drm/dp: Add helpers to calculate the link BW overhead

Add helpers drivers can use to calculate the BW allocation overhead -
due to SSC, FEC, DSC and data alignment on symbol cycles - and the
channel coding efficiency - due to the 8b/10b, 128b/132b encoding. On
128b/132b links the FEC overhead is part of the coding efficiency, so
not accounted for in the BW allocation overhead.

The drivers can use these functions to calculate a ratio, controlling
the stream symbol insertion rate of the source device in each SST TU
or MST MTP frame. Drivers can calculate this

m/n = (pixel_data_rate * drm_dp_bw_overhead()) /
      (link_data_rate * drm_dp_bw_channel_coding_efficiency())

ratio for a given link and pixel stream and with that the

slots_per_mtp = CEIL(64 * m / n)

allocated slots per MTP for the stream in a link frame and with
that the

pbn = slots_per_mtp * drm_mst_get_pbn_divider()

allocated PBNs for the stream on the MST link path.

Take drm_dp_bw_overhead() into use in drm_dp_calc_pbn_mode(), for
drivers calculating the PBN value directly.

v2:
- Add dockbook description to drm_dp_bw_channel_coding_efficiency().
  (LKP).
- Clarify the way m/n ratio is calculated in the commit log.
v3:
- Fix compile breakage for !CONFIG_BACKLIGHT_CLASS_DEVICE. (LKP)
- Account for FEC_PM overhead (+ 0.0015625 %), add comment
  with the formula to calculate the total FEC overhead. (Ville)
v4:
- Rename DRM_DP_OVERHEAD_SSC to DRM_DP_OVERHEAD_SSC_REF_CLK. (Ville)
v5:
- Clarify in the commit log what MTP means.
- Simplify the commit log's formula to calculate PBN.

Cc: Lyude Paul <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: kernel test robot <[email protected]>
Cc: [email protected]
Reviewed-by: Stanislav Lisovskiy <[email protected]> (v2)
Acked-by: Maxime Ripard <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: ideak

drivers/gpu/drm/display/drm_dp_helper.c

@@ -3897,3 +3897,135 @@ int drm_panel_dp_aux_backlight(struct drm_panel *panel, struct drm_dp_aux *aux)
 EXPORT_SYMBOL(drm_panel_dp_aux_backlight);
 
