[ Upstream commit be03b0651ffd8bab69dfd574c6818b446c0753ce ]

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@linux.intel.c... Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250") Cc: stable stable@kernel.org Acked-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de Signed-off-by: Uwe Kleine-König u.kleine-koenig@pengutronix.de Signed-off-by: Ilpo Järvinen ilpo.jarvinen@linux.intel.com Link: https://lore.kernel.org/r/f4d774be-1437-a550-8334-19d8722ab98c@linux.intel.c... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org ---

Here's the backport for 5.18. I think it applies fine to older kernel versions too.

drivers/tty/serial/8250/8250_port.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 1fbd5bf264be..7e295d2701b2 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1535,6 +1535,8 @@ static inline void __stop_tx(struct uart_8250_port *p)

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