Messages in this thread | | | Subject | Re: [PATCH 18 of 38] x86: unify pci iommu setup and allow swiotlb to compile for 32 bit | From | Ian Campbell <> | Date | Fri, 21 Nov 2008 14:21:32 +0000 |
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On Wed, 2008-11-19 at 11:19 +0900, FUJITA Tomonori wrote: > > The problem that I talked about in the previous mail: > > > max_slots = mask + 1 > > ? ALIGN(mask + 1, 1 << IO_TLB_SHIFT) >> IO_TLB_SHIFT > > : 1UL << (BITS_PER_LONG - IO_TLB_SHIFT); > > Since the popular value of the mask is 0xffffffff. So the above code > (mask + 1 ?) works wrongly if the size of mask is 32bit (well, > accidentally the result of max_slots is identical though).
I've just been looking at this again and I don't think it is an accident that this evaluates to the correct value when mask + 1 == 0.
The patch which adds the "mask + 1 ? ... : 1UL << ..." stuff is:
commit b15a3891c916f32a29832886a053a48be2741d4d Author: Jan Beulich <jbeulich@novell.com> Date: Thu Mar 13 09:13:30 2008 +0000 avoid endless loops in lib/swiotlb.c Commit 681cc5cd3efbeafca6386114070e0bfb5012e249 ("iommu sg merging: swiotlb: respect the segment boundary limits") introduced two possibilities for entering an endless loop in lib/swiotlb.c: - if max_slots is zero (possible if mask is ~0UL) [...]
I think the existing code is the nicest way to handle this corner case and it is necessary anyway to handle the ~0UL case on 64 bit.
Ian.
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