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s390/time: steer clocksource on STP sync events

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On STP sync events the TOD clock will jump in time, either forward or
backward. The TOD clocksource claims to be continuous but in case of
an STP sync with a negative offset it is not.

Subtract the offset injected by the STP sync check from the result of
the TOD clocksource to make it continuous again. Add code to drift the
offset towards zero with a fixed rate, steering 1 second in ~9 hours.

Suggested-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
This commit is contained in:
Martin Schwidefsky
2016-10-11 12:49:50 +02:00
parent 2ace06ec0d
commit 75c7b6f3f6
7 changed files with 106 additions and 7 deletions
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11
arch/s390/kernel/vdso64/clock_gettime.S
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@@ -83,8 +83,17 @@ __kernel_clock_gettime:
tmll %r4,0x0001 /* pending update ? loop */
jnz 5b
stcke 0(%r15) /* Store TOD clock */
lgf %r2,__VDSO_TK_SHIFT(%r5) /* Timekeeper shift */
lg %r1,1(%r15)
lg %r0,__VDSO_TS_END(%r5) /* TOD steering end time */
slgr %r0,%r1 /* now - ts_steering_end */
ltgr %r0,%r0 /* past end of steering ? */
jm 17f
srlg %r0,%r0,15 /* 1 per 2^16 */
tm __VDSO_TS_DIR+3(%r5),0x01 /* steering direction? */
jz 18f
lcgr %r0,%r0 /* negative TOD offset */
18: algr %r1,%r0 /* add steering offset */
17: lgf %r2,__VDSO_TK_SHIFT(%r5) /* Timekeeper shift */
sg %r1,__VDSO_XTIME_STAMP(%r5) /* TOD - cycle_last */
msgf %r1,__VDSO_TK_MULT(%r5) /* * tk->mult */
alg %r1,__VDSO_XTIME_NSEC(%r5) /* + tk->xtime_nsec */