drm/bridge: tc358768: fix TCLK_ZEROCNT computation

Correct computation of TCLK_ZEROCNT register.

This register must be set to a value that ensure that
(TCLK-PREPARECNT + TCLK-ZERO) > 300ns

with the actual value of (TCLK-PREPARECNT + TCLK-ZERO) being

(1 to 2) + (TCLK_ZEROCNT + 1)) x HSByteClkCycle + (PHY output delay)

with PHY output delay being about

(2 to 3) 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/20230427142934.55435-5-francesco@dolcini.it

diff --git a/drivers/gpu/drm/bridge/tc358768.c b/drivers/gpu/drm/bridge/tc358768.c
index 429bd49..487bfe3 100644 (file)
--- a/drivers/gpu/drm/bridge/tc358768.c
+++ b/drivers/gpu/drm/bridge/tc358768.c
@@ -743,10 +743,10 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
 
        /* 38ns < TCLK_PREPARE < 95ns */
        val = tc358768_ns_to_cnt(65, dsibclk_nsk) - 1;
-       /* TCLK_PREPARE > 300ns */
-       val2 = tc358768_ns_to_cnt(300 + tc358768_to_ns(3 * ui_nsk),
-                                 dsibclk_nsk);
-       val |= (val2 - tc358768_to_ns(phy_delay_nsk - dsibclk_nsk)) << 8;
+       /* TCLK_PREPARE + TCLK_ZERO > 300ns */
+       val2 = tc358768_ns_to_cnt(300 - tc358768_to_ns(2 * ui_nsk),
+                                 dsibclk_nsk) - 2;
+       val |= val2 << 8;
        dev_dbg(priv->dev, "TCLK_HEADERCNT: 0x%x\n", val);
        tc358768_write(priv, TC358768_TCLK_HEADERCNT, val);