[PATCH v2] tty: serial: samsung_tty: loopback mode support

[Chanho Park]

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_ to avoid confusion.

We can test it by linux-serial-test program
with -k option. The tool will set TIOCM_LOOP mode when the option is
specified.
-k, --loopback     Use internal hardware loop back

Signed-off-by: Chanho Park <chanho61.park at samsung.com>
---
Changes from v1:
- Drop TIOCM_LOOP return from s3c24xx_serial_get_mctrl as pointed out by
  Ilpo. Documentation/driver-api/serial/driver.rst indicates the bit is
  only for set_mctrl.
- Change the loopback bit definition from S3C2443_UCON_LOOPBACK to
  S3C2410_UCON_LOOPBACK because it has been supported since s3c2410.
- Remove the unnecessary footnote and put a blank line before Signed-off-by

 drivers/tty/serial/samsung_tty.c | 8 ++++++++
 include/linux/serial_s3c.h       | 2 +-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
index d5ca904def34..03ef4ff506fd 100644
--- a/drivers/tty/serial/samsung_tty.c
+++ b/drivers/tty/serial/samsung_tty.c
@@ -1012,6 +1012,7 @@ 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;
@@ -1019,6 +1020,13 @@ 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)
diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
index dec15f5b3dec..1672cf0810ef 100644
--- a/include/linux/serial_s3c.h
+++ b/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  | \
-- 
2.36.1