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SubjectRe: [PATCH v4 3/6] clk: fractional-divider: keep mwidth and nwidth internally
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On Wed, 2015-07-22 at 16:45 -0700, Stephen Boyd wrote:
> 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?

I care about memory foot print of kernel's .text as well.

--
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy


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