Messages in this thread |  | | Date | Sun, 19 Jul 2015 18:58:05 -0700 | | From | Guenter Roeck <> | | Subject | Re: [PATCH] iommu-common: Do not use 64 bit constant 0xffffffffffffffffl for computing align_mask |
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On 07/19/2015 01:25 PM, Rasmus Villemoes wrote: > On Sun, Jul 19 2015, Guenter Roeck <linux@roeck-us.net> wrote: > >> On Sun, Jul 19, 2015 at 02:20:14PM +0200, Sowmini Varadhan wrote: >>> >>> Using a 64 bit constant generates "warning: integer constant is too >>> large for 'long' type" on 32 bit platforms. Instead use ~0l to get >>> the desired effect. >>> >>> Detected by Andrew Morton who has confirmed that this patch >>> fixes the warning on i386/gcc-4.4.3, i386/gcc-4.4.0 and arm/gcc-4.4.4. >>> >>> Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com> >>> --- >>> lib/iommu-common.c | 2 +- >>> 1 files changed, 1 insertions(+), 1 deletions(-) >>> >>> diff --git a/lib/iommu-common.c b/lib/iommu-common.c >>> index df30632..fd1297d 100644 >>> --- a/lib/iommu-common.c >>> +++ b/lib/iommu-common.c >>> @@ -119,7 +119,7 @@ unsigned long iommu_tbl_range_alloc(struct device *dev, >>> unsigned long align_mask = 0; >>> >>> if (align_order > 0) >>> - align_mask = 0xffffffffffffffffl >> (64 - align_order); >>> + align_mask = ~0l >> (64 - align_order); >>> >> Wonder if this just hides the real problem. Unless align_order >> is very large, the resulting mask on 32 bit systems may be 0. >> Is this really the idea ? > > Probably not, but that's not what would happen on x86: the shift > only depends on the lower 5 or 6 bits - I don't know if other platforms > also has that behaviour. So for align_order == 2 and x86_32 we'd > effectively end up with a shift of 62 & 31 == 30 (though technically > undefined behaviour), and the desired mask of 0x3. >
#include <stdio.h>
main(int argc, char **argv) { unsigned long am1, am2, am3; int align_order = 2;
am1 = 0xffffffffffffffffl >> (64 - align_order); am2 = ~0l >> (64 - align_order); am3 = ~0ul >> (64 - align_order);
printf("0x%lx 0x%lx 0x%lx\n", am1, am2, am3); }
yields an output of
0x3 0xffffffffffffffff 0x3
So either case ~0l appears to be wrong; it should be ~0ul. I don't know if ~0ull makes a difference for some architectures.
Guenter
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