linux-kernel - Re: USB transfer_buffer allocations on 64bit systems

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Date:	Mon, 12 Apr 2010 10:59:57 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Daniel Mack <daniel@...aq.de>, linux-kernel@...r.kernel.org,
	Pedro Ribeiro <pedrib@...il.com>, <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...e.de>, <alsa-devel@...a-project.org>,
	<linux-usb@...r.kernel.org>
Subject: Re: USB transfer_buffer allocations on 64bit systems

Alan Stern <stern@...land.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@...ux.intel.com -- Speaking for myself only.
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