linux-kernel - [041/121] serial/mxs-auart: increase time to wait for transmitter to become idle

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <lsq.1378608721.183443551@decadent.org.uk>
Date:	Sun, 08 Sep 2013 03:52:01 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
Subject: [041/121] serial/mxs-auart: increase time to wait for transmitter
 to become idle

3.2.51-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>

commit 079a036f4283e2b0e5c26080b8c5112bc0cc1831 upstream.

Without this patch the driver waits ~1 ms for the UART to become idle. At
115200n8 this time is (theoretically) enough to transfer 11.5 characters
(= 115200 bits/s / (10 Bits/char) * 1ms). As the mxs-auart has a fifo size
of 16 characters the clock is gated too early. The problem is worse for
lower baud rates.

This only happens to really shut down the transmitter in the middle of a
transfer if /dev/ttyAPPx isn't opened in userspace (e.g. by a getty) but
was at least once (because the bootloader doesn't disable the transmitter).

So increase the timeout to 20 ms which should be enough for 9600n8, too.
Moreover skip gating the clock if the timeout is elapsed.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/tty/serial/mxs-auart.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

--- a/drivers/tty/serial/mxs-auart.c
+++ b/drivers/tty/serial/mxs-auart.c
@@ -543,7 +543,7 @@ auart_console_write(struct console *co,
 	struct mxs_auart_port *s;
 	struct uart_port *port;
 	unsigned int old_ctrl0, old_ctrl2;
-	unsigned int to = 1000;
+	unsigned int to = 20000;
 
 	if (co->index >	MXS_AUART_PORTS || co->index < 0)
 		return;
@@ -564,18 +564,23 @@ auart_console_write(struct console *co,
 
 	uart_console_write(port, str, count, mxs_auart_console_putchar);
 
-	/*
-	 * Finally, wait for transmitter to become empty
-	 * and restore the TCR
-	 */
+	/* Finally, wait for transmitter to become empty ... */
 	while (readl(port->membase + AUART_STAT) & AUART_STAT_BUSY) {
+		udelay(1);
 		if (!to--)
 			break;
-		udelay(1);
 	}
 
-	writel(old_ctrl0, port->membase + AUART_CTRL0);
-	writel(old_ctrl2, port->membase + AUART_CTRL2);
+	/*
+	 * ... and restore the TCR if we waited long enough for the transmitter
+	 * to be idle. This might keep the transmitter enabled although it is
+	 * unused, but that is better than to disable it while it is still
+	 * transmitting.
+	 */
+	if (!(readl(port->membase + AUART_STAT) & AUART_STAT_BUSY)) {
+		writel(old_ctrl0, port->membase + AUART_CTRL0);
+		writel(old_ctrl2, port->membase + AUART_CTRL2);
+	}
 
 	clk_disable(s->clk);
 }

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/