linux-kernel - Re: [PATCH v3 1/7] clk: introduce clk_div

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Date:	Tue, 7 Jul 2015 16:26:18 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, Sascha Hauer <kernel@...gutronix.de>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	Tero Kristo <t-kristo@...com>,
	Russell King <linux@....linux.org.uk>,
	Dinh Nguyen <dinguyen@...nsource.altera.com>
Subject: Re: [PATCH v3 1/7] clk: introduce clk_div_mask() helper

On 07/07, Andy Shevchenko wrote:
> On Thu, 2015-06-18 at 12:48 -0700, Stephen Boyd wrote:
> > On 03/31, Andy Shevchenko wrote:
> > > diff --git a/include/linux/clk-provider.h b/include/linux/clk
> > > -provider.h
> > > index 5591ea7..20b0b67 100644
> > > --- a/include/linux/clk-provider.h
> > > +++ b/include/linux/clk-provider.h
> > > @@ -353,6 +353,11 @@ struct clk_divider {
> > >  	spinlock_t	*lock;
> > >  };
> > >  
> > > +static inline unsigned long clk_div_mask(u8 width)
> > > +{
> > > +	return (1 << width) - 1;
> > > +}
> > > +
> > 
> > Why not just change drivers to use GENMASK? It's a proven and
> > tested way to generate a bitmask.
> 
> Too many unneeded calculations I suppose.

That's what compiler optimizations are for.

> 
> Compare:
> 	mask = clk_div_mask(mm) << ms;
> 	which is simple ((1 << mm) - 1) << ms
> and
> 	mask = GENMASK(mm + ms - 1, ms);
> 	which is (~0 << ms) & (~0 >> (BITS_PER_LONG - 1 - (mm + ms -
> 1)))

And if mm is 32 then we hit undefined behavior.

> 
> > 
> > So I'd rather see drivers converted to use that macro directly
> > especially because the mask may need to start at some bit that
> > isn't 0.
> 
> If you think the above is not a burden, I can do the conversion to
> GENMASK.
> 
> Though it might make sense when ms = 0 explicitly.
> 

Yes let's use GENMASK. I imagine the extra few instructions are
negligible.

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