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authorJohn Stultz <john.stultz@linaro.org>2015-03-11 21:16:29 -0700
committerIngo Molnar <mingo@kernel.org>2015-03-12 10:16:38 +0100
commit6086e346fdea1ae64d974c94c1acacc2605567ae (patch)
tree9da654f78319b3ebc583896db20c2413a62c6b8f /kernel/time/clocksource.c
parent0bbdb4258bd116d8cd5d209e8d5b29bae516e5b3 (diff)
clocksource: Simplify the clocks_calc_max_nsecs() logic
The previous clocks_calc_max_nsecs() code 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 with signed 64-bit values, where as the intervals are always unsigned. This resulted in overly conservative intervals, which other safety margins were then added to, reducing the intended interval length. Signed-off-by: John Stultz <john.stultz@linaro.org> Cc: Dave Jones <davej@codemonkey.org.uk> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Prarit Bhargava <prarit@redhat.com> Cc: Richard Cochran <richardcochran@gmail.com> Cc: Stephen Boyd <sboyd@codeaurora.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1426133800-29329-2-git-send-email-john.stultz@linaro.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel/time/clocksource.c')
-rw-r--r--kernel/time/clocksource.c15
1 files changed, 3 insertions, 12 deletions
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 4892352f0e49..2148f413256c 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