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x86/vdso: Document vgtod_ts better

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After reading do_hres() and do_course() and scratching my head a
bit, I figured out why the arithmetic is strange.  Document it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/f66f53d81150bbad47d7b282c9207a71a3ce1c16.1538689401.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2018-10-04 14:44:45 -07:00 committed by Ingo Molnar
parent 89fe0a1f1c
commit bcc4a62a73
1 changed files with 9 additions and 0 deletions
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9
arch/x86/include/asm/vgtod.h
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@ -13,6 +13,15 @@ typedef u64 gtod_long_t;
typedef unsigned long gtod_long_t;
#endif
/*
* There is one of these objects in the vvar page for each
* vDSO-accelerated clockid. For high-resolution clocks, this encodes
* the time corresponding to vsyscall_gtod_data.cycle_last. For coarse
* clocks, this encodes the actual time.
*
* To confuse the reader, for high-resolution clocks, nsec is left-shifted
* by vsyscall_gtod_data.shift.
*/
struct vgtod_ts {
u64 sec;
u64 nsec;