From: "Ilpo J�rvinen" ilpo.jarvinen@linux.intel.com
There's a potential race before THRE/TEMT deasserts when DMA Tx is starting up (or the next batch of continuous Tx is being submitted). This can lead to misdetecting Tx empty condition.
It is entirely normal for THRE/TEMT to be set for some time after the DMA Tx had been setup in serial8250_tx_dma(). As Tx side is definitely not empty at that point, it seems incorrect for serial8250_tx_empty() claim Tx is empty.
Fix the race by also checking in serial8250_tx_empty() whether there's DMA Tx active.
Note: This fix only addresses in-kernel race mainly to make using TCSADRAIN/FLUSH robust. Userspace can still cause other races but they seem userspace concurrency control problems.
Fixes: 9ee4b83e51f74 ("serial: 8250: Add support for dmaengine") Cc: stable@vger.kernel.org Signed-off-by: Ilpo Järvinen ilpo.jarvinen@linux.intel.com Link: https://lore.kernel.org/r/20230317113318.31327-3-ilpo.jarvinen@linux.intel.c... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org (cherry picked from commit 146a37e05d620cef4ad430e5d1c9c077fe6fa76f) Signed-off-by: Ilpo Järvinen ilpo.jarvinen@linux.intel.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/tty/serial/8250/8250.h | 12 ++++++++++++ drivers/tty/serial/8250/8250_port.c | 12 +++++++++--- 2 files changed, 21 insertions(+), 3 deletions(-)
--- a/drivers/tty/serial/8250/8250.h +++ b/drivers/tty/serial/8250/8250.h @@ -305,6 +305,13 @@ extern int serial8250_rx_dma(struct uart extern void serial8250_rx_dma_flush(struct uart_8250_port *); extern int serial8250_request_dma(struct uart_8250_port *); extern void serial8250_release_dma(struct uart_8250_port *); + +static inline bool serial8250_tx_dma_running(struct uart_8250_port *p) +{ + struct uart_8250_dma *dma = p->dma; + + return dma && dma->tx_running; +} #else static inline int serial8250_tx_dma(struct uart_8250_port *p) { @@ -320,6 +327,11 @@ static inline int serial8250_request_dma return -1; } static inline void serial8250_release_dma(struct uart_8250_port *p) { } + +static inline bool serial8250_tx_dma_running(struct uart_8250_port *p) +{ + return false; +} #endif
static inline int ns16550a_goto_highspeed(struct uart_8250_port *up) --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1922,19 +1922,25 @@ static int serial8250_tx_threshold_handl static unsigned int serial8250_tx_empty(struct uart_port *port) { struct uart_8250_port *up = up_to_u8250p(port); + unsigned int result = 0; unsigned long flags; unsigned int lsr;
serial8250_rpm_get(up);
spin_lock_irqsave(&port->lock, flags); - lsr = serial_port_in(port, UART_LSR); - up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS; + if (!serial8250_tx_dma_running(up)) { + lsr = serial_port_in(port, UART_LSR); + up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS; + + if ((lsr & BOTH_EMPTY) == BOTH_EMPTY) + result = TIOCSER_TEMT; + } spin_unlock_irqrestore(&port->lock, flags);
serial8250_rpm_put(up);
- return (lsr & BOTH_EMPTY) == BOTH_EMPTY ? TIOCSER_TEMT : 0; + return result; }
unsigned int serial8250_do_get_mctrl(struct uart_port *port)