linux-kernel - Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86

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Message-ID: <alpine.LFD.2.01.0906301456170.3605@localhost.localdomain>
Date:	Tue, 30 Jun 2009 15:10:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Yinghai Lu <yinghai@...nel.org>
cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Mikael Pettersson <mikpe@...uu.se>,
	Matthew Wilcox <matthew@....cx>,
	Grant Grundler <grundler@...isc-linux.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-pci@...r.kernel.org
Subject: Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit
 x86



On Tue, 30 Jun 2009, Yinghai Lu wrote:
?
> +#define round_up(x, y) ({ __typeof__(x) __mask = (y)-1; \
> +			 ((x)+__mask) & ~__mask; })
> +#define round_down(x, y) ({ __typeof__(x) __mask = (y)-1; (x) & ~__mask; })

Yes, except we might as well simplify it. Do it without the statement 
expressions, using just a single 'y'. Like this:

  #define __round_mask(x,y) ((__typeof__(x))((y)-1))
  #define round_up(x,y) (((x) | __round_mask(x,y))+1)
  #define round_down(x,y) ((x) & ~__round_mask(x,y))

(Yeah, it uses 'x' twice, but the second one is for 'typeof', which 
doesn't actually cause the value to be evaluated, so it's ok).

Now those 'round_xyz()' operations will always return a value of a type 
that is the same as the type of 'x' except it's gone through the normal C 
integer promotion rules (ie if 'x' is a smaller type than 'int', then it 
will be promoted to 'int').

Not very well tested, but it _looks_ correct, and uses Peter's trick, and 
willlet the compiler notice that

	round_up(x,y)-1

is the same thing as

	x | (y-1)

which it _could_ have perhaps figured out before, but now it's way more 
obvious.
		Linus
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