firmware: imx: scu: Ensure sequential TX

SCU requires that all messages words are written sequentially but linux MU
driver implements multiple independent channels for each register so ordering
between different channels must be ensured by SCU API interface.

Wait for tx_done before every send to ensure that no queueing happens at the
mailbox channel level.

Fixes: edbee095 ("firmware: imx: add SCU firmware driver support")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: : Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

drivers/firmware/imx/imx-scu.c

@@ -29,6 +29,7 @@ struct imx_sc_chan {
 	struct mbox_client cl;
 	struct mbox_chan *ch;
 	int idx;
+	struct completion tx_done;
 };
 
 struct imx_sc_ipc {
@@ -100,6 +101,14 @@ int imx_scu_get_handle(struct imx_sc_ipc **ipc)
 }
 EXPORT_SYMBOL(imx_scu_get_handle);
 
+/* Callback called when the word of a message is ack-ed, eg read by SCU */
+static void imx_scu_tx_done(struct mbox_client *cl, void *mssg, int r)
+{
+	struct imx_sc_chan *sc_chan = container_of(cl, struct imx_sc_chan, cl);
+
+	complete(&sc_chan->tx_done);
+}
+
 static void imx_scu_rx_callback(struct mbox_client *c, void *msg)
 {
 	struct imx_sc_chan *sc_chan = container_of(c, struct imx_sc_chan, cl);
@@ -149,6 +158,19 @@ static int imx_scu_ipc_write(struct imx_sc_ipc *sc_ipc, void *msg)
 
 	for (i = 0; i < hdr->size; i++) {
 		sc_chan = &sc_ipc->chans[i % 4];
+
+		/*
+		 * SCU requires that all messages words are written
+		 * sequentially but linux MU driver implements multiple
+		 * independent channels for each register so ordering between
+		 * different channels must be ensured by SCU API interface.
+		 *
+		 * Wait for tx_done before every send to ensure that no
+		 * queueing happens at the mailbox channel level.
+		 */
+		wait_for_completion(&sc_chan->tx_done);
+		reinit_completion(&sc_chan->tx_done);
+
 		ret = mbox_send_message(sc_chan->ch, &data[i]);
 		if (ret < 0)
 			return ret;
@@ -247,6 +269,11 @@ static int imx_scu_probe(struct platform_device *pdev)
 		cl->knows_txdone = true;
 		cl->rx_callback = imx_scu_rx_callback;
 
+		/* Initial tx_done completion as "done" */
+		cl->tx_done = imx_scu_tx_done;
+		init_completion(&sc_chan->tx_done);
+		complete(&sc_chan->tx_done);
+
 		sc_chan->sc_ipc = sc_ipc;
 		sc_chan->idx = i % 4;
 		sc_chan->ch = mbox_request_channel_byname(cl, chan_name);