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serial: qcom_geni: fix kfifo underflow when flush precedes DMA completion IRQ

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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.

Fixes: 2aaa43c707 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: stable <stable@kernel.org>
Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260506-serial-dma-stale-tx-buf-v1-1-e3ccb360d719@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Viken Dadhaniya 2026-05-06 10:15:21 +05:30 committed by Greg Kroah-Hartman
parent 9a9254c4a2
commit 452d6fa37a
1 changed files with 13 additions and 1 deletions
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14
drivers/tty/serial/qcom_geni_serial.c
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@ -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;