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Subject[PATCH 4.19 216/234] serial: 8250: Store to lsr_save_flags after lsr read
From: Ilpo Järvinen <>

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.

Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250")
Cc: stable <>
Acked-by: Uwe Kleine-König <>
Signed-off-by: Uwe Kleine-König <>
Signed-off-by: Ilpo Järvinen <>
Signed-off-by: Greg Kroah-Hartman <>
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

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