linux-kernel - [PATCH 3.16 229/294] tty: nozomi: avoid a harmless gcc warning

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Date:   Mon, 06 Nov 2017 23:03:02 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Arnd Bergmann" <arnd@...db.de>
Subject: [PATCH 3.16 229/294] tty: nozomi: avoid a harmless gcc warning

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

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

From: Arnd Bergmann <arnd@...db.de>

commit a4f642a8a3c2838ad09fe8313d45db46600e1478 upstream.

The nozomi wireless data driver has its own helper function to
transfer data from a FIFO, doing an extra byte swap on big-endian
architectures, presumably to bring the data back into byte-serial
order after readw() or readl() perform their implicit byteswap.

This helper function is used in the receive_data() function to
first read the length into a 32-bit variable, which causes
a compile-time warning:

drivers/tty/nozomi.c: In function 'receive_data':
drivers/tty/nozomi.c:857:9: warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]

The problem is that gcc is unsure whether the data was actually
read or not. We know that it is at this point, so we can replace
it with a single readl() to shut up that warning.

I am leaving the byteswap in there, to preserve the existing
behavior, even though this seems fishy: Reading the length of
the data into a cpu-endian variable should normally not use
a second byteswap on big-endian systems, unless the hardware
is aware of the CPU endianess.

There appears to be a lot more confusion about endianess in this
driver, so it probably has not worked on big-endian systems in
a long time, if ever, and I have no way to test it. It's well
possible that this driver has not been used by anyone in a while,
the last patch that looks like it was tested on the hardware is
from 2008.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/tty/nozomi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/tty/nozomi.c
+++ b/drivers/tty/nozomi.c
@@ -823,7 +823,7 @@ static int receive_data(enum port_type i
 	struct tty_struct *tty = tty_port_tty_get(&port->port);
 	int i, ret;

-	read_mem32((u32 *) &size, addr, 4);
+	size = __le32_to_cpu(readl(addr));
 	/*  DBG1( "%d bytes port: %d", size, index); */

 	if (tty && test_bit(TTY_THROTTLED, &tty->flags)) {