 #endif
+
+/* See DP Standard v2.1 2.6.4.4.1.1, 2.8.4.4, 2.8.7 */
+static int drm_dp_link_symbol_cycles(int lane_count, int pixels, int bpp_x16,
+				     int symbol_size, bool is_mst)
+{
+	int cycles = DIV_ROUND_UP(pixels * bpp_x16, 16 * symbol_size * lane_count);
+	int align = is_mst ? 4 / lane_count : 1;
+
+	return ALIGN(cycles, align);
+}
+
+static int drm_dp_link_dsc_symbol_cycles(int lane_count, int pixels, int slice_count,
+					 int bpp_x16, int symbol_size, bool is_mst)
+{
+	int slice_pixels = DIV_ROUND_UP(pixels, slice_count);
+	int slice_data_cycles = drm_dp_link_symbol_cycles(lane_count, slice_pixels,
+							  bpp_x16, symbol_size, is_mst);
+	int slice_eoc_cycles = is_mst ? 4 / lane_count : 1;
+
+	return slice_count * (slice_data_cycles + slice_eoc_cycles);
+}
+
+/**
+ * drm_dp_bw_overhead - Calculate the BW overhead of a DP link stream
+ * @lane_count: DP link lane count
+ * @hactive: pixel count of the active period in one scanline of the stream
+ * @dsc_slice_count: DSC slice count if @flags/DRM_DP_LINK_BW_OVERHEAD_DSC is set
+ * @bpp_x16: bits per pixel in .4 binary fixed point
+ * @flags: DRM_DP_OVERHEAD_x flags
+ *
+ * Calculate the BW allocation overhead of a DP link stream, depending
+ * on the link's
+ * - @lane_count
+ * - SST/MST mode (@flags / %DRM_DP_OVERHEAD_MST)
+ * - symbol size (@flags / %DRM_DP_OVERHEAD_UHBR)
+ * - FEC mode (@flags / %DRM_DP_OVERHEAD_FEC)
+ * - SSC/REF_CLK mode (@flags / %DRM_DP_OVERHEAD_SSC_REF_CLK)
+ * as well as the stream's
+ * - @hactive timing
+ * - @bpp_x16 color depth
+ * - compression mode (@flags / %DRM_DP_OVERHEAD_DSC).
+ * Note that this overhead doesn't account for the 8b/10b, 128b/132b
+ * channel coding efficiency, for that see
+ * @drm_dp_link_bw_channel_coding_efficiency().
+ *
+ * Returns the overhead as 100% + overhead% in 1ppm units.
+ */
+int drm_dp_bw_overhead(int lane_count, int hactive,
+		       int dsc_slice_count,
+		       int bpp_x16, unsigned long flags)
+{
+	int symbol_size = flags & DRM_DP_BW_OVERHEAD_UHBR ? 32 : 8;
+	bool is_mst = flags & DRM_DP_BW_OVERHEAD_MST;
+	u32 overhead = 1000000;
+	int symbol_cycles;
+
+	/*
+	 * DP Standard v2.1 2.6.4.1
+	 * SSC downspread and ref clock variation margin:
+	 *   5300ppm + 300ppm ~ 0.6%
+	 */
+	if (flags & DRM_DP_BW_OVERHEAD_SSC_REF_CLK)
+		overhead += 6000;
+
+	/*
+	 * DP Standard v2.1 2.6.4.1.1, 3.5.1.5.4:
+	 * FEC symbol insertions for 8b/10b channel coding:
+	 * After each 250 data symbols on 2-4 lanes:
+	 *   250 LL + 5 FEC_PARITY_PH + 1 CD_ADJ   (256 byte FEC block)
+	 * After each 2 x 250 data symbols on 1 lane:
+	 *   2 * 250 LL + 11 FEC_PARITY_PH + 1 CD_ADJ (512 byte FEC block)
+	 * After 256 (2-4 lanes) or 128 (1 lane) FEC blocks:
+	 *   256 * 256 bytes + 1 FEC_PM
+	 * or
+	 *   128 * 512 bytes + 1 FEC_PM
+	 * (256 * 6 + 1) / (256 * 250) = 2.4015625 %
+	 */
+	if (flags & DRM_DP_BW_OVERHEAD_FEC)
+		overhead += 24016;
+
+	/*
+	 * DP Standard v2.1 2.7.9, 5.9.7
+	 * The FEC overhead for UHBR is accounted for in its 96.71% channel
+	 * coding efficiency.
+	 */
+	WARN_ON((flags & DRM_DP_BW_OVERHEAD_UHBR) &&
+		(flags & DRM_DP_BW_OVERHEAD_FEC));
+
+	if (flags & DRM_DP_BW_OVERHEAD_DSC)
+		symbol_cycles = drm_dp_link_dsc_symbol_cycles(lane_count, hactive,
+							      dsc_slice_count,
+							      bpp_x16, symbol_size,
+							      is_mst);
+	else
+		symbol_cycles = drm_dp_link_symbol_cycles(lane_count, hactive,
+							  bpp_x16, symbol_size,
+							  is_mst);
+
+	return DIV_ROUND_UP_ULL(mul_u32_u32(symbol_cycles * symbol_size * lane_count,
+					    overhead * 16),
+				hactive * bpp_x16);
+}
+EXPORT_SYMBOL(drm_dp_bw_overhead);
+
+/**
+ * drm_dp_bw_channel_coding_efficiency - Get a DP link's channel coding efficiency
+ * @is_uhbr: Whether the link has a 128b/132b channel coding
+ *
+ * Return the channel coding efficiency of the given DP link type, which is
+ * either 8b/10b or 128b/132b (aka UHBR). The corresponding overhead includes
+ * the 8b -> 10b, 128b -> 132b pixel data to link symbol conversion overhead
+ * and for 128b/132b any link or PHY level control symbol insertion overhead
+ * (LLCP, FEC, PHY sync, see DP Standard v2.1 3.5.2.18). For 8b/10b the
+ * corresponding FEC overhead is BW allocation specific, included in the value
+ * returned by drm_dp_bw_overhead().
+ *
+ * Returns the efficiency in the 100%/coding-overhead% ratio in
+ * 1ppm units.
+ */
+int drm_dp_bw_channel_coding_efficiency(bool is_uhbr)
+{
+	if (is_uhbr)
+		return 967100;
+	else
+		/*
+		 * Note that on 8b/10b MST the efficiency is only
+		 * 78.75% due to the 1 out of 64 MTPH packet overhead,
+		 * not accounted for here.
+		 */
+		return 800000;
+}
+EXPORT_SYMBOL(drm_dp_bw_channel_coding_efficiency);

