[PATCH 25/44] drm/dp: Add drm_dp_max_dprx_data_rate()

[Imre Deak]

Copy intel_dp_max_data_rate() to DRM core. It will be needed by a
follow-up DP tunnel patch, checking the maximum rate the DPRX (sink)
supports. Accordingly use the drm_dp_max_dprx_data_rate() name for
clarity. This patchset will also switch calling the new DRM function
in i915 instead of intel_dp_max_data_rate().

Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 58 +++++++++++++++++++++++++
 include/drm/display/drm_dp_helper.h     |  2 +
 2 files changed, 60 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index b1ca3a1100dab..24911243d4d3a 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -4058,3 +4058,61 @@ int drm_dp_bw_channel_coding_efficiency(bool is_uhbr)
 		return 800000;
 }
 EXPORT_SYMBOL(drm_dp_bw_channel_coding_efficiency);
+
+/*
+ * Given a link rate and lanes, get the data bandwidth.
+ *
+ * Data bandwidth is the actual payload rate, which depends on the data
+ * bandwidth efficiency and the link rate.
+ *
+ * For 8b/10b channel encoding, SST and non-FEC, the data bandwidth efficiency
+ * is 80%. For example, for a 1.62 Gbps link, 1.62*10^9 bps * 0.80 * (1/8) =
+ * 162000 kBps. With 8-bit symbols, we have 162000 kHz symbol clock. Just by
+ * coincidence, the port clock in kHz matches the data bandwidth in kBps, and
+ * they equal the link bit rate in Gbps multiplied by 100000. (Note that this no
+ * longer holds for data bandwidth as soon as FEC or MST is taken into account!)
+ *
+ * For 128b/132b channel encoding, the data bandwidth efficiency is 96.71%. For
+ * example, for a 10 Gbps link, 10*10^9 bps * 0.9671 * (1/8) = 1208875
+ * kBps. With 32-bit symbols, we have 312500 kHz symbol clock. The value 1000000
+ * does not match the symbol clock, the port clock (not even if you think in
+ * terms of a byte clock), nor the data bandwidth. It only matches the link bit
+ * rate in units of 10000 bps.
+ *
+ * Note that protocol layers above the DPRX link level considered here can
+ * further limit the maximum data rate. Such layers are the MST topology (with
+ * limits on the link between the source and first branch device as well as on
+ * the whole MST path until the DPRX link) and (Thunderbolt) DP tunnels -
+ * which in turn can encapsulate an MST link with its own limit - with each
+ * SST or MST encapsulated tunnel sharing the BW of a tunnel group.
+ *
+ * TODO: Add support for querying the max data rate with the above limits as
+ * well.
+ *
+ * Returns the maximum data rate in kBps units.
+ */
+int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes)
+{
+	int ch_coding_efficiency =
+		drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(max_link_rate));
+	int max_link_rate_kbps = max_link_rate * 10;
+
+	/*
+	 * UHBR rates always use 128b/132b channel encoding, and have
+	 * 97.71% data bandwidth efficiency. Consider max_link_rate the
+	 * link bit rate in units of 10000 bps.
+	 */
+	/*
+	 * Lower than UHBR rates always use 8b/10b channel encoding, and have
+	 * 80% data bandwidth efficiency for SST non-FEC. However, this turns
+	 * out to be a nop by coincidence:
+	 *
+	 *	int max_link_rate_kbps = max_link_rate * 10;
+	 *	max_link_rate_kbps = DIV_ROUND_DOWN_ULL(max_link_rate_kbps * 8, 10);
+	 *	max_link_rate = max_link_rate_kbps / 8;
+	 */
+	return DIV_ROUND_DOWN_ULL(mul_u32_u32(max_link_rate_kbps * max_lanes,
+					      ch_coding_efficiency),
+				  1000000 * 8);
+}
+EXPORT_SYMBOL(drm_dp_max_dprx_data_rate);
diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
index 863b2e7add29e..454ae7517419a 100644
--- a/include/drm/display/drm_dp_helper.h
+++ b/include/drm/display/drm_dp_helper.h
@@ -813,4 +813,6 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
 		       int bpp_x16, unsigned long flags);
 int drm_dp_bw_channel_coding_efficiency(bool is_uhbr);
 
+int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes);
+
 #endif /* _DRM_DP_HELPER_H_ */
-- 
2.39.2