drm/bridge: tc358768: fix TCLK_TRAILCNT computation

Correct computation of TCLK_TRAILCNT register.

The driver does not implement non-continuous clock mode, so the actual
value doesn't make a practical difference yet. However this change also
ensures that the value does not write to reserved registers bits in case
of under/overflow.

This register must be set to a value that ensures that

TCLK-TRAIL > 60ns
 and
TEOT <= (105 ns + 12 x UI)

with the actual value of TCLK-TRAIL being

(TCLK_TRAILCNT + (1 to 2)) xHSByteClkCycle +
 (2 + (1 to 2)) * HSBYTECLKCycle - (PHY output delay)

with PHY output delay being about

(2 to 3) x MIPIBitClk cycle in the BitClk conversion.

Fixes: ff1ca6397b ("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/20230427142934.55435-2-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-5-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-2-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-5-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-2-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-5-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-2-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-3-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-5-francesco@dolcini.it

drivers/gpu/drm/bridge/tc358768.c

@@ -10,6 +10,7 @@
 #include <linux/i2c.h>
 #include <linux/kernel.h>
 #include <linux/media-bus-format.h>
+#include <linux/minmax.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
 #include <linux/regulator/consumer.h>
@@ -639,6 +640,7 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
 	struct mipi_dsi_device *dsi_dev = priv->output.dev;
 	unsigned long mode_flags = dsi_dev->mode_flags;
 	u32 val, val2, lptxcnt, hact, data_type;
+	s32 raw_val;
 	const struct drm_display_mode *mode;
 	u32 dsibclk_nsk, dsiclk_nsk, ui_nsk, phy_delay_nsk;
 	u32 dsiclk, dsibclk, video_start;
@@ -750,9 +752,9 @@ static void tc358768_bridge_pre_enable(struct drm_bridge *bridge)
 	dev_dbg(priv->dev, "TCLK_HEADERCNT: 0x%x\n", val);
 	tc358768_write(priv, TC358768_TCLK_HEADERCNT, val);
 
-	/* TCLK_TRAIL > 60ns + 3*UI */
-	val = 60 + tc358768_to_ns(3 * ui_nsk);
-	val = tc358768_ns_to_cnt(val, dsibclk_nsk) - 5;
+	/* TCLK_TRAIL > 60ns AND TEOT <= 105 ns + 12*UI */
+	raw_val = tc358768_ns_to_cnt(60 + tc358768_to_ns(2 * ui_nsk), dsibclk_nsk) - 5;
+	val = clamp(raw_val, 0, 127);
 	dev_dbg(priv->dev, "TCLK_TRAILCNT: 0x%x\n", val);
 	tc358768_write(priv, TC358768_TCLK_TRAILCNT, val);