm_dbg_kms(&i915->drm, "UHBR check failed(required bw %d available %d)\n", - output_bpp * adjusted_mode->crtc_clock, symbol_clock * 72); + output_bpp * adjusted_mode->crtc_clock, available_bw); return -EINVAL; } }