linux-kernel - [PATCH 1/5] tty: add port flag to suppress ready signalling on open

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Message-Id: <20201130153742.9163-2-johan@kernel.org>
Date:   Mon, 30 Nov 2020 16:37:38 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>,
        "Mychaela N . Falconia" <falcon@...ecalypso.org>,
        linux-serial@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>
Subject: [PATCH 1/5] tty: add port flag to suppress ready signalling on open

Add a NORDY port flag to suppress raising the modem-control lines on
open to signal DTE readiness.

This can be used to implement a NORDY termios control flag to complement
HUPCL, which controls lowering of the modem-control lines on final
close.

Initially drivers can export the flag through sysfs, which also allows
control over the lines on first open.

This can be use to prevent undesirable side-effects on open for
applications where the DTR and RTS lines are used for non-standard
purposes such as generating power-on and reset pulses.

Signed-off-by: Johan Hovold <johan@...nel.org>
---
 drivers/tty/tty_port.c |  2 +-
 include/linux/tty.h    | 14 ++++++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index ea80bf872f54..2613debc1d06 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -415,7 +415,7 @@ EXPORT_SYMBOL(tty_port_carrier_raised);
  */
 void tty_port_raise_dtr_rts(struct tty_port *port)
 {
-	if (port->ops->dtr_rts)
+	if (port->ops->dtr_rts && !tty_port_nordy(port))
 		port->ops->dtr_rts(port, 1);
 }
 EXPORT_SYMBOL(tty_port_raise_dtr_rts);
diff --git a/include/linux/tty.h b/include/linux/tty.h
index a99e9b8e4e31..31414efd8661 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -267,6 +267,7 @@ struct tty_port {
 #define TTY_PORT_CHECK_CD	4	/* carrier detect enabled */
 #define TTY_PORT_KOPENED	5	/* device exclusively opened by
 					   kernel */
+#define TTY_PORT_NORDY		6	/* do not raise DTR/RTS on open */
 
 /*
  * Where all of the state associated with a tty is kept while the tty
@@ -683,6 +684,19 @@ static inline void tty_port_set_kopened(struct tty_port *port, bool val)
 		clear_bit(TTY_PORT_KOPENED, &port->iflags);
 }
 
+static inline bool tty_port_nordy(struct tty_port *port)
+{
+	return test_bit(TTY_PORT_NORDY, &port->iflags);
+}
+
+static inline void tty_port_set_nordy(struct tty_port *port, bool val)
+{
+	if (val)
+		set_bit(TTY_PORT_NORDY, &port->iflags);
+	else
+		clear_bit(TTY_PORT_NORDY, &port->iflags);
+}
+
 extern struct tty_struct *tty_port_tty_get(struct tty_port *port);
 extern void tty_port_tty_set(struct tty_port *port, struct tty_struct *tty);
 extern int tty_port_carrier_raised(struct tty_port *port);
-- 
2.26.2