serial: imx: fix data breakage on termios change

imx_set_termios(): avoid writing baud rate divider registers when the
values to be written are the same as current. Any writing seems to
restart transmission/receiving logic in the hardware, that leads to
data breakage even when rate doesn't in fact change. E.g., user
switches RTS/CTS handshake and suddenly gets broken bytes.
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Link: https://lore.kernel.org/r/1567017475-11919-5-git-send-email-sorganov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -1545,7 +1545,7 @@ imx_uart_set_termios(struct uart_port *port, struct ktermios *termios,
 	unsigned int baud, quot;
 	unsigned int old_csize = old ? old->c_cflag & CSIZE : CS8;
 	unsigned long div;
-	unsigned long num, denom;
+	unsigned long num, denom, old_ubir, old_ubmr;
 	uint64_t tdiv64;
 
 	/*
@@ -1670,8 +1670,21 @@ imx_uart_set_termios(struct uart_port *port, struct ktermios *termios,
 	ufcr = (ufcr & (~UFCR_RFDIV)) | UFCR_RFDIV_REG(div);
 	imx_uart_writel(sport, ufcr, UFCR);
 
-	imx_uart_writel(sport, num, UBIR);
-	imx_uart_writel(sport, denom, UBMR);
+	/*
+	 *  Two registers below should always be written both and in this
+	 *  particular order. One consequence is that we need to check if any of
+	 *  them changes and then update both. We do need the check for change
+	 *  as even writing the same values seem to "restart"
+	 *  transmission/receiving logic in the hardware, that leads to data
+	 *  breakage even when rate doesn't in fact change. E.g., user switches
+	 *  RTS/CTS handshake and suddenly gets broken bytes.
+	 */
+	old_ubir = imx_uart_readl(sport, UBIR);
+	old_ubmr = imx_uart_readl(sport, UBMR);
+	if (old_ubir != num || old_ubmr != denom) {
+		imx_uart_writel(sport, num, UBIR);
+		imx_uart_writel(sport, denom, UBMR);
+	}
 
 	if (!imx_uart_is_imx1(sport))
 		imx_uart_writel(sport, sport->port.uartclk / div / 1000,