ARM: EXYNOS: Allow to use architected timer
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Cover letter [PATCH 0/2] ARM: EXYNOS: Allow to use architected timer
Quoted 10 lines · On Tuesday 06 October 2015 11:56:33 Pavel Fedin wrote:
Hello!
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IIRC the use of the CP15 timer is not reliable on this platform. I don't have a link ready, but this has come up a few times.
I know that Samsung SoC guys are not very keen on this, therefore i actually made this optional. If
you want, you could omit the second patch, and by default Exynos platform will use MCT. But if someone wants to use KVM, it would be enough only to modify device tree in order to enable CP15. Would this be a good compromise?
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Regarding KVM usage, would it be possible to emulate the architected timer from qemu instead?
Unfortunately no. Accesses to virtual timer registers cannot be trapped. It is possible to trap
physical timer accesses, and trick the kernel into using it, but it's slow, and it is a different story, which you can read here: http://www.spinics.net/lists/kvm/msg121714.html.
So, i think that having an option to use CP15 would be a good solution.
My feeling is that is shouldn't be keyed off the presence of the device in DT though. Maybe we can find a way that allows you to put the device into DT but not have it used by default unless the user explicitly enables it, e.g. though a kernel command line option like "force_broken_archtimer"?
Or maybe we could find a way to keep using MCT in the host but use the arch timer in the guest only? At least that would not potentially break anything that works today.
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It is known to work, and i use this configuration here on SMDK5410 without any problems.
For all I can tell, nobody every /saw/ the problem, we just know that it hasn't passed verification (or something like that) and you shouldn't use it in production.
Arnd