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SubjectRe: [PATCH V2 18/19] clocksource: add C-SKY clocksource drivers
On Wed, Jul 04, 2018 at 04:35:43PM +0200, Thomas Gleixner wrote:
> 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.
Timer's frequency is about 50Mhz-100Mhz and LO word wrap arounds timer is
about 42s ~ 84s.

Our Branch prediction buffer will let the CPU speculative execute
continually. So the "bt" won't be the performance issue.

And I could modify it like this:

static inline u64 get_ccvr(void)
{
u32 lo, hi, t;

t = mfcr(PTIM_CCVR_LO);
hi = mfcr(PTIM_CCVR_HI);
lo = mfcr(PTIM_CCVR_LO);

if (lo < t) hi++;

return ((u64)hi << 32) | lo;
}

t = mfcr(PTIM_CCVR_LO);
50: c1c36023 mfcr r3, cr<3, 14>
hi = mfcr(PTIM_CCVR_HI);
54: c1c26021 mfcr r1, cr<2, 14>
lo = mfcr(PTIM_CCVR_LO);
58: c1c3602c mfcr r12, cr<3, 14>
if (lo < t) hi++;
5c: 64f0 cmphs r12, r3
5e: c4210c21 incf r1, r1, 1
return ((u64)hi << 32) | lo;
62: 3200 movi r2, 0

Hmm? (No jump at all)

Guo Ren

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