[PATCH 7/8] mailbox: mpfs: add bounded wait for BUSY to clear before sending request

[Jamie Gibbons]

The MPFS mailbox driver currently checks the BUSY bit at the start of
mpfs_mbox_send() and immediately returns -EBUSY if the controller is
busy.

On MPFS, BUSY may be transiently asserted during early boot even though
no other U-Boot service is actively executing. In Linux, returning
-EBUSY here is retryable via the mailbox framework and scheduler, but in
U-Boot this results in a hard failure.

Replace the immediate BUSY check with a bounded wait using
regmap_read_poll_timeout(), waiting for the controller to become idle
before issuing a new request. This preserves the intent of the BUSY
check while avoiding spurious early-boot failures in U-Boot’s
synchronous, polled execution model.

The timeout is conservative and based on observed MPFS behaviour, where
BUSY clears within a few milliseconds.

Signed-off-by: Jamie Gibbons <jamie.gibbons at microchip.com>
---
 drivers/mailbox/mpfs-mbox.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/mailbox/mpfs-mbox.c b/drivers/mailbox/mpfs-mbox.c
index 165d9d89630..8b7a2719330 100644
--- a/drivers/mailbox/mpfs-mbox.c
+++ b/drivers/mailbox/mpfs-mbox.c
@@ -65,8 +65,11 @@ static int mpfs_mbox_send(struct mbox_chan *chan, const void *data)
 
 	u32 *word_buf = (u32 *)msg->cmd_data;
 
-	if (mpfs_mbox_busy(chan))
-		return -EBUSY;
+	ret = regmap_read_poll_timeout(mbox->control_scb, SERVICES_SR_OFFSET,
+				       value, !(value & SERVICE_SR_BUSY_MASK),
+				       1, 20);
+	if (ret)
+		return ret;
 
 	for (idx = 0; idx < (msg->cmd_data_size / BYTES_4); idx++)
 		writel(word_buf[idx], mbox->mbox_base + msg->mbox_offset + idx * BYTES_4);
-- 
2.43.0