linux-kernel - [PATCH 3.18 51/68] tty/isicom: fix big-endian compile warning

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Date:   Fri,  5 May 2017 11:32:36 -0700
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: [PATCH 3.18 51/68] tty/isicom: fix big-endian compile warning

3.18-stable review patch.  If anyone has any objections, please let me know.

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

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

commit f3e2d56dce47dbd0bb3f69f84741b439542fef37 upstream.

Building an arm allmodconfig kernel triggers a lengthy but harmless
warning in the isicom driver:

drvers/tty/isicom.c: In function 'isicom_send_break':
uapi/linux/swab.h:13:15: warning: integer overflow in expression [-Woverflow]
  (((__u16)(x) & (__u16)0x00ffU) << 8) |   \
               ^
uapi/linux/swab.h:107:2: note: in expansion of macro '___constant_swab16'
  ___constant_swab16(x) :   \
  ^
uapi/linux/byteorder/big_endian.h:34:43: note: in expansion of macro '__swab16'
 #define __cpu_to_le16(x) ((__force __le16)__swab16((x)))
                                           ^
linux/byteorder/generic.h:89:21: note: in expansion of macro '__cpu_to_le16'
 #define cpu_to_le16 __cpu_to_le16
                     ^
include/asm/io.h:270:6: note: in expansion of macro 'cpu_to_le16'
      cpu_to_le16(v),__io(p)); })
      ^
drivers/tty/isicom.c:1058:2: note: in expansion of macro 'outw'
  outw((length & 0xff00), base);
  ^

Apparently, the problem is related to the fact that the value 0xff00,
when used as a 16-bit number, is negative and passed into bitwise
operands of the generic byte swapping code.

Marking the input argument as unsigned in both technically correct
and avoids the warning.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/tty/isicom.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/tty/isicom.c
+++ b/drivers/tty/isicom.c
@@ -1055,7 +1055,7 @@ static int isicom_send_break(struct tty_
 
 	outw(0x8000 | ((port->channel) << (card->shift_count)) | 0x3, base);
 	outw((length & 0xff) << 8 | 0x00, base);
-	outw((length & 0xff00), base);
+	outw((length & 0xff00u), base);
 	InterruptTheCard(base);
 
 	unlock_card(card);