Re: [PATCH] U300 sched_clock implementation

[Linus Walleij]

Adding in Paul M since it was his patch that was supposed to fix up a
generic solution...

2009/7/7 Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>:
> On Tue, Jun 02, 2009 at 11:00:12AM +0200, Peter Zijlstra wrote:
>> On Mon, 2009-06-01 at 09:46 +0200, Linus Walleij wrote:
>> > 2009/5/25 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
>> > > On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote:
>> > >> 2009/5/24 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
>> > >>
>> > >> > On Sat, 2009-05-23 at 23:46 +0200, Linus Walleij wrote:
>> > >> >
>> > >> >> This overrides the global sched_clock() symbol in the Linux
>> > >> >> scheduler with a local implementation which takes advantage of
>> > >> >> the timesource in U300 giving a scheduling resolution of 1us. The
>> > >> >> solution is the same as found in the OMAP2 core code.
>> > >> >
>> > >> > We assume sched_clock() to return time in ns (e-9) resolution.
>> > >>
>> > >> Yep okay and in this case:
>> > >>
>> > >> >> +       ret = (unsigned long long) u300_get_cycles();
>> > >> >> +       ret = (ret * clocksource_u300_1mhz.mult_orig) >>
>> > >> >> +               clocksource_u300_1mhz.shift;
>> > >> >> +       return ret;
>> > >>
>> > >> (mult_orig >> shift) == 1000
>> > >
>> > > Ah, ok -- missed that little detail ;-)
>> > >
>> > >> So for each cycle in cyclecount register we return 1000 * cycles
>> > >> i.e 1000ns.
>> > >>
>> > >> If it looks nicer we can of course simply:
>> > >> return (unsigned long long) u300_get_cycles * 1000;
>> > >>
>> > >> But the question here is whether this resolution is enough for
>> > >> sched_clock() or if it is irrelevant to override sched_clock()
>> > >> if it cannot schedule with better precision than 1000 ns.
>> > >
>> > > No anything better than jiffies is good, 1us certainly is worth the
>> > > trouble.
>> >
>> > Can I interpret this as Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> ?
>>
>> I think its best if we continue with the patch Paul Mundt has been
>> proposing.
>
> [added Tim Bird to CC]
>
> So what do we do?  There's apparantly been zero movement on this for
> over a month, and Tim Bird is reposting his patch adding __notrace
> to ARMs existing sched_clock implementations.
>
> Given that it seems the generic approach has died a death, I suggest we
> merge Linus' U300 patch, and get Tim to redo his patch to take account
> of that, and apply both.
>
> Then, if the generic approach eventually happens, everything can then be
> fixed up.
>
> Alternatively, if there is movement on the generic approach...
>
> Discuss.
>

I would really like to see Pauls work finalized, it looked very promising,
and I think there was actually a rough consensus about his last patch.
But I guess that will be in the 2.6.32 merge window earliest?

Yours,
Linus Walleij
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