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 <davidu@nvidia.com>
[treding@nvidia.com: update specification references, add comment]
Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --git a/drivers/gpu/drm/tegra/mipi-phy.c b/drivers/gpu/drm/tegra/mipi-phy.c
index 486d19d..ba2ae65 100644 (file)
--- a/drivers/gpu/drm/tegra/mipi-phy.c
+++ b/drivers/gpu/drm/tegra/mipi-phy.c
@@ -12,9 +12,9 @@
 #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)
@@ -34,7 +34,20 @@ int mipi_dphy_timing_get_default(struct mipi_dphy_timing *timing,
        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;
@@ -46,8 +59,8 @@ int mipi_dphy_timing_get_default(struct mipi_dphy_timing *timing,
 }
 
 /*
- * 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)