drm/tegra: dsi: Adjust D-PHY timing

Compliance testing shows that HS Trail is off by -12%. Increase the HS
Trail time to make this test pass.

Signed-off-by: David Ung <[e-mail]>
[[e-mail]: update specification references, add comment]
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/mipi-phy.c

···
 #include "mipi-phy.h"
 
 /*
- * Default D-PHY timings based on MIPI D-PHY specification. Derived from
- * the valid ranges specified in Section 5.9 of the D-PHY specification
- * with minor adjustments.
+ * Default D-PHY timings based on MIPI D-PHY specification. Derived from the
+ * valid ranges specified in Section 6.9, Table 14, Page 40 of the D-PHY
+ * specification (v1.2) with minor adjustments.
  */
 int mipi_dphy_timing_get_default(struct mipi_dphy_timing *timing,
 				 unsigned long period)
···
 	timing->hszero = 145 + 5 * period;
 	timing->hssettle = 85 + 6 * period;
 	timing->hsskip = 40;
-	timing->hstrail = max(8 * period, 60 + 4 * period);
+
+	/*
+	 * The MIPI D-PHY specification (Section 6.9, v1.2, Table 14, Page 40)
+	 * contains this formula as:
+	 *
+	 *     T_HS-TRAIL = max(n * 8 * period, 60 + n * 4 * period)
+	 *
+	 * where n = 1 for forward-direction HS mode and n = 4 for reverse-
+	 * direction HS mode. There's only one setting and this function does
+	 * not parameterize on anything other that period, so this code will
+	 * assumes that reverse-direction HS mode is supported and uses n = 4.
+	 */
+	timing->hstrail = max(4 * 8 * period, 60 + 4 * 4 * period);
+
 	timing->init = 100000;
 	timing->lpx = 60;
 	timing->taget = 5 * timing->lpx;
···
 }
 
 /*
- * Validate D-PHY timing according to MIPI Alliance Specification for D-PHY,
- * Section 5.9 "Global Operation Timing Parameters".
+ * Validate D-PHY timing according to MIPI D-PHY specification (v1.2, Section
+ * Section 6.9 "Global Operation Timing Parameters").
  */
 int mipi_dphy_timing_validate(struct mipi_dphy_timing *timing,
 			      unsigned long period)