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

[ Upstream commit 7bfb915a597a301abb892f620fe5c283a9fdbd77 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 27a26092493adb723bca7814721f3a3a048dd25c..a7d5fa892be267230865df69a431e1da971a661c 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);                         \
        }                                                                     \
                                                                              \