roundup_pow_of_two() may not handle 64-bit integers

Brian Norris computersforpeace at
Thu May 15 12:22:38 EST 2014


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

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.


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

More information about the Linuxppc-dev mailing list