tty: serial: samsung_tty: loopback mode support

Internal loopback mode can be supported by setting
UCON register's Loopback Mode bit. The mode & bit can be supported
since s3c2410 and later SoCs. The prefix of LOOPBACK / BIT(5) naming
should be also changed to S3C2410_ in order to avoid confusion.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Link: https://lore.kernel.org/r/20220629004141.51484-1-chanho61.park@samsung.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/samsung_tty.c

@@ -1018,6 +1018,7 @@ static unsigned int s3c24xx_serial_get_mctrl(struct uart_port *port)
 static void s3c24xx_serial_set_mctrl(struct uart_port *port, unsigned int mctrl)
 {
 	unsigned int umcon = rd_regl(port, S3C2410_UMCON);
+	unsigned int ucon = rd_reg(port, S3C2410_UCON);
 
 	if (mctrl & TIOCM_RTS)
 		umcon |= S3C2410_UMCOM_RTS_LOW;
@@ -1025,6 +1026,13 @@ static void s3c24xx_serial_set_mctrl(struct uart_port *port, unsigned int mctrl)
 		umcon &= ~S3C2410_UMCOM_RTS_LOW;
 
 	wr_regl(port, S3C2410_UMCON, umcon);
+
+	if (mctrl & TIOCM_LOOP)
+		ucon |= S3C2410_UCON_LOOPBACK;
+	else
+		ucon &= ~S3C2410_UCON_LOOPBACK;
+
+	wr_regl(port, S3C2410_UCON, ucon);
 }
 
 static void s3c24xx_serial_break_ctl(struct uart_port *port, int break_state)

include/linux/serial_s3c.h

@@ -83,7 +83,7 @@
 #define S3C2410_UCON_RXIRQMODE	  (1<<0)
 #define S3C2410_UCON_RXFIFO_TOI	  (1<<7)
 #define S3C2443_UCON_RXERR_IRQEN  (1<<6)
-#define S3C2443_UCON_LOOPBACK	  (1<<5)
+#define S3C2410_UCON_LOOPBACK	  (1<<5)
 
 #define S3C2410_UCON_DEFAULT	  (S3C2410_UCON_TXILEVEL  | \
 				   S3C2410_UCON_RXILEVEL  | \