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SubjectRe: [PATCH V2 18/19] clocksource: add C-SKY clocksource drivers
On Wed, 4 Jul 2018, Guo Ren wrote:
> On Tue, Jul 03, 2018 at 11:39:05AM +0200, Thomas Gleixner wrote:
> > > +static inline u64 get_ccvr(void)
> > > +{
> > > + u32 lo, hi, t;
> > > +
> > > + do {
> > > + hi = mfcr(PTIM_CCVR_HI);
> > > + lo = mfcr(PTIM_CCVR_LO);
> > > + t = mfcr(PTIM_CCVR_HI);
> > > + } while(t != hi);
> >
> > No idea which frequency this timer ticks at, but if the 32 bit wrap does
> > not come too fast, then you really should avoid that loop. That function is
> > called very frequently.
>
> 0000006c <clksrc_read>:
> hi = mfcr(PTIM_CCVR_HI);
> 6c: c1c26023 mfcr r3, cr<2, 14>
> lo = mfcr(PTIM_CCVR_LO);
> 70: c1c36021 mfcr r1, cr<3, 14>
> t = mfcr(PTIM_CCVR_HI);
> 74: c1c26022 mfcr r2, cr<2, 14>
> } while(t != hi);
> 78: 648e cmpne r3, r2
> 7a: 0bf9 bt 0x6c // 6c <clksrc_read>
>
> When two read cr<2, 14> is not equal, we'll retry. So only when
> CCVR_LO is at 0xffffffff between the two read of CCVR_HI. That's very
> very small probability event for "bt 0x6c".
>
> Don't worry about the "do {...} whie(t != hi)", it's no performance issue.

But _three_ mfcr plus a conditional jump which _cannot_ be predicted are a
performance issue. When you can replace that with a single mfcr, then you
win a lot, really. The time keeping and the sched clock code can handle
that nicely unless you really have fast wrap arounds on the LO word.

Thanks,

tglx


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