Messages in this thread | | | Date | Wed, 10 Dec 2008 16:31:45 +0200 | From | Gilad Ben-Yossef <> | Subject | Re: [PATCH] USB: use stack allocation for struct usb_ctrlrequest |
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Wu Fengguang wrote:
> >>> e.g. what if the memory is allocated from ZONE_HIGHMEM? >>> >>> >> In many cases there is no problem to DMA high memory. If you happen to >> be working with a device that does have problems with full 32 bit >> addresses then GFP_DMA would be the right flag, not GFP_NOIO. >> > > For 64bit systems, we can easily go beyond 4GB physical memory. > So at least we should add GFP_DMA32 in addition to GFP_NOIO? > > I don't think so. 64bit systems don't have ZONE_HIMEM - this is why I was referring to 32 bit.
More to the point, many devices don't have any issues accessing full 64 bit addressing mode and many 64bit machines have IOMMU that will take care of those devices that DO have a problem. Adding this flag will just penalize the MM sub-system for no reason. For the rare case where there is a device that cannot do 64 bit addressing in a 64 bit machine with no IOMMU, AFAIK bounce buffers are used to overcome the issue.
Cheers, Gilad
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