linux/kernel.h: fix DIV_ROUND_CLOSEST with unsigned divisors
Commit 263a523d18
("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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c4e18497d8
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@ -77,13 +77,15 @@
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/*
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* Divide positive or negative dividend by positive divisor and round
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* to closest integer. Result is undefined for negative divisors.
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* to closest integer. Result is undefined for negative divisors and
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* for negative dividends if the divisor variable type is unsigned.
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*/
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#define DIV_ROUND_CLOSEST(x, divisor)( \
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{ \
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typeof(x) __x = x; \
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typeof(divisor) __d = divisor; \
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(((typeof(x))-1) > 0 || (__x) > 0) ? \
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(((typeof(x))-1) > 0 || \
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((typeof(divisor))-1) > 0 || (__x) > 0) ? \
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(((__x) + ((__d) / 2)) / (__d)) : \
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(((__x) - ((__d) / 2)) / (__d)); \
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} \
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