linux-kernel - [PATCH] staging: iio: ad2s1200: Fix sign extension

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Date:	Fri, 23 Jan 2015 00:09:56 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Lars-Peter Clausen <lars@...afoo.de>,
	Jonathan Cameron <jic23@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Hartmut Knaack <knaack.h@....de>,
	Peter Meerwald <pmeerw@...erw.net>, linux-iio@...r.kernel.org,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: [PATCH] staging: iio: ad2s1200: Fix sign extension

The line above makes vel a 12-bit quantity (st->rx[] is u8). The
intention is to sign-extend vel using bit 11 as the sign bit. But
because of C's promotion rules "vel = (vel << 4) >> 4;" is actually a
no-op, since vel is promoted to int before the inner
shift. sign_extend32 works equally well for 8 and 16 bits types, so
use that.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 drivers/staging/iio/resolver/ad2s1200.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/iio/resolver/ad2s1200.c b/drivers/staging/iio/resolver/ad2s1200.c
index 017d2f8379b7..c17893b4918c 100644
--- a/drivers/staging/iio/resolver/ad2s1200.c
+++ b/drivers/staging/iio/resolver/ad2s1200.c
@@ -18,6 +18,7 @@
 #include <linux/delay.h>
 #include <linux/gpio.h>
 #include <linux/module.h>
+#include <linux/bitops.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>
@@ -68,7 +69,7 @@ static int ad2s1200_read_raw(struct iio_dev *indio_dev,
 		break;
 	case IIO_ANGL_VEL:
 		vel = (((s16)(st->rx[0])) << 4) | ((st->rx[1] & 0xF0) >> 4);
-		vel = (vel << 4) >> 4;
+		vel = sign_extend32(vel, 11);
 		*val = vel;
 		break;
 	default:
-- 
2.1.3

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