serial: 8250_dw: Store LSR into lsr_saved_flags in dw8250_tx_wait_empty()

stable inclusion
from stable-v4.19.256
commit c3ca8a752a422ba1f9580f2e1d41ad650252d8ac
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5Q0SQ
CVE: NA

--------------------------------

[ Upstream commit af14f300 ]

Make sure LSR flags are preserved in dw8250_tx_wait_empty(). This
function is called from a low-level out function and therefore cannot
call serial_lsr_in() as it would lead to infinite recursion.

It is borderline if the flags need to be saved here at all since this
code relates to writing LCR register which usually implies no important
characters should be arriving.

Fixes: 914eaf93 ("serial: 8250_dw: Allow TX FIFO to drain before writing to UART_LCR")
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220608095431.18376-7-ilpo.jarvinen@linux.intel.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

drivers/tty/serial/8250/8250_dw.c

@@ -140,12 +140,15 @@ static void dw8250_check_lcr(struct uart_port *p, int value)
 /* Returns once the transmitter is empty or we run out of retries */
 static void dw8250_tx_wait_empty(struct uart_port *p)
 {
+	struct uart_8250_port *up = up_to_u8250p(p);
 	unsigned int tries = 20000;
 	unsigned int delay_threshold = tries - 1000;
 	unsigned int lsr;
 
 	while (tries--) {
 		lsr = readb (p->membase + (UART_LSR << p->regshift));
+		up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
+
 		if (lsr & UART_LSR_TEMT)
 			break;