[PATCH 4.19 216/234] serial: 8250: Store to lsr_save_flags after lsr read

[Greg Kroah-Hartman]

From: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>

commit be03b0651ffd8bab69dfd574c6818b446c0753ce upstream.

Not all LSR register flags are preserved across reads. Therefore, LSR
readers must store the non-preserved bits into lsr_save_flags.

This fix was initially mixed into feature commit f6f586102add ("serial:
8250: Handle UART without interrupt on TEMT using em485"). However,
that feature change had a flaw and it was reverted to make room for
simpler approach providing the same feature. The embedded fix got
reverted with the feature change.

Re-add the lsr_save_flags fix and properly mark it's a fix.

Link: https://lore.kernel.org/all/1d6c31d-d194-9e6a-ddf9-5f29af829f3@xxxxxxxxxxxxxxx/T/#m1737eef986bd20cf19593e344cebd7b0244945fc
Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250")
Cc: stable <stable@xxxxxxxxxx>
Acked-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/f4d774be-1437-a550-8334-19d8722ab98c@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/serial/8250/8250_port.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1522,6 +1522,8 @@ static inline void __stop_tx(struct uart
 
 	if (em485) {
 		unsigned char lsr = serial_in(p, UART_LSR);
+		p->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
+
 		/*
 		 * To provide required timeing and allow FIFO transfer,
 		 * __stop_tx_rs485() must be called only when both FIFO and