Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)

[Marco Stornelli]

Il 18/11/2011 00:05, Andrew Morton ha scritto:
> On Wed, 16 Nov 2011 14:56:06 -0500
> Andrei Warkentin<andreiw@xxxxxxxxxx>  wrote:
>
> > 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1. It does
> > in case the argument is a variable but in case it's a constant it behaves
> > wrong and returns 0. Probably nobody ever did it so this was never noticed,
> > however net/drivers/vmxnet3 with latest GCC does and breaks on unicpu systems.
> >
> > This is similar to Rolf's patch to roundup_pow_of_two(1).
> >
> > Cc: Rolf Eike Beer<eike-kernel@xxxxxxxxx>
> > Cc: opensuse-kernel@xxxxxxxxxxxx
> > Reviewed-by: Jesper Juhl<jj@xxxxxxxxxxxxx>
> > Signed-off-by: Andrei Warkentin<andreiw@xxxxxxxxxx>
> > ---
> >   include/linux/log2.h |    2 +-
> >   1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/linux/log2.h b/include/linux/log2.h
> > index 25b8086..ccda848 100644
> > --- a/include/linux/log2.h
> > +++ b/include/linux/log2.h
> > @@ -185,7 +185,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
> >   #define rounddown_pow_of_two(n)			\
> >   (						\
> >   	__builtin_constant_p(n) ? (		\
> > -		(n == 1) ? 0 :			\
> > +		(n == 1) ? 1 :			\
> >   		(1UL<<  ilog2(n))) :		\
> >   	__rounddown_pow_of_two(n)		\
> >    )
>
> I assume that nobody has gone off and checked whether all current
> callers will survive this change.  If they had, they'd have looked in
> drivers/char/ramoops.c and seen:
>
> 	rounddown_pow_of_two(pdata->mem_size);
> 	rounddown_pow_of_two(pdata->record_size);
>
> These operations are no-ops.  It should be
>
> 	pdata->mem_size = rounddown_pow_of_two(pdata->mem_size);
> 	pdata->record_size = rounddown_pow_of_two(pdata->record_size);
>
> Marco or Sergio: please fix, test and send it over sometime?

Ok, I'll look at it.

Marco
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