[PATCH] drm/display/dp: Use static values for min_hblank

[Murthy, Arun R]

> -----Original Message-----
> From: Jani Nikula <jani.nikula at linux.intel.com>
> Sent: Tuesday, June 24, 2025 3:20 PM
> To: Murthy, Arun R <arun.r.murthy at intel.com>; intel-gfx at lists.freedesktop.org;
> intel-xe at lists.freedesktop.org
> Cc: Deak, Imre <imre.deak at intel.com>; Murthy, Arun R
> <arun.r.murthy at intel.com>
> Subject: Re: [PATCH] drm/display/dp: Use static values for min_hblank
> 
> On Tue, 24 Jun 2025, Arun R Murthy <arun.r.murthy at intel.com> wrote:
> > Use recommended static values as per wa_14021694213 for min_hblank to
> > avoid underruns.
> 
> Subject prefix should be "drm/i915/dp".
Realized just after sending the patch, will keep an eye in future.

> 
> > Bspec: 74379
> > Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c | 68
> > +++++----------------------------
> >  1 file changed, 10 insertions(+), 58 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c
> > b/drivers/gpu/drm/i915/display/intel_dp.c
> > index
> >
> f48912f308df7fd26c9d089e8f1bd096bfc8df95..feac034d1789c05994b210aabb
> b5
> > 3d4b407cecf6 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -3115,68 +3115,20 @@ int intel_dp_compute_min_hblank(struct
> intel_crtc_state *crtc_state,
> >  				const struct drm_connector_state *conn_state)
> {
> >  	struct intel_display *display = to_intel_display(crtc_state);
> > -	const struct drm_display_mode *adjusted_mode =
> > -					&crtc_state->hw.adjusted_mode;
> > -	struct intel_connector *connector = to_intel_connector(conn_state-
> >connector);
> > -	int symbol_size = intel_dp_is_uhbr(crtc_state) ? 32 : 8;
> > -	/*
> > -	 * min symbol cycles is 3(BS,VBID, BE) for 128b/132b and
> > -	 * 5(BS, VBID, MVID, MAUD, BE) for 8b/10b
> > -	 */
> > -	int min_sym_cycles = intel_dp_is_uhbr(crtc_state) ? 3 : 5;
> > -	bool is_mst = intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST);
> > -	int num_joined_pipes = intel_crtc_num_joined_pipes(crtc_state);
> > -	int min_hblank;
> > -	int max_lane_count = 4;
> > -	int hactive_sym_cycles, htotal_sym_cycles;
> > -	int dsc_slices = 0;
> > -	int link_bpp_x16;
> >
> >  	if (DISPLAY_VER(display) < 30)
> >  		return 0;
> >
> > -	/* MIN_HBLANK should be set only for 8b/10b MST or for 128b/132b
> SST/MST */
> 
> Has this changed?
> 
No, still holds good. Will retain this change.

> > -	if (!is_mst && !intel_dp_is_uhbr(crtc_state))
> > -		return 0;
> > -
> > -	if (crtc_state->dsc.compression_enable) {
> > -		dsc_slices = intel_dp_dsc_get_slice_count(connector,
> > -							  adjusted_mode-
> >crtc_clock,
> > -							  adjusted_mode-
> >crtc_hdisplay,
> > -							  num_joined_pipes);
> > -		if (!dsc_slices) {
> > -			drm_dbg(display->drm, "failed to calculate dsc slice
> count\n");
> > -			return -EINVAL;
> > -		}
> > -	}
> > -
> > -	if (crtc_state->dsc.compression_enable)
> > -		link_bpp_x16 = crtc_state->dsc.compressed_bpp_x16;
> > -	else
> > -		link_bpp_x16 =
> fxp_q4_from_int(intel_dp_output_bpp(crtc_state->output_format,
> > -								   crtc_state-
> >pipe_bpp));
> > -
> > -	/* Calculate min Hblank Link Layer Symbol Cycle Count for 8b/10b MST
> & 128b/132b */
> > -	hactive_sym_cycles = drm_dp_link_symbol_cycles(max_lane_count,
> > -						       adjusted_mode->hdisplay,
> > -						       dsc_slices,
> > -						       link_bpp_x16,
> > -						       symbol_size, is_mst);
> > -	htotal_sym_cycles = adjusted_mode->htotal * hactive_sym_cycles /
> > -			     adjusted_mode->hdisplay;
> > -
> > -	min_hblank = htotal_sym_cycles - hactive_sym_cycles;
> > -	/* minimum Hblank calculation:
> https://groups.vesa.org/wg/DP/document/20494 */
> > -	min_hblank = max(min_hblank, min_sym_cycles);
> > -
> > -	/*
> > -	 * adjust the BlankingStart/BlankingEnd framing control from
> > -	 * the calculated value
> > -	 */
> > -	min_hblank = min_hblank - 2;
> > -
> > -	min_hblank = min(10, min_hblank);
> > -	crtc_state->min_hblank = min_hblank;
> > +	/* min_hblank formula is undergoing a change and the recomendation
> is to use static value */
> > +	if (intel_dp_is_uhbr(crtc_state)) {
> > +		if (crtc_state->dsc.compression_enable &&
> > +		    crtc_state->output_format ==
> INTEL_OUTPUT_FORMAT_YCBCR420 &&
> > +		    crtc_state->pipe_bpp == 18)
> > +			crtc_state->min_hblank = 2;
> > +		else
> > +			crtc_state->min_hblank = 3;
> > +	} else
> > +		crtc_state->min_hblank = 10;
> 
> Per coding style, the else also needs braces. Checkpatch will tell you this, please
> run it before posting.
> 
> This is not new stuff, and it's a waste of both your time and mine to
> accummulate versions on this.
Sorry for that, 
Started using b4 tool and the option check does run the checkpatch. b4 prep --check does run checkpatch, not sure why this was not caught.
Will take a look at the options considered for checkpatch in my setup.

Thanks and Regards,
Arun R Murthy
--------------------
> >
> >  	return 0;
> >  }
> >
> > ---
> > base-commit: da56936fad6f5e1d5f0cef8b50277bfb071eefe7
> > change-id: 20250624-min_hblank-8af8d2626ff8
> >
> > Best regards,
> 
> --
> Jani Nikula, Intel