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).
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| @@ -77,13 +77,15 @@ · 76 unmodified lines | |||
| 77 | 77 | ||
| 78 | /* | 78 | /* |
| 79 | * Divide positive or negative dividend by positive divisor and round | 79 | * Divide positive or negative dividend by positive divisor and round |
| 80 | * to closest integer. Result is undefined for negative divisors. | 80 | * to closest integer. Result is undefined for negative divisors and |
| 81 | * for negative dividends if the divisor variable type is unsigned. | ||
| 81 | */ | 82 | */ |
| 82 | #define DIV_ROUND_CLOSEST(x, divisor)( \ | 83 | #define DIV_ROUND_CLOSEST(x, divisor)( \ |
| 83 | { \ | 84 | { \ |
| 84 | typeof(x) __x = x; \ | 85 | typeof(x) __x = x; \ |
| 85 | typeof(divisor) __d = divisor; \ | 86 | typeof(divisor) __d = divisor; \ |
| 86 | (((typeof(x))-1) > 0 || (__x) > 0) ? \ | 87 | (((typeof(x))-1) > 0 || \ |
| 88 | ((typeof(divisor))-1) > 0 || (__x) > 0) ? \ | ||
| 87 | (((__x) + ((__d) / 2)) / (__d)) : \ | 89 | (((__x) + ((__d) / 2)) / (__d)) : \ |
| 88 | (((__x) - ((__d) / 2)) / (__d)); \ | 90 | (((__x) - ((__d) / 2)) / (__d)); \ |
| 89 | } \ | 91 | } \ |