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

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