Re: [PATCH] timekeeping: Move persistent clock registration code from ARM to kernel

[John Stultz]

On Thu, Nov 13, 2014 at 2:46 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Mon, 10 Nov 2014, Anatol Pomozov wrote:
>> On Mon, Nov 10, 2014 at 1:53 AM, Thierry Reding
>> <thierry.reding@xxxxxxxxx> wrote:
>> > On Fri, Nov 07, 2014 at 11:34:15AM -0800, Anatol Pomozov wrote:
>> >> ARM timekeeping functionality allows to register persistent/boot clock dynamically.
>> >> This code is arch-independent and can be useful on other plaforms as well.
>> >>
>> >> As a byproduct of this change, tegra20_timer becomes ARM64 compatible.
>> >>
>> >> Tested: backported the change to chromeos-3.14 kernel ran on tegra 64bit
>> >> board, made sure high-resolution clock works.
>> >
>> > Using this on an upstream kernel doesn't work, though, because 64-bit
>> > ARM doesn't implement struct delay_timer which the driver needs since
>> > v3.17.
>> >
>> > But I suppose the delay timer infrastructure could be moved into the
>> > core similar to the persistent and boot clock as this patch does.
>>
>> Thanks. It makes sense, I will send it in a separate patch, once this
>> one will be reviewed. On our kernel I haven't seen this issue as we
>> still use 3.14.
>
> That's why you should test/compile your stuff on latest greatest and
> not on a year old conglomorate of unknown provenance. :)
>
> Aside of that I really wonder why we need that persistent_clock stuff
> at all. We already have mechanisms to register persistent clocks AKA
> RTCs after the early boot process and update the wall clock time
> before we actually need it. Nothing in early boot depends on correct
> wall clock at all.
>
> So instead of adding more extra persistent clock nonsense, can we just
> move all of that to the place where it belongs, i.e. RTC?

Sigh.. I've got this on an eventual todo list.. The big problem though
is that the RTC infrastructure can't be called with irqs off, so its
not as optimal for measuring suspend time. Some of the suspend-time
measurement with clocksources that don't halt is interesting here.

So we need to add to the RTC infrastructure special accessors that are
safe when irqs are off, and we can then deprecate the persistent clock
bits. There's still evaluation quirks with setting the time earlier in
boot or not (possibly some rng effects as well there), but that could
be worked out if we had the suspend timing via safe RTC interfaces
sorted.

thanks
-john
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