drm/i915/dp: Guarantee a minimum HBlank time

Mandate a minimum Hblank symbol cycle count between BlankingStart and
BlankingEnd in 8b/10b MST and 128b/132b mode.

v2: Affine calculation/updation of min HBlank to dp_mst (Jani)
v3: moved min_hblank from struct intel_dp to intel_crtc_state (Jani)
v4: use max/min functions, change intel_xx *intel_xx to intel_xx *xx
    (Jani)
    Limit hblank to 511 and accommodate BS/BE in calculated value
    (Srikanth)
v5: Some spelling corrections (Suraj)
v6: Removed DP2.1 in comment as this is applicable for both DP2.1 and
    DP1.4 (Suraj)
v7: crtc_state holds the logical values and the register value
    computation is moved to mst_enable() (Jani)
v8: Limit max hblank to 0x10, disable min_hblank on mst_disable (Jani)

Bspec: 74379
Signed-off-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250122-hblank-v9-1-90afda006685@intel.com

drivers/gpu/drm/i915/display/intel_crtc_state_dump.c

@@ -249,6 +249,7 @@ void intel_crtc_state_dump(const struct intel_crtc_state *pipe_config,
 			   str_enabled_disabled(pipe_config->has_sel_update),
 			   str_enabled_disabled(pipe_config->has_panel_replay),
 			   str_enabled_disabled(pipe_config->enable_psr2_sel_fetch));
+		drm_printf(&p, "minimum HBlank: %d\n", pipe_config->min_hblank);
 	}
 
 	drm_printf(&p, "audio: %i, infoframes: %i, infoframes enabled: 0x%x\n",

drivers/gpu/drm/i915/display/intel_display_types.h

@@ -1097,6 +1097,7 @@ struct intel_crtc_state {
 
 	int max_link_bpp_x16;	/* in 1/16 bpp units */
 	int pipe_bpp;		/* in 1 bpp units */
+	int min_hblank;
 	struct intel_link_m_n dp_m_n;
 
 	/* m2_n2 for eDP downclock */

drivers/gpu/drm/i915/display/intel_dp_mst.c

@@ -209,6 +209,28 @@ static int intel_dp_mst_dsc_get_slice_count(const struct intel_connector *connec
 					    num_joined_pipes);
 }
 
+static void intel_dp_mst_compute_min_hblank(struct intel_crtc_state *crtc_state,
+					    struct intel_connector *connector,
+					    int bpp_x16)
+{
+	struct intel_encoder *encoder = connector->encoder;
+	struct intel_display *display = to_intel_display(encoder);
+	const struct drm_display_mode *adjusted_mode =
+					&crtc_state->hw.adjusted_mode;
+	int symbol_size = intel_dp_is_uhbr(crtc_state) ? 32 : 8;
+	int hblank;
+
+	if (DISPLAY_VER(display) < 20)
+		return;
+
+	/* Calculate min Hblank Link Layer Symbol Cycle Count for 8b/10b MST & 128b/132b */
+	hblank = DIV_ROUND_UP((DIV_ROUND_UP
+			       (adjusted_mode->htotal - adjusted_mode->hdisplay, 4) * bpp_x16),
+			      symbol_size);
+
+	crtc_state->min_hblank = hblank;
+}
+
 int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
 				   struct intel_crtc_state *crtc_state,
 				   struct drm_connector_state *conn_state,
@@ -278,6 +300,9 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
 
 		local_bw_overhead = intel_dp_mst_bw_overhead(crtc_state,
 							     false, dsc_slice_count, link_bpp_x16);
+
+		intel_dp_mst_compute_min_hblank(crtc_state, connector, link_bpp_x16);
+
 		intel_dp_mst_compute_m_n(crtc_state,
 					 local_bw_overhead,
 					 link_bpp_x16,
@@ -967,6 +992,7 @@ static void mst_stream_disable(struct intel_atomic_state *state,
 	struct intel_dp *intel_dp = to_primary_dp(encoder);
 	struct intel_connector *connector =
 		to_intel_connector(old_conn_state->connector);
+	enum transcoder trans = old_crtc_state->cpu_transcoder;
 
 	drm_dbg_kms(display->drm, "active links %d\n",
 		    intel_dp->active_mst_links);
@@ -977,6 +1003,8 @@ static void mst_stream_disable(struct intel_atomic_state *state,
 	intel_hdcp_disable(intel_mst->connector);
 
 	intel_dp_sink_disable_decompression(state, connector, old_crtc_state);
+
+	intel_de_write(display, DP_MIN_HBLANK_CTL(trans), 0x00);
 }
 
 static void mst_stream_post_disable(struct intel_atomic_state *state,
@@ -1249,7 +1277,7 @@ static void mst_stream_enable(struct intel_atomic_state *state,
 	enum transcoder trans = pipe_config->cpu_transcoder;
 	bool first_mst_stream = intel_dp->active_mst_links == 1;
 	struct intel_crtc *pipe_crtc;
-	int ret, i;
+	int ret, i, min_hblank;
 
 	drm_WARN_ON(display->drm, pipe_config->has_pch_encoder);
 
@@ -1264,6 +1292,29 @@ static void mst_stream_enable(struct intel_atomic_state *state,
 			       TRANS_DP2_VFREQ_PIXEL_CLOCK(crtc_clock_hz & 0xffffff));
 	}
 
+	if (DISPLAY_VER(display) >= 20) {
+		/*
+		 * adjust the BlankingStart/BlankingEnd framing control from
+		 * the calculated value
+		 */
+		min_hblank = pipe_config->min_hblank - 2;
+
+		/* Maximum value to be programmed is limited to 0x10 */
+		min_hblank = min(0x10, min_hblank);
+
+		/*
+		 * Minimum hblank accepted for 128b/132b would be 5 and for
+		 * 8b/10b would be 3 symbol count
+		 */
+		if (intel_dp_is_uhbr(pipe_config))
+			min_hblank = max(min_hblank, 5);
+		else
+			min_hblank = max(min_hblank, 3);
+
+		intel_de_write(display, DP_MIN_HBLANK_CTL(trans),
+			       min_hblank);
+	}
+
 	enable_bs_jitter_was(pipe_config);
 
 	intel_ddi_enable_transcoder_func(encoder, pipe_config);

drivers/gpu/drm/i915/i915_reg.h

@@ -3198,6 +3198,10 @@
 #define _TRANS_DP2_VFREQLOW_D			0x630a8
 #define TRANS_DP2_VFREQLOW(trans)		_MMIO_TRANS(trans, _TRANS_DP2_VFREQLOW_A, _TRANS_DP2_VFREQLOW_B)
 
+#define _DP_MIN_HBLANK_CTL_A			0x600ac
+#define _DP_MIN_HBLANK_CTL_B			0x610ac
+#define DP_MIN_HBLANK_CTL(trans)		_MMIO_TRANS(trans, _DP_MIN_HBLANK_CTL_A, _DP_MIN_HBLANK_CTL_B)
+
 /* SNB eDP training params */
 /* SNB A-stepping */
 #define  EDP_LINK_TRAIN_400MV_0DB_SNB_A		(0x38 << 22)