Re: [PATCH] ib_ehca: fix compile failure on ppc64

[Kyle McMartin <kmcmartin-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Mon, Feb 27, 2012 at 09:18:35PM -0800, Roland Dreier wrote:
> On Mon, Feb 27, 2012 at 2:02 PM, Kyle McMartin <kmcmartin-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> >  static u64 ehca_get_max_hwpage_size(struct ehca_shca *shca)
> >  {
> > -       return 1UL << ilog2(shca->hca_cap_mr_pgsize);
> > +       u32 pgsize = shca->hca_cap_mr_pgsize;
> > +       return 1UL << ilog2(pgsize);
> >  }
> 
> How about using rounddown_pow_of_two(shca->hca_cap_mr_pgsize)?
> Does that build?  Because that seems to me to be a cleaner way of
> expressing this anyway...
> 

Yeah, that works too, good catch, I should have noticed that sooner.

> Something about that ilog2() implementation seems to trigger gcc
> bugs, I seem to remember some other bogus compile failure on
> sparc involving it as well...
> 

I suspect it's the 64 deep trinary operators ;-) (It seems to trip up on
the __builtin_constant_p and hit the second ____ilog2_NaN) It's a pretty
weird .config dependent bug, I'm very confused by it.

Suggested-by: Roland Dreier <roland-BHEL68pLQRGGvPXPguhicg@public.gmane.org>
Signed-off-by: Kyle McMartin <kmcmarti-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>

--- a/drivers/infiniband/hw/ehca/ehca_mrmw.c
+++ b/drivers/infiniband/hw/ehca/ehca_mrmw.c
@@ -112,7 +112,7 @@ static u32 ehca_encode_hwpage_size(u32 pgsize)
 
 static u64 ehca_get_max_hwpage_size(struct ehca_shca *shca)
 {
-	return 1UL << ilog2(shca->hca_cap_mr_pgsize);
+	return rounddown_pow_of_two(shca->hca_cap_mr_pgsize);
 }
 
 static struct ehca_mr *ehca_mr_new(void)
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