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 <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: kernel test robot <lkp@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> (v2)
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-1-imre.deak@intel.com

1 files changed, 17 insertions, 6 deletions

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index a462229908fc..0854fe428f17 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/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);