linux-kernel - roundup_pow_of_two() may not handle 64-bit integers

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Message-ID: <20140515022238.GL28907@ld-irv-0074>
Date:	Wed, 14 May 2014 19:22:38 -0700
From:	Brian Norris <computersforpeace@...il.com>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Brian Norris <computersforpeace@...il.com>
Subject: roundup_pow_of_two() may not handle 64-bit integers

Hi,

I'm looking to use roundup_pow_of_two() (actually, order_base_2())
from <linux/log2.h>, but it seems that it only supports 64-bit integers
if your toolchain uses a 64-bit 'unsigned long' type. This is strange,
considering that ilog2() is explicitly designed for 32-bit or 64-bit
compatibility.

I also note that there is at least one location in which this limitation
currently might be problematic: in pnv_pci_ioda2_set_bypass()
(arch/powerpc/platforms/powernv/pci-ioda.c). It looks like this could be
a problem if using large amounts of DRAM on a 32-bit PPC build, with
64-bit physical addresses.

(There may be other cases like this one, but I haven't closely studied
all callers of roundup_pow_of_two().)

I'm thinking of cooking a patch to improve roundup_pow_of_two() (and
thus order_base_2()), but I'd like to solicit comments on the basic
problem first.

Regards,
Brian

P.S. And of course, rounddown_pow_of_two() has the same issue.
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