DP 2.0 UHBR link rates always use 128b/132b channel encoding, which has
a different data bandwidth efficiency from 8b/10b. The computation is
slightly convoluted due to the units we use; this is all explained in
the added comment.
v2: Clarified comment (Manasi)
Reviewed-by: Manasi Navare <manasi.d.navare@xxxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
---
drivers/gpu/drm/i915/display/intel_dp.c | 51 ++++++++++++++++++++++---
drivers/gpu/drm/i915/display/intel_dp.h | 2 +-
2 files changed, 46 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 38d69e3d55ff..8e42c30b459c 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -210,6 +210,10 @@ int intel_dp_max_lane_count(struct intel_dp *intel_dp)
return intel_dp->max_link_lane_count;
}
+/*
+ * The required data bandwidth for a mode with given pixel clock and bpp. This
+ * is the required net bandwidth independent of the data bandwidth efficiency.
+ */
int
intel_dp_link_required(int pixel_clock, int bpp)
{
@@ -217,16 +221,51 @@ intel_dp_link_required(int pixel_clock, int bpp)
return DIV_ROUND_UP(pixel_clock * bpp, 8);
}
+/*
+ * 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.
+ */
int
-intel_dp_max_data_rate(int max_link_clock, int max_lanes)
+intel_dp_max_data_rate(int max_link_rate, int max_lanes)
{
- /* max_link_clock is the link symbol clock (LS_Clk) in kHz and not the
- * link rate that is generally expressed in Gbps. Since, 8 bits of data
- * is transmitted every LS_Clk per lane, there is no need to account for
- * the channel encoding that is done in the PHY layer here.
+ if (max_link_rate >= 1000000) {
+ /*
+ * 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.
+ */
+ int max_link_rate_kbps = max_link_rate * 10;
+ max_link_rate_kbps = DIV_ROUND_CLOSEST_ULL(max_link_rate_kbps * 9671, 10000);
+ max_link_rate = max_link_rate_kbps / 8;
+ }
+
+ /*
+ * 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, and can be skipped:
+ *
+ * int max_link_rate_kbps = max_link_rate * 10;
+ * max_link_rate_kbps = DIV_ROUND_CLOSEST_ULL(max_link_rate_kbps * 8, 10);
+ * max_link_rate = max_link_rate_kbps / 8;
*/
- return max_link_clock * max_lanes;
+ return max_link_rate * max_lanes;
}
bool intel_dp_can_bigjoiner(struct intel_dp *intel_dp)
diff --git a/drivers/gpu/drm/i915/display/intel_dp.h b/drivers/gpu/drm/i915/display/intel_dp.h
index 1345d588fc6d..ae0f776bffab 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.h
+++ b/drivers/gpu/drm/i915/display/intel_dp.h
@@ -88,7 +88,7 @@ bool intel_dp_source_supports_hbr3(struct intel_dp *intel_dp);
bool intel_dp_get_colorimetry_status(struct intel_dp *intel_dp);
int intel_dp_link_required(int pixel_clock, int bpp);
-int intel_dp_max_data_rate(int max_link_clock, int max_lanes);
+int intel_dp_max_data_rate(int max_link_rate, int max_lanes);
bool intel_dp_can_bigjoiner(struct intel_dp *intel_dp);
bool intel_dp_needs_vsc_sdp(const struct intel_crtc_state *crtc_state,
const struct drm_connector_state *conn_state);
--
2.20.1
