[PATCH 2/2] mailbox: qcom-cpucp: handle NULL data in send_data callback

mailbox_clear_channel() calls mbox_send_message() with NULL data to
notify the remote side that the RX channel has been cleared.
qcom_cpucp_mbox_send_data() blindly dereferenced the data pointer,
causing a NULL pointer dereference kernel panic when invoked from
this path under PREEMPT_RT.

Add an explicit NULL check and return early without writing to the
TX register, which is the correct behaviour for a channel-clear
notification.

Fixes: 0e2a9a03106c ("mailbox: Add support for QTI CPUCP mailbox controller")
Signed-off-by: Jia Yang <[email protected]>
---
 drivers/mailbox/qcom-cpucp-mbox.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/mailbox/qcom-cpucp-mbox.c b/drivers/mailbox/qcom-cpucp-mbox.c
index 0f7fe189e8b1..298b357c0f9a 100644
--- a/drivers/mailbox/qcom-cpucp-mbox.c
+++ b/drivers/mailbox/qcom-cpucp-mbox.c
@@ -117,6 +117,14 @@ static int qcom_cpucp_mbox_send_data(struct mbox_chan *chan, void *data)
 	unsigned long chan_id = channel_number(chan);
 	u32 *val = data;
 
+	/*
+	 * mailbox_clear_channel() calls mbox_send_message() with NULL data to
+	 * signal the remote side that the channel has been cleared.  Nothing
+	 * needs to be written to the TX register in that case, so just return.
+	 */
+	if (!val)
+		return 0;
+
 	writel(*val, cpucp->tx_base + APSS_CPUCP_TX_MBOX_CMD(chan_id) + APSS_CPUCP_MBOX_CMD_OFF);
 
 	return 0;

-- 
2.43.0