Hello 2017-11-24 19:35 GMT+09:00 David Laight <David.Laight@xxxxxxxxxx>: > From: Jaewon Kim >> Sent: 24 November 2017 05:59 >> >> dma-coherent uses bitmap APIs which internally consider align based on the >> requested size. If most of allocations are small size like KBs, using >> alignment scheme seems to be good for anti-fragmentation. But if large >> allocation are commonly used, then an allocation could be failed because >> of the alignment. To avoid the allocation failure, we had to increase total >> size. >> >> This is a example, total size is 30MB, only few memory at front is being >> used, and 9MB is being requsted. Then 9MB will be aligned to 16MB. The >> first try on offset 0MB will be failed because others already are using >> them. The second try on offset 16MB will be failed because of ouf of bound. >> >> So if the alignment is not necessary on a specific dma-coherent memory >> region, we can set no-align property. Then dma-coherent will ignore the >> alignment only for the memory region. > > ISTM that the alignment needs to be a property of the request, not of the > device. Certainly the device driver code is most likely to know the specific > alignment requirements of any specific allocation. > Sorry but I'm not fully understand on 'a property of the request'. Actually dma-coherent APIs does not get alignment through argument but it internally uses get_order to determine alignment according to a requested size. I think if you meant that dma-coherent APIs should work in that way because drivers calling to dma-coherent APIs have been assuming the alignment for a long time. I still think few memory region could be managed without alignment if author knows well and adds no-align into its device tree. But it's OK if open source community worried about the no-alignment. Thank you > We've some hardware that would need large allocations to be 16k aligned. > We actually use multiple 16k allocations because any large buffers are > accessed directly from userspace (mmap and vm_iomap_memory) and the > card has its own page tables (with 16k pages). > > David > -- 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>
