linux-kernel - Re: [PATCH] iommu-common: Do not use 64 bit constant 0xffffffffffffffffl for computing align

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Date:	Mon, 20 Jul 2015 16:28:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	linux-kernel@...r.kernel.org, benh@...nel.crashing.org,
	davem@...emloft.net
Subject: Re: [PATCH] iommu-common: Do not use 64 bit  constant
 0xffffffffffffffffl for computing align_mask

On Mon, 20 Jul 2015 19:57:18 +0200 Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> On Mon, Jul 20 2015, Guenter Roeck <linux@...ck-us.net> wrote:
> 
> > So either case ~0l appears to be wrong; it should be ~0ul.
> 
> Yes, right-shifting -1 of any type is probably always wrong, as it will
> always give -1 again.

Not for unsigned types.

The kernel uses "-1UL" and "-1ULL" quite a lot - it's a convenient way
of saying "all ones, regardless of size".  Also, assigning plain old
"-1" to an unsigned variable will make that variable all-ones
regardless of size.

In this case I expect we could do

	align_mask = -1UL >> (64 - align_order);

but I don't know about that 64.  Maybe it should be BITS_PER_LONG?
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