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Re: [PATCH v4 3/6] clk: fractional-divider: keep mwidth and nwidth internally

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  Re: [PATCH v4 3/6] clk: fractional-divider: keep mwidth and nwidth  internally Stephen Boyd <sboyd@codeaurora.org> - 2015-07-23 01:50 +0200
    Re: [PATCH v4 3/6] clk: fractional-divider: keep mwidth and nwidth  internally Andy Shevchenko <andriy.shevchenko@linux.intel.com> - 2015-07-23 09:30 +0200

#1190367 — Re: [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?

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#1190680

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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