If cmdq client is multi thread user, racing will occur without mutex
protection. It will make the C message queued in mailbox's queue
always need D message's triggering.

Thread A		Thread B		  Thread C		Thread D...
-----------------------------------------------------------------------------------
mbox_send_message()
	send_data()
			mbox_send_message()
				*exit
						mbox_send_message()
							*exit
mbox_client_txdone()
	tx_tick()
			mbox_client_txdone()
				tx_tick()
						mbox_client_txdone()
							tx_tick()
msg_submit()
	send_data()
			msg_submit()
				*exit
						msg_submit()
							*exit
-----------------------------------------------------------------------------------

Signed-off-by: Bibby Hsieh <redacted>
---
 drivers/soc/mediatek/mtk-cmdq-helper.c | 3 +++
 include/linux/soc/mediatek/mtk-cmdq.h  | 1 +
 2 files changed, 4 insertions(+)

diff --git a/drivers/soc/mediatek/mtk-cmdq-helper.c b/drivers/soc/mediatek/mtk-cmdq-helper.c
index 9add0fd5fa6c..9e35e0beffaa 100644
--- a/drivers/soc/mediatek/mtk-cmdq-helper.c
+++ b/drivers/soc/mediatek/mtk-cmdq-helper.c

@@ -81,6 +81,7 @@ struct cmdq_client *cmdq_mbox_create(struct device *dev, int index, u32 timeout)
 	client->client.dev = dev;
 	client->client.tx_block = false;
 	client->chan = mbox_request_channel(&client->client, index);
+	mutex_init(&client->mutex);
 
 	if (IS_ERR(client->chan)) {
 		long err;

@@ -352,9 +353,11 @@ int cmdq_pkt_flush_async(struct cmdq_pkt *pkt, cmdq_async_flush_cb cb,
 		spin_unlock_irqrestore(&client->lock, flags);
 	}
 
+	mutex_lock(&client->mutex);
 	mbox_send_message(client->chan, pkt);
 	/* We can send next packet immediately, so just call txdone. */
 	mbox_client_txdone(client->chan, 0);
+	mutex_unlock(&client->mutex);
 
 	return 0;
 }

diff --git a/include/linux/soc/mediatek/mtk-cmdq.h b/include/linux/soc/mediatek/mtk-cmdq.h
index a74c1d5acdf3..0f9071cd1bc7 100644
--- a/include/linux/soc/mediatek/mtk-cmdq.h
+++ b/include/linux/soc/mediatek/mtk-cmdq.h

@@ -28,6 +28,7 @@ struct cmdq_client {
 	struct mbox_chan *chan;
 	struct timer_list timer;
 	u32 timeout_ms; /* in unit of microsecond */
+	struct mutex mutex;
 };
 
 /**