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authorGuenter Roeck <linux@roeck-us.net>2012-12-20 15:05:42 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-12-20 17:40:20 -0800
commitc4e18497d8fd92eef2c6e7eadcc1a107ccd115ea (patch)
tree9513b220cbdd633ad7c42299d61f2b46b9fb1561 /fs
parent154b454edaf6d94a69016db6c342c57fa935bbe9 (diff)
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linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors
Commit 263a523d18bc ("linux/kernel.h: Fix warning seen with W=1 due to change in DIV_ROUND_CLOSEST") fixes a warning seen with W=1 due to change in DIV_ROUND_CLOSEST. Unfortunately, the C compiler converts divide operations with unsigned divisors to unsigned, even if the dividend is signed and negative (for example, -10 / 5U = 858993457). The C standard says "If one operand has unsigned int type, the other operand is converted to unsigned int", so the compiler is not to blame. As a result, DIV_ROUND_CLOSEST(0, 2U) and similar operations now return bad values, since the automatic conversion of expressions such as "0 - 2U/2" to unsigned was not taken into account. Fix by checking for the divisor variable type when deciding which operation to perform. This fixes DIV_ROUND_CLOSEST(0, 2U), but still returns bad values for negative dividends divided by unsigned divisors. Mark the latter case as unsupported. One observed effect of this problem is that the s2c_hwmon driver reports a value of 4198403 instead of 0 if the ADC reads 0. Other impact is unpredictable. Problem is seen if the divisor is an unsigned variable or constant and the dividend is less than (divisor/2). Signed-off-by: Guenter Roeck <linux@roeck-us.net> Reported-by: Juergen Beisert <jbe@pengutronix.de> Tested-by: Juergen Beisert <jbe@pengutronix.de> Cc: Jean Delvare <khali@linux-fr.org> Cc: <stable@vger.kernel.org> [3.7.x] Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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