From: Andy Lutomirski <luto@kernel.org>
Date: Thu, 4 Oct 2018 21:44:45 +0000 (-0700)
Subject: x86/vdso: Document vgtod_ts better
X-Git-Tag: v5.4-rc1~2407^2~1
X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=bcc4a62a73cb65327d7268fbfa3a786d603f52dc;p=platform%2Fkernel%2Flinux-rpi.git

x86/vdso: Document vgtod_ts better

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>
---

diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index d17b092..69d05c6 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -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;