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

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Message-ID: <87oaj6ogox.fsf@rasmusvillemoes.dk>
Date:	Mon, 20 Jul 2015 19:57:18 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.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, 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. Probably one should add a smatch/sparse warning
for that.

> I don't know if ~0ull makes a difference for some architectures.

I highly doubt it. The result is truncated to unsigned long
anyway. Assuming align_order always has a value between 0 and
BITS_PER_LONG, GENMASK should be exactly what is wanted.

Rasmus
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