drm/bridge: tc358768: Fix DSI command tx

Wait for the command transmission to be completed in the DSI transfer
function polling for the dc_start bit to go back to idle state after the
transmission is started.

This is documented in the datasheet and failures to do so lead to
commands corruption.

Fixes: ff1ca63 ("drm/bridge: Add tc358768 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20240926141246.48282-1-francesco@dolcini.it
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240926141246.48282-1-francesco@dolcini.it

drivers/gpu/drm/bridge/tc358768.c

@@ -125,6 +125,9 @@
 #define TC358768_DSI_CONFW_MODE_CLR	(6 << 29)
 #define TC358768_DSI_CONFW_ADDR_DSI_CONTROL	(0x3 << 24)
 
+/* TC358768_DSICMD_TX (0x0600) register */
+#define TC358768_DSI_CMDTX_DC_START	BIT(0)
+
 static const char * const tc358768_supplies[] = {
 	"vddc", "vddmipi", "vddio"
 };
@@ -229,6 +232,21 @@ static void tc358768_update_bits(struct tc358768_priv *priv, u32 reg, u32 mask,
 		tc358768_write(priv, reg, tmp);
 }
 
+static void tc358768_dsicmd_tx(struct tc358768_priv *priv)
+{
+	u32 val;
+
+	/* start transfer */
+	tc358768_write(priv, TC358768_DSICMD_TX, TC358768_DSI_CMDTX_DC_START);
+	if (priv->error)
+		return;
+
+	/* wait transfer completion */
+	priv->error = regmap_read_poll_timeout(priv->regmap, TC358768_DSICMD_TX, val,
+					       (val & TC358768_DSI_CMDTX_DC_START) == 0,
+					       100, 100000);
+}
+
 static int tc358768_sw_reset(struct tc358768_priv *priv)
 {
 	/* Assert Reset */
@@ -516,8 +534,7 @@ static ssize_t tc358768_dsi_host_transfer(struct mipi_dsi_host *host,
 		}
 	}
 
-	/* start transfer */
-	tc358768_write(priv, TC358768_DSICMD_TX, 1);
+	tc358768_dsicmd_tx(priv);
 
 	ret = tc358768_clear_error(priv);
 	if (ret)