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

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Date:	Tue, 13 Jan 2015 22:59:47 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	gregkh@...uxfoundation.org
Cc:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH] tty/isicom: fix big-endian compile warning

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>

diff --git a/drivers/tty/isicom.c b/drivers/tty/isicom.c
index 59ed783c4bcd..2054427992e0 100644
--- a/drivers/tty/isicom.c
+++ b/drivers/tty/isicom.c
@@ -1055,7 +1055,7 @@ static int isicom_send_break(struct tty_struct *tty, int length)
 
 	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);

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