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: ff1ca6397b1d ("drm/bridge: Add tc358768 driver")
Cc: [e-mail]
Signed-off-by: Francesco Dolcini <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Neil Armstrong <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/bridge/tc358768.c

···
 #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"
 };
···
 	tmp |= val & mask;
 	if (tmp != orig)
 		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)
···
 		}
 	}
 
-	/* start transfer */
-	tc358768_write(priv, TC358768_DSICMD_TX, 1);
+	tc358768_dsicmd_tx(priv);
 
 	ret = tc358768_clear_error(priv);
 	if (ret)