serial: 8250: Fix fifo underflow on flush

commit 9e512eaaf8f4008c44ede3dfc0fbc9d9c5118583 upstream.

When flushing the serial port's buffer, uart_flush_buffer() calls
kfifo_reset() but if there is an outstanding DMA transfer then the
completion function will consume data from the kfifo via
uart_xmit_advance(), underflowing and leading to ongoing DMA as the
driver tries to transmit another 2^32 bytes.

This is readily reproduced with serial-generic and amidi sending even
short messages as closing the device on exit will wait for the fifo to
drain and in the underflow case amidi hangs for 30 seconds on exit in
tty_wait_until_sent().  A trace of that gives:

     kworker/1:1-84    [001]    51.769423: bprint:               serial8250_tx_dma: tx_size=3 fifo_len=3
           amidi-763   [001]    51.769460: bprint:               uart_flush_buffer: resetting fifo
 irq/21-fe530000-76    [000]    51.769474: bprint:               __dma_tx_complete: tx_size=3
 irq/21-fe530000-76    [000]    51.769479: bprint:               serial8250_tx_dma: tx_size=4096 fifo_len=4294967293
 irq/21-fe530000-76    [000]    51.781295: bprint:               __dma_tx_complete: tx_size=4096
 irq/21-fe530000-76    [000]    51.781301: bprint:               serial8250_tx_dma: tx_size=4096 fifo_len=4294963197
 irq/21-fe530000-76    [000]    51.793131: bprint:               __dma_tx_complete: tx_size=4096
 irq/21-fe530000-76    [000]    51.793135: bprint:               serial8250_tx_dma: tx_size=4096 fifo_len=4294959101
 irq/21-fe530000-76    [000]    51.804949: bprint:               __dma_tx_complete: tx_size=4096

Since the port lock is held in when the kfifo is reset in
uart_flush_buffer() and in __dma_tx_complete(), adding a flush_buffer
hook to adjust the outstanding DMA byte count is sufficient to avoid the
kfifo underflow.

Fixes: 9ee4b83e51f74 ("serial: 8250: Add support for dmaengine")
Cc: stable <stable@kernel.org>
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Link: https://lore.kernel.org/r/20250208124148.1189191-1-jkeeping@inmusicbrands.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
index d9500a25e03abf34b4bb3a183542497e075fadf0..6e9d6c489813f49b8aa3f4f08dc5cabb123ce805 100644 (file)
--- a/drivers/tty/serial/8250/8250.h
+++ b/drivers/tty/serial/8250/8250.h
@@ -328,6 +328,7 @@ static inline int is_omap1510_8250(struct uart_8250_port *pt)
 
 #ifdef CONFIG_SERIAL_8250_DMA
 extern int serial8250_tx_dma(struct uart_8250_port *);
+extern void serial8250_tx_dma_flush(struct uart_8250_port *);
 extern int serial8250_rx_dma(struct uart_8250_port *);
 extern void serial8250_rx_dma_flush(struct uart_8250_port *);
 extern int serial8250_request_dma(struct uart_8250_port *);
@@ -360,6 +361,7 @@ static inline int serial8250_tx_dma(struct uart_8250_port *p)
 {
        return -1;
 }
+static inline void serial8250_tx_dma_flush(struct uart_8250_port *p) { }
 static inline int serial8250_rx_dma(struct uart_8250_port *p)
 {
        return -1;

diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
index ec3cd723256fb13f485ffb17894aa11b852800fb..7a92e3397908ef0141a5a5754874f7026ab22254 100644 (file)
--- a/drivers/tty/serial/8250/8250_dma.c
+++ b/drivers/tty/serial/8250/8250_dma.c
@@ -137,6 +137,22 @@ err:
        return ret;
 }
 
+void serial8250_tx_dma_flush(struct uart_8250_port *p)
+{
+       struct uart_8250_dma *dma = p->dma;
+
+       if (!dma->tx_running)
+               return;
+
+       /*
+        * kfifo_reset() has been called by the serial core, avoid
+        * advancing and underflowing in __dma_tx_complete().
+        */
+       dma->tx_size = 0;
+
+       dmaengine_terminate_async(dma->rxchan);
+}
+
 int serial8250_rx_dma(struct uart_8250_port *p)
 {
        struct uart_8250_dma            *dma = p->dma;

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 2d595a646bb89c493fb5c73a0d4a3d2ac209f591..be50781ef0b52ec89370b7237161826b1d40ef7a 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2512,6 +2512,14 @@ static unsigned int npcm_get_divisor(struct uart_8250_port *up,
        return DIV_ROUND_CLOSEST(port->uartclk, 16 * baud + 2) - 2;
 }
 
+static void serial8250_flush_buffer(struct uart_port *port)
+{
+       struct uart_8250_port *up = up_to_u8250p(port);
+
+       if (up->dma)
+               serial8250_tx_dma_flush(up);
+}
+
 static unsigned int serial8250_do_get_divisor(struct uart_port *port,
                                              unsigned int baud,
                                              unsigned int *frac)
@@ -3238,6 +3246,7 @@ static const struct uart_ops serial8250_pops = {
        .break_ctl      = serial8250_break_ctl,
        .startup        = serial8250_startup,
        .shutdown       = serial8250_shutdown,
+       .flush_buffer   = serial8250_flush_buffer,
        .set_termios    = serial8250_set_termios,
        .set_ldisc      = serial8250_set_ldisc,
        .pm             = serial8250_pm,