[PATCH v2] serial: core: only stop transmit when HW fifo is empty

[Jonas Gorski <jonas.gorski@gmail.com>]

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>
---
(this is v2 of the bcm63xx-uart fix attempt)

v1 -> v2
* replace workaround with fix for core issue
* add Cc: for stable

I'm somewhat confident this is the core issue causing the broken output
with bcm63xx-uart, and there is no actual need for the UART_TX_NOSTOP.

I wouldn't be surprised if this also fixes mxs-uart for which
UART_TX_NOSTOP was introduced.

If it does, there is no need for the flag anymore.
 include/linux/serial_core.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 55b1f3ba48ac..bb0f2d4ac62f 100644
--- 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);			      \
 	}								      \
 									      \
-- 
2.34.1