On Tue, Nov 4, 2014 at 2:27 PM, Michal Nazarewicz <mina86@xxxxxxxxxx> wrote: > On Tue, Nov 04 2014, Gregory Fong wrote: >> The alignment in cma_alloc() is done w.r.t. the bitmap. This is a >> problem when, for example: >> >> - a device requires 16M (order 12) alignment >> - the CMA region is not 16 M aligned >> >> In such a case, can result with the CMA region starting at, say, >> 0x2f800000 but any allocation you make from there will be aligned from >> there. Requesting an allocation of 32 M with 16 M alignment, will >> result in an allocation from 0x2f800000 to 0x31800000, which doesn't >> work very well if your strange device requires 16M alignment. >> >> This doesn't have the behavior I would expect, which would be for the >> allocation to be aligned w.r.t. the start of memory. I realize that >> aligning the CMA region is an option, but don't see why cma_alloc() >> aligns to the start of the CMA region. Is there a good reason for >> having cma_alloc() alignment work this way? > > No, it's a bug. The alignment should indicate alignment of physical > address not position in CMA region. > Ah, now I see that Marek submitted this patch from you back in 2011 that would have allowed the bitmap lib to support an alignment offset: http://thread.gmane.org/gmane.linux.kernel/1121103/focus=1121100 Any idea why this didn't make it into the later changesets? If not, I'll resubmit it and to use it to fix this bug. Thanks, Gregory -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>