[PATCH v1 8/9] drm/bridge: tc358768: fix THS_TRAILCNT computation

[Francesco Dolcini]

From: Francesco Dolcini <francesco.dolcini at toradex.com>

Correct computation of THS_TRAILCNT register.

This register must be set to a value that ensure that
THS_TRAIL > 60 ns + 4 x UI
 and
THS_TRAIL > 8 x UI
 and
THS_TRAIL < TEOT
 with
TEOT = 105 ns + (12 x UI)

with the actual value of THS_TRAIL being

(1 + THS_TRAILCNT) x ByteClk cycle + ((1 to 2) + 2) xHSBYTECLK cycle +
 - (PHY output delay)

with PHY output delay being about

(8 + (5 to 6)) x MIPIBitClk cycle in the BitClk conversion.

Fixes: ff1ca6397b1d ("drm/bridge: Add tc358768 driver")
Signed-off-by: Francesco Dolcini <francesco.dolcini at toradex.com>
---
 drivers/gpu/drm/bridge/tc358768.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
index 854fc04f08d0..947c7dca567a 100644
--- a/drivers/gpu/drm/bridge/tc358768.c
+++ b/drivers/gpu/drm/bridge/tc358768.c
@@ -779,9 +779,10 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
 	dev_dbg(priv->dev, "TCLK_POSTCNT: 0x%x\n", val);
 	tc358768_write(priv, TC358768_TCLK_POSTCNT, val);
 
-	/* 60ns + 4*UI < THS_PREPARE < 105ns + 12*UI */
-	val = tc358768_ns_to_cnt(60 + tc358768_to_ns(15 * ui_nsk),
-				 dsibclk_nsk) - 5;
+	/* max(60ns + 4*UI, 8*UI) < THS_TRAILCNT < 105ns + 12*UI */
+	raw_val = tc358768_ns_to_cnt(60 + tc358768_to_ns(18 * ui_nsk),
+				     dsibclk_nsk) - 4;
+	val = clamp(raw_val, 0, 15);
 	dev_dbg(priv->dev, "THS_TRAILCNT: 0x%x\n", val);
 	tc358768_write(priv, TC358768_THS_TRAILCNT, val);
 
-- 
2.25.1