serial: 8250: Store to lsr_save_flags after lsr read
authorIlpo Järvinen <ilpo.jarvinen@linux.intel.com>
Fri, 20 May 2022 10:35:41 +0000 (13:35 +0300)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 22 Jun 2022 12:22:04 +0000 (14:22 +0200)
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@linux.intel.com/T/#m1737eef986bd20cf19593e344cebd7b0244945fc
Fixes: e490c9144cfa ("tty: Add software emulated RS485 support for 8250")
Cc: stable <stable@kernel.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@penugtronix.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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/tty/serial/8250/8250_port.c

index 2285ef9..df9731f 100644 (file)
@@ -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