The previous clocks_calc_max_nsecs had some unecessarily complex bit logic to find the max interval that could cause multiplication overflows. Since this is not in the hot path, just do the divide to make it easier to read.
The previous implementation also had a subtle issue that it avoided overflows into signed 64bit values, where as the intervals are always unsigned. This resulted in overly conservative intervals, which other saftey margins were then added to, reducing the intended interval length. Cc: Dave Jones <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Richard Cochran <[email protected]> Cc: Prarit Bhargava <[email protected]> Cc: Stephen Boyd <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Peter Zijlstra <[email protected]> Signed-off-by: John Stultz <[email protected]> --- kernel/time/clocksource.c | 15 +++------------ 1 file changed, 3 insertions(+), 12 deletions(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 4892352..11323f4 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -476,19 +476,10 @@ u64 clocks_calc_max_nsecs(u32 mult, u32 shift, u32 maxadj, u64 mask) /* * Calculate the maximum number of cycles that we can pass to the - * cyc2ns function without overflowing a 64-bit signed result. The - * maximum number of cycles is equal to ULLONG_MAX/(mult+maxadj) - * which is equivalent to the below. - * max_cycles < (2^63)/(mult + maxadj) - * max_cycles < 2^(log2((2^63)/(mult + maxadj))) - * max_cycles < 2^(log2(2^63) - log2(mult + maxadj)) - * max_cycles < 2^(63 - log2(mult + maxadj)) - * max_cycles < 1 << (63 - log2(mult + maxadj)) - * Please note that we add 1 to the result of the log2 to account for - * any rounding errors, ensure the above inequality is satisfied and - * no overflow will occur. + * cyc2ns function without overflowing a 64-bit result. */ - max_cycles = 1ULL << (63 - (ilog2(mult + maxadj) + 1)); + max_cycles = ULLONG_MAX; + do_div(max_cycles, mult+maxadj); /* * The actual maximum number of cycles we can defer the clocksource is -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
