serial: core: only stop transmit when HW fifo is empty

If the circular buffer is empty, it just means we fit all characters to
send into the HW fifo, but not that the hardware finished transmitting
them.

So if we immediately call stop_tx() after that, this may abort any
pending characters in the HW fifo, and cause dropped characters on the
console.

Fix this by only stopping tx when the tx HW fifo is actually empty.

Fixes: 8275b48b2780 ("tty: serial: introduce transmit helpers")
Cc: stable@vger.kernel.org
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Link: https://lore.kernel.org/r/20240303150807.68117-1-jonas.gorski@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 55b1f3ba48ac1725f110747b8d05ce6bbfc1de0b..bb0f2d4ac62f68b0b8b3a47bddc1ff19f61c358d 100644 (file)
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -786,7 +786,8 @@ enum UART_TX_FLAGS {
        if (pending < WAKEUP_CHARS) {                                         \
                uart_write_wakeup(__port);                                    \
                                                                              \
-               if (!((flags) & UART_TX_NOSTOP) && pending == 0)              \
+               if (!((flags) & UART_TX_NOSTOP) && pending == 0 &&            \
+                   __port->ops->tx_empty(__port))                            \
                        __port->ops->stop_tx(__port);                         \
        }                                                                     \
                                                                              \