[Intel-gfx] [PATCH v3 02/11] drm/dp_mst: Fix PBN divider calculation for UHBR rates

[Imre Deak]

The current way of calculating the pbn_div value, the link BW per each
MTP slot, worked only for DP 1.4 link rates. Fix things up for UHBR
rates calculating with the correct channel coding efficiency based on
the link rate.

v2:
- Return the fractional pbn_div value from drm_dp_get_vc_payload_bw().
v3:
- Fix rounding up quotient while calculating req_slots. (Ville)

Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Lyude Paul <lyude at redhat.com>
Cc: dri-devel at lists.freedesktop.org
Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c | 10 +++++++---
 include/drm/display/drm_dp_helper.h           | 13 +++++++++++++
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index 000d05e80352a..8ca01a6bf645d 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -3585,14 +3585,18 @@ static int drm_dp_send_up_ack_reply(struct drm_dp_mst_topology_mgr *mgr,
 fixed20_12 drm_dp_get_vc_payload_bw(const struct drm_dp_mst_topology_mgr *mgr,
 				    int link_rate, int link_lane_count)
 {
+	int ch_coding_efficiency =
+		drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(link_rate));
 	fixed20_12 ret;
 
 	if (link_rate == 0 || link_lane_count == 0)
 		drm_dbg_kms(mgr->dev, "invalid link rate/lane count: (%d / %d)\n",
 			    link_rate, link_lane_count);
 
-	/* See DP v2.0 2.6.4.2, VCPayload_Bandwidth_for_OneTimeSlotPer_MTP_Allocation */
-	ret.full = dfixed_const(link_rate * link_lane_count / 54000);
+	/* See DP v2.0 2.6.4.2, 2.7.6.3 VCPayload_Bandwidth_for_OneTimeSlotPer_MTP_Allocation */
+	ret.full = DIV_ROUND_DOWN_ULL(mul_u32_u32(link_rate * link_lane_count,
+						  ch_coding_efficiency),
+				      (1000000ULL * 8 * 5400) >> 12);
 
 	return ret;
 }
@@ -4342,7 +4346,7 @@ int drm_dp_atomic_find_time_slots(struct drm_atomic_state *state,
 		}
 	}
 
-	req_slots = DIV_ROUND_UP(pbn, dfixed_trunc(topology_state->pbn_div));
+	req_slots = DIV_ROUND_UP(dfixed_const(pbn), topology_state->pbn_div.full);
 
 	drm_dbg_atomic(mgr->dev, "[CONNECTOR:%d:%s] [MST PORT:%p] TU %d -> %d\n",
 		       port->connector->base.id, port->connector->name,
diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index c5f1079acb3b1..863b2e7add29e 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -252,6 +252,19 @@ drm_edp_backlight_supported(const u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE])
 	return !!(edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP);
 }
 
+/**
+ * drm_dp_is_uhbr_rate - Determine if a link rate is UHBR
+ * @link_rate: link rate in 10kbits/s units
+ *
+ * Determine if the provided link rate is an UHBR rate.
+ *
+ * Returns: %True if @link_rate is an UHBR rate.
+ */
+static inline bool drm_dp_is_uhbr_rate(int link_rate)
+{
+	return link_rate >= 1000000;
+}
+
 /*
  * DisplayPort AUX channel
  */
-- 
2.39.2