[PATCH v2 2/4] drm/i915/dp: Fix UHBR link M/N values

[Imre Deak]

Signed-off-by: Imre Deak <imre.deak at intel.com>
---
 drivers/gpu/drm/i915/display/intel_display.c | 16 ++++++++-----
 drivers/gpu/drm/i915/display/intel_dp.c      | 24 ++++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_dp.h      |  2 ++
 3 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
index 24aebdb715e7d..c059eb0170a5b 100644
--- a/drivers/gpu/drm/i915/display/intel_display.c
+++ b/drivers/gpu/drm/i915/display/intel_display.c
@@ -2411,6 +2411,7 @@ intel_link_compute_m_n(u16 bits_per_pixel, int nlanes,
 		       struct intel_link_m_n *m_n)
 {
 	u32 data_clock = bits_per_pixel * pixel_clock;
+	u32 link_symbol_clock = intel_dp_link_symbol_clock(link_clock);
 	u32 data_m;
 	u32 data_n;
 
@@ -2431,7 +2432,7 @@ intel_link_compute_m_n(u16 bits_per_pixel, int nlanes,
 		    0x8000000);
 
 	compute_m_n(&m_n->link_m, &m_n->link_n,
-		    pixel_clock, link_clock,
+		    pixel_clock, link_symbol_clock,
 		    0x80000);
 }
 
@@ -3943,20 +3944,23 @@ int intel_dotclock_calculate(int link_freq,
 			     const struct intel_link_m_n *m_n)
 {
 	/*
-	 * The calculation for the data clock is:
+	 * The calculation for the data clock -> pixel clock is:
 	 * pixel_clock = ((m/n)*(link_clock * nr_lanes))/bpp
 	 * But we want to avoid losing precison if possible, so:
 	 * pixel_clock = ((m * link_clock * nr_lanes)/(n*bpp))
 	 *
-	 * and the link clock is simpler:
-	 * link_clock = (m * link_clock) / n
+	 * and for link freq (10kbs units) -> pixel clock it is:
+	 * link_symbol_clock = link_freq * 10 / link_symbol_size
+	 * pixel_clock = (m * link_symbol_clock) / n
+	 *    or for more precision:
+	 * pixel_clock = (m * link_freq * 10) / (n * link_symbol_size)
 	 */
 
 	if (!m_n->link_n)
 		return 0;
 
-	return DIV_ROUND_UP_ULL(mul_u32_u32(m_n->link_m, link_freq),
-				m_n->link_n);
+	return DIV_ROUND_UP_ULL(mul_u32_u32(m_n->link_m, link_freq * 10),
+				m_n->link_n * intel_dp_link_symbol_size(link_freq));
 }
 
 int intel_crtc_dotclock(const struct intel_crtc_state *pipe_config)
diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
index 37d7c9c2d6953..43b5fbb5b67e0 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.c
+++ b/drivers/gpu/drm/i915/display/intel_dp.c
@@ -132,6 +132,30 @@ bool intel_dp_is_uhbr(const struct intel_crtc_state *crtc_state)
 	return intel_dp_is_uhbr_rate(crtc_state->port_clock);
 }
 
+/**
+ * intel_dp_link_symbol_size - get the link symbol size for a given link rate
+ * @rate: link rate in 10kbit/s units
+ *
+ * Returns the link symbol size in bits/symbol units depending on the link
+ * rate -> channel coding.
+ */
+int intel_dp_link_symbol_size(int rate)
+{
+	return intel_dp_is_uhbr_rate(rate) ? 32 : 10;
+}
+
+/**
+ * intel_dp_link_symbol_clock - convert link rate to link symbol clock
+ * @rate: link rate in 10kbit/s units
+ *
+ * Returns the link symbol clock frequency in kHz units depending on the
+ * link rate and channel coding.
+ */
+int intel_dp_link_symbol_clock(int rate)
+{
+	return DIV_ROUND_CLOSEST(rate * 10, intel_dp_link_symbol_size(rate));
+}
+
 static void intel_dp_set_default_sink_rates(struct intel_dp *intel_dp)
 {
 	intel_dp->sink_rates[0] = 162000;
diff --git a/drivers/gpu/drm/i915/display/intel_dp.h b/drivers/gpu/drm/i915/display/intel_dp.h
index e80da67554196..64dbf8f192708 100644
--- a/drivers/gpu/drm/i915/display/intel_dp.h
+++ b/drivers/gpu/drm/i915/display/intel_dp.h
@@ -82,6 +82,8 @@ bool intel_dp_has_hdmi_sink(struct intel_dp *intel_dp);
 bool intel_dp_is_edp(struct intel_dp *intel_dp);
 bool intel_dp_is_uhbr_rate(int rate);
 bool intel_dp_is_uhbr(const struct intel_crtc_state *crtc_state);
+int intel_dp_link_symbol_size(int rate);
+int intel_dp_link_symbol_clock(int rate);
 bool intel_dp_is_port_edp(struct drm_i915_private *dev_priv, enum port port);
 enum irqreturn intel_dp_hpd_pulse(struct intel_digital_port *dig_port,
 				  bool long_hpd);
-- 
2.39.2