FAILED: patch "[PATCH] serial: qcom_geni: fix kfifo underflow when flush precedes" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 452d6fa37ae9b021f4f6d397dbae077f7296f6f4
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026060404-computing-retiree-825a@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 452d6fa37ae9b021f4f6d397dbae077f7296f6f4 Mon Sep 17 00:00:00 2001
From: Viken Dadhaniya <[email protected]>
Date: Wed, 6 May 2026 10:15:21 +0530
Subject: [PATCH] serial: qcom_geni: fix kfifo underflow when flush precedes
DMA completion IRQ

When uart_flush_buffer() runs before the DMA completion IRQ is delivered, the following race can occur (all steps serialized by uart_port_lock):

1. DMA starts: tx_remaining = N, kfifo contains N bytes
2. DMA completes in hardware; IRQ is pending but not yet delivered
3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(), making kfifo_len() = 0 while tx_remaining remains N
4. uart_flush_buffer() releases the port lock
5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls uart_xmit_advance(uport, tx_remaining) on an empty kfifo

uart_xmit_advance() increments kfifo->out by tx_remaining. Since kfifo_reset() already set both in and out to 0, out wraps past in, causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next start_tx_dma() call then submits a DMA transfer of stale buffer data.

Fix this by snapshotting kfifo_len() at the start of handle_tx_dma() and skipping uart_xmit_advance() when fifo_len < tx_remaining, which indicates the kfifo was reset by a preceding flush.

index 5139a9d21b2b..17da115b1e78 100644
@@ -1031,8 +1031,20 @@ static void qcom_geni_serial_handle_tx_dma(struct uart_port *uport)
 {
 	struct qcom_geni_serial_port *port = to_dev_port(uport);
 	struct tty_port *tport = &uport->state->port;
+	unsigned int fifo_len = kfifo_len(&tport->xmit_fifo);
+
+	/*
+	 * Only advance the kfifo if it still contains the bytes that were
+	 * transferred. uart_flush_buffer() may have run before this IRQ
+	 * fired: it calls kfifo_reset() under the port lock, making
+	 * fifo_len = 0 while tx_remaining remains non-zero. Calling
+	 * uart_xmit_advance() in that case would underflow kfifo->out past
+	 * kfifo->in, making kfifo_len() wrap to UART_XMIT_SIZE - tx_remaining
+	 * and triggering a spurious large DMA transfer of stale data.
+	 */
+	if (fifo_len >= port->tx_remaining)
+		uart_xmit_advance(uport, port->tx_remaining);
 
-	uart_xmit_advance(uport, port->tx_remaining);
 	geni_se_tx_dma_unprep(&port->se, port->tx_dma_addr, port->tx_remaining);
 	port->tx_dma_addr = 0;
 	port->tx_remaining = 0;

From: Viken Dadhaniya <[email protected]>

[ Upstream commit 452d6fa37ae9b021f4f6d397dbae077f7296f6f4 ]

When uart_flush_buffer() runs before the DMA completion IRQ is delivered, the following race can occur (all steps serialized by uart_port_lock):

1. DMA starts: tx_remaining = N, kfifo contains N bytes
2. DMA completes in hardware; IRQ is pending but not yet delivered
3. uart_flush_buffer() acquires the port lock and calls kfifo_reset(), making kfifo_len() = 0 while tx_remaining remains N
4. uart_flush_buffer() releases the port lock
5. DMA IRQ fires; handle_tx_dma() acquires the port lock and calls uart_xmit_advance(uport, tx_remaining) on an empty kfifo

uart_xmit_advance() increments kfifo->out by tx_remaining. Since kfifo_reset() already set both in and out to 0, out wraps past in, causing kfifo_len() to return UART_XMIT_SIZE - tx_remaining. The next start_tx_dma() call then submits a DMA transfer of stale buffer data.

Fix this by snapshotting kfifo_len() at the start of handle_tx_dma() and skipping uart_xmit_advance() when fifo_len < tx_remaining, which indicates the kfifo was reset by a preceding flush.

---
 drivers/tty/serial/qcom_geni_serial.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

index f820a09cb5c39b..b97faf0c804bfd 100644
@@ -962,8 +962,21 @@ static void qcom_geni_serial_handle_tx_dma(struct uart_port *uport)
 {
 	struct qcom_geni_serial_port *port = to_dev_port(uport);
 	struct circ_buf *xmit = &uport->state->xmit;
+	unsigned int chars_pending = uart_circ_chars_pending(xmit);
+
+	/*
+	 * Only advance the buffer if it still contains the bytes that were
+	 * transferred. uart_flush_buffer() may have run before this IRQ
+	 * fired: it clears the circular buffer under the port lock, making
+	 * chars_pending = 0 while tx_remaining remains non-zero. Calling
+	 * uart_xmit_advance() in that case would advance xmit->tail past
+	 * xmit->head, making uart_circ_chars_pending() wrap to
+	 * UART_XMIT_SIZE - tx_remaining and triggering a spurious large DMA
+	 * transfer of stale data.
+	 */
+	if (chars_pending >= port->tx_remaining)
+		uart_xmit_advance(uport, port->tx_remaining);
 
-	uart_xmit_advance(uport, port->tx_remaining);
 	geni_se_tx_dma_unprep(&port->se, port->tx_dma_addr, port->tx_remaining);
 	port->tx_dma_addr = 0;
 	port->tx_remaining = 0;

2.53.0