Re: USB transfer_buffer allocations on 64bit systems

[Andi Kleen <andi@firstfloor.org>]

Alan Stern <stern@rowland.harvard.edu> writes:
>> 
>> The fix is to use usb_buffer_alloc() for that purpose which ensures
>> memory that is suitable for DMA. And on x86_64, this also means that the
>> upper 32 bits of the address returned are all 0's.
>
> That is not a good fix.  usb_buffer_alloc() provides coherent memory, 
> which is not what we want.  I believe the correct fix is to specify the 
> GFP_DMA32 flag in the kzalloc() call.

The traditional way to handle this is to leave it to swiotlb in
pci_map_*. pci_map_* is needed anyways if you run with a IOMMU.

Also note at least on x86 systems coherent memory is the same as non coherent
memory. And GFP_DMA32 is a x86 specific flag, doesn't necessarily 
do any good anywhere else.

So if you add x86isms anyways you could as well use dma_alloc_coherent()
directly which is actually better at this than a simple GFP_DMA32
and as a bonus handles the IOMMUs correctly too.

Or just use GFP_KERNEL and pci_map_* later.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.