On 10/28/2015 11:30 AM, Alex Smith wrote:
On 28 October 2015 at 18:21, Leonid Yegoshin <leonid.yegos...@imgtec.com> wrote:
1) I don't see that in code - there is no check that kernel uses actually uses
R4K clocksource as primary (A), and if kernel uses R4K count as a clocksource
and later switches to some more precise clocksource then there is no change in
VDSO gettimeofday handling (B).
Incorrect. The vdso_clock_mode flag in arch_clocksource_data that I
mentioned in my previous email is copied into the VDSO data page by
update_vsyscall(), which is called when the clocksource changes.
OK, I see this, good.
2) The fact that MIPS kernel as today uses CP0_COUNT in any core as the same
clocksource is correct only until first power saving event with CPU clock
disabled (skipping Octeon). After that it is an incorrect use without an
accurate synchronization and that synchronization doesn't exist.
And I remember that today kernel uses only CPU0 CP0_COUNT to update time... may
be wrong, need to check, but that could be a good code.
Maybe I'm missing something but I don't see anything in the generic
timekeeping code that handles the same clocksource being
unsynchronised or running at a different rate on different CPUs.
(I would like to skip here the generic timekeeping code capabilities, just to
restrict a discussion to HW capabilities)
Given that, if you think there is an issue that prevents the VDSO from
using it then that would surely affect the kernel as well and needs to
be fixed separately?
If it really is necessary to prevent the VDSO from using a certain
clocksource even though the kernel is using it, it should be a simple
matter of setting clocksource.archdata.vdso_clock_mode to
VDSO_CLOCK_NONE. This is how this patch stops it using the CP0 count
when RDHWR is broken.
OK, just put kernel-build time check that it is not SMP without GIC clocksource
or it is Octeon. It would be enough to stop a mess.
If you feel it's necessary then please do.
Please resend a patch with this fix.
- Leonid.
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