[PATCH v3 5/6] firmware: samsung: acpm: Fix infinite loop on sequence number exhaustion

[Tudor Ambarus]

Sashiko identified a possible infinite loop [1].

ACPM IPC sequence numbers are tracked via a 64-bit bitmap. Previously,
acpm_prepare_xfer() used a do...while loop to search for a free
sequence number.

If all 63 available sequence numbers are leaked due to transient
hardware timeouts or mailbox failures, the bitmap becomes full.
The next call to acpm_prepare_xfer() would enter an infinite loop.

Fix this by utilizing the kernel's optimized bitmap search functions
(find_next_zero_bit / find_first_zero_bit). If the pool is completely
exhausted, log the failure and return -EBUSY to allow the kernel to
fail gracefully instead of hanging.

Cc: stable at vger.kernel.org
Fixes: a88927b534ba ("firmware: add Exynos ACPM protocol driver")
Closes: https://sashiko.dev/#/patchset/20260420-acpm-tmu-v3-0-3dc8e93f0b26%40linaro.org [1]
Signed-off-by: Tudor Ambarus <tudor.ambarus at linaro.org>
---
 drivers/firmware/samsung/exynos-acpm.c | 36 +++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/drivers/firmware/samsung/exynos-acpm.c b/drivers/firmware/samsung/exynos-acpm.c
index e4d8d1120192..b8a4978b091b 100644
--- a/drivers/firmware/samsung/exynos-acpm.c
+++ b/drivers/firmware/samsung/exynos-acpm.c
@@ -12,6 +12,7 @@
 #include <linux/container_of.h>
 #include <linux/delay.h>
 #include <linux/device.h>
+#include <linux/find.h>
 #include <linux/firmware/samsung/exynos-acpm-protocol.h>
 #include <linux/io.h>
 #include <linux/iopoll.h>
@@ -370,29 +371,40 @@ static int acpm_wait_for_queue_slots(struct acpm_chan *achan, u32 next_tx_front)
  * TX queue.
  * @achan:	ACPM channel info.
  * @xfer:	reference to the transfer being prepared.
+ *
+ * Return: 0 on success, -EBUSY if the sequence number pool is exhausted.
  */
-static void acpm_prepare_xfer(struct acpm_chan *achan,
-			      const struct acpm_xfer *xfer)
+static int acpm_prepare_xfer(struct acpm_chan *achan,
+			     const struct acpm_xfer *xfer)
 {
 	struct acpm_rx_data *rx_data;
 	u32 *txd = (u32 *)xfer->txd;
+	unsigned long size = ACPM_SEQNUM_MAX - 1;
+	unsigned long bit;
+
+	bit = find_next_zero_bit(achan->bitmap_seqnum, size, achan->seqnum);
+	if (bit >= size) {
+		bit = find_first_zero_bit(achan->bitmap_seqnum, size);
+		if (bit >= size) {
+			dev_err_ratelimited(achan->acpm->dev,
+					    "ACPM sequence number pool exhausted\n");
+			return -EBUSY;
+		}
+	}
 
-	/* Prevent chan->seqnum from being re-used */
-	do {
-		if (++achan->seqnum == ACPM_SEQNUM_MAX)
-			achan->seqnum = 1;
-	} while (test_bit(achan->seqnum - 1, achan->bitmap_seqnum));
+	/* Flag the index based on seqnum. (seqnum: 1~63, bitmap: 0~62) */
+	achan->seqnum = bit + 1;
+	set_bit(bit, achan->bitmap_seqnum);
 
 	txd[0] |= FIELD_PREP(ACPM_PROTOCOL_SEQNUM, achan->seqnum);
 
 	/* Clear data for upcoming responses */
-	rx_data = &achan->rx_data[achan->seqnum - 1];
+	rx_data = &achan->rx_data[bit];
 	memset(rx_data->cmd, 0, sizeof(*rx_data->cmd) * rx_data->n_cmd);
 	/* zero means no response expected */
 	rx_data->rxcnt = xfer->rxcnt;
 
-	/* Flag the index based on seqnum. (seqnum: 1~63, bitmap: 0~62) */
-	set_bit(achan->seqnum - 1, achan->bitmap_seqnum);
+	return 0;
 }
 
 /**
@@ -460,7 +472,9 @@ int acpm_do_xfer(struct acpm_handle *handle, const struct acpm_xfer *xfer)
 		if (ret)
 			return ret;
 
-		acpm_prepare_xfer(achan, xfer);
+		ret = acpm_prepare_xfer(achan, xfer);
+		if (ret)
+			return ret;
 
 		/* Write TX command. */
 		__iowrite32_copy(achan->tx.base + achan->mlen * tx_front,

-- 
2.54.0.545.g6539524ca2-goog