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SubjectRe: [PATCH v4 3/6] clk: fractional-divider: keep mwidth and nwidth internally
On 07/22, Andy Shevchenko wrote:
> On Tue, 2015-07-21 at 17:41 -0700, Stephen Boyd wrote:
> > On 07/14, Andy Shevchenko wrote:
> > > diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk
> > > -fractional-divider.c
> > > index 7cfcc56..16f42ae 100644
> > > --- a/drivers/clk/clk-fractional-divider.c
> > > +++ b/drivers/clk/clk-fractional-divider.c
> > > @@ -128,9 +128,11 @@ struct clk
> > > *clk_register_fractional_divider(struct device *dev,
> > >
> > > fd->reg = reg;
> > > fd->mshift = mshift;
> > > - fd->mmask = (BIT(mwidth) - 1) << mshift;
> > > + fd->mwidth = mwidth;
> > > + fd->mmask = GENMASK(mwidth - 1, 0) << mshift;
> > > fd->nshift = nshift;
> > > - fd->nmask = (BIT(nwidth) - 1) << nshift;
> > > + fd->nwidth = nwidth;
> > > + fd->nmask = GENMASK(nwidth - 1, 0) << nshift;
> >
> > Please do the shifts in the GENMASK.
>
> It's not optimal. Waste of performance.
>
> 32-bit case on 32-bit machine (similar to other cases on x86).
>
> a) GENMASK(x - 1, 0) << y
>
> movl $32, %ecx
> subb 4(%esp), %cl
> movl $-1, %eax
> shrl %cl, %eax
> movzbl 8(%esp), %ecx
> sall %cl, %eax
> ret
>
> b) GENMASK(x + y - 1, y)
>
> pushl %esi
> pushl %ebx
> movl $1, %edx
> movzbl 12(%esp), %eax
> movzbl 16(%esp), %esi
> movl $-1, %ebx
> subl %eax, %edx
> movl %ebx, %eax
> subl %esi, %edx
> leal 31(%edx), %ecx
> shrl %cl, %eax
> movl %esi, %ecx
> sall %cl, %ebx
> andl %ebx, %eax
> popl %ebx
> popl %esi
> ret
>

This is not hotpath code, so why do we care?

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
a Linux Foundation Collaborative Project


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