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author | Nils Carlson <nils.carlson@ericsson.com> | 2011-06-15 22:08:54 (GMT) |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2011-06-16 03:04:02 (GMT) |
commit | 273ef9509b7903e50f36aaf9f1d5dc9087fca506 (patch) | |
tree | 4d1800cadbb85647b4db5cf5cb855dc43465536c /arch/mips | |
parent | 31b5f8eeece4c0d70b649bfac7759cf7e3f915dd (diff) | |
download | linux-fsl-qoriq-273ef9509b7903e50f36aaf9f1d5dc9087fca506.tar.xz |
drivers/char/hpet.c: fix periodic-emulation for delayed interrupts
When interrupts are delayed due to interrupt masking or due to other
interrupts being serviced the HPET periodic-emuation would fail. This
happened because given an interval t and a time for the current interrupt
m we would compute the next time as t + m. This works until we are
delayed for > t, in which case we would be writing a new value which is in
fact in the past.
This can be solved by computing the next time instead as (k * t) + m where
k is large enough to be in the future. The exact computation of k is
described in a comment to the code.
More detail:
Assuming an interval of 5 between each expected interrupt we have a normal
case of
t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t5: interrupt, read t5 from comparator, set next interrupt t5 + 5
t10: interrupt, read t10 from comparator, set next interrupt t10 + 5
...
So, what happens when the interrupt is serviced too late?
t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t11: delayed interrupt serviced, read t5 from comparator, set next
interrupt t5 + 5, which is in the past!
... counter loops ...
t10: Much much later, get the next interrupt.
This can happen either because we have interrupts masked for too long
(some stupid driver goes on a printk rampage) or just because we are
pushing the limits of the interval (too small a period), or both most
probably.
My solution is to read the main counter as well and set the next interrupt
to occur at the right interval, for example:
t0: interrupt, read t0 from comparator, set next interrupt t0 + 5
t11: delayed interrupt serviced, read t5 from comparator, set next
interrupt t15 as t10 has been missed.
t15: back on track.
Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'arch/mips')
0 files changed, 0 insertions, 0 deletions