From bbec5998d7bd349730f59c959a8b00cfff816e34 Mon Sep 17 00:00:00 2001 From: John Keeping Date: Sat, 8 Feb 2025 12:41:44 +0000 Subject: [PATCH] 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 Signed-off-by: John Keeping Link: https://lore.kernel.org/r/20250208124148.1189191-1-jkeeping@inmusicbrands.com Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250.h | 2 ++ drivers/tty/serial/8250/8250_dma.c | 16 ++++++++++++++++ drivers/tty/serial/8250/8250_port.c | 9 +++++++++ 3 files changed, 27 insertions(+) diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h index 04bf2b297fdc..12b9a134fb80 100644 --- a/drivers/tty/serial/8250/8250.h +++ b/drivers/tty/serial/8250/8250.h @@ -300,6 +300,7 @@ 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 *); @@ -316,6 +317,7 @@ 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 bdc7bb7a1b92..0b4139c0a975 100644 --- a/drivers/tty/serial/8250/8250_dma.c +++ b/drivers/tty/serial/8250/8250_dma.c @@ -126,6 +126,22 @@ 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 5d8022cdb50a..6b3ff20a0656 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -2473,6 +2473,14 @@ 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) @@ -3119,6 +3127,7 @@ 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,