drm/bridge: tc358768: fix THS_TRAILCNT computation

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@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-9-francesco@dolcini.it

diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
index d8209433d5f43defa7ab0025902584e90a61390b..966a25cb0b108ff08e922ca27a07f3196c7fce9c 100644 (file)
--- a/drivers/gpu/drm/bridge/tc358768.c
+++ b/drivers/gpu/drm/bridge/tc358768.c
@@ -780,9 +780,10 @@ 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);