linux-kernel - [PATCH] serial: When UART is suspended, set RTS to false

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Date:   Thu, 24 Mar 2022 10:56:20 -0400
From:   Al Cooper <alcooperx@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Al Cooper <alcooperx@...cast.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org
Subject: [PATCH] serial: When UART is suspended, set RTS to false

From: Al Cooper <alcooperx@...cast.net>

When flow control is enabled, the UART should set RTS to false
during suspend to stop incoming data. Currently, the suspend
routine sets the mctrl register in the uart to zero, but leaves
the shadow version in the uart_port struct alone so that resume
can restore it. This causes a problem later in suspend when
serial8250_do_shutdown() is called which uses the shadow mctrl
register to clear some additional bits but ends up restoring RTS.
The solution is to clear RTS from the shadow version before
serial8250_do_shutdown() is called and restore it after.

Signed-off-by: Al Cooper <alcooperx@...cast.net>
---
 drivers/tty/serial/serial_core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 0db90be4c3bc..a788d1c899d4 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2173,6 +2173,7 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
 	if (tty_port_initialized(port)) {
 		const struct uart_ops *ops = uport->ops;
 		int tries;
+		unsigned int mctrl;
 
 		tty_port_set_suspended(port, 1);
 		tty_port_set_initialized(port, 0);
@@ -2180,6 +2181,9 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
 		spin_lock_irq(&uport->lock);
 		ops->stop_tx(uport);
 		ops->set_mctrl(uport, 0);
+		/* save mctrl so it can be restored on resume */
+		mctrl = uport->mctrl;
+		uport->mctrl = 0;
 		ops->stop_rx(uport);
 		spin_unlock_irq(&uport->lock);
 
@@ -2193,6 +2197,7 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *uport)
 				uport->name);
 
 		ops->shutdown(uport);
+		uport->mctrl = mctrl;
 	}
 
 	/*
-- 
2.17.1