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -4726,17 +4726,28 @@ EXPORT_SYMBOL(drm_dp_check_act_status);
 int drm_dp_calc_pbn_mode(int clock, int bpp)
 {
 	/*
-	 * margin 5300ppm + 300ppm ~ 0.6% as per spec, factor is 1.006
 	 * The unit of 54/64Mbytes/sec is an arbitrary unit chosen based on
 	 * common multiplier to render an integer PBN for all link rate/lane
 	 * counts combinations
 	 * calculate
-	 * peak_kbps *= (1006/1000)
-	 * peak_kbps *= (64/54)
-	 * peak_kbps *= 8    convert to bytes
+	 * peak_kbps = clock * bpp / 16
+	 * peak_kbps *= SSC overhead / 1000000
+	 * peak_kbps /= 8    convert to Kbytes
+	 * peak_kBps *= (64/54) / 1000    convert to PBN
 	 */
-	return DIV_ROUND_UP_ULL(mul_u32_u32(clock * bpp, 64 * 1006 >> 4),
-				1000 * 8 * 54 * 1000);
+	/*
+	 * TODO: Use the actual link and mode parameters to calculate
+	 * the overhead. For now it's assumed that these are
+	 * 4 link lanes, 4096 hactive pixels, which don't add any
+	 * significant data padding overhead and that there is no DSC
+	 * or FEC overhead.
+	 */
+	int overhead = drm_dp_bw_overhead(4, 4096, 0, bpp,
+					  DRM_DP_BW_OVERHEAD_MST |
+					  DRM_DP_BW_OVERHEAD_SSC_REF_CLK);
+
+	return DIV64_U64_ROUND_UP(mul_u32_u32(clock * bpp, 64 * overhead >> 4),
+				  1000000ULL * 8 * 54 * 1000);
 }
 EXPORT_SYMBOL(drm_dp_calc_pbn_mode);
 

include/drm/display/drm_dp_helper.h

@@ -788,4 +788,15 @@ bool drm_dp_downstream_rgb_to_ycbcr_conversion(const u8 dpcd[DP_RECEIVER_CAP_SIZ
 					       const u8 port_cap[4], u8 color_spc);
 int drm_dp_pcon_convert_rgb_to_ycbcr(struct drm_dp_aux *aux, u8 color_spc);
 
+#define DRM_DP_BW_OVERHEAD_MST		BIT(0)
+#define DRM_DP_BW_OVERHEAD_UHBR		BIT(1)
+#define DRM_DP_BW_OVERHEAD_SSC_REF_CLK	BIT(2)
+#define DRM_DP_BW_OVERHEAD_FEC		BIT(3)
+#define DRM_DP_BW_OVERHEAD_DSC		BIT(4)
+
+int drm_dp_bw_overhead(int lane_count, int hactive,
+		       int dsc_slice_count,
+		       int bpp_x16, unsigned long flags);
+int drm_dp_bw_channel_coding_efficiency(bool is_uhbr);
+
 #endif /* _DRM_DP_HELPER_H_ */