linux-kernel - Re: [PATCH: 2/2] [SERIAL] avoid stalling suspend if serial port won't drain

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Date:	Tue, 8 Jan 2008 11:57:03 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Linux Kernel List <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH: 2/2] [SERIAL] avoid stalling suspend if serial port won't drain

Some ports seem to be unable to drain their transmitters on shut down.
Such a problem can occur if the port is programmed for hardware imposed
flow control, characters are in the FIFO but the CTS signal is inactive.

Normally, this isn't a problem because most places where we wait for the
transmitter to drain have a time-out.  However, there is no timeout in
the suspend path.

Give a port 30ms to drain; this is an arbitary value chosen to avoid
long delays if there are many such ports in the system, while giving a
reasonable chance for a single port to drain.  Should a port not drain
within this timeout, issue a warning.

Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>

diff --git a/drivers/serial/serial_core.c b/drivers/serial/serial_core.c
index 3bb5d24..4ce219c 100644
--- a/drivers/serial/serial_core.c
+++ b/drivers/serial/serial_core.c
@@ -1977,6 +1977,7 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *port)
 
 	if (state->info && state->info->flags & UIF_INITIALIZED) {
 		const struct uart_ops *ops = port->ops;
+		int tries;
 
 		state->info->flags = (state->info->flags & ~UIF_INITIALIZED)
 				     | UIF_SUSPENDED;
@@ -1990,9 +1991,14 @@ int uart_suspend_port(struct uart_driver *drv, struct uart_port *port)
 		/*
 		 * Wait for the transmitter to empty.
 		 */
-		while (!ops->tx_empty(port)) {
+		for (tries = 3; !ops->tx_empty(port) && tries; tries--) {
 			msleep(10);
 		}
+		if (!tries)
+			printk(KERN_ERR "%s%s%s%d: Unable to drain transmitter\n",
+			       port->dev ? port->dev->bus_id : "",
+			       port->dev ? ": " : "",
+			       drv->dev_name, port->line);
 
 		ops->shutdown(port);
 	}


-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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