Re: USB transfer_buffer allocations on 64bit systems

From: Oliver Neukum
Date: Wed Apr 07 2010 - 12:56:38 EST

Am Mittwoch, 7. April 2010 16:59:47 schrieb Alan Stern:
> > 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.
> Of course, some EHCI hardware is capable of using 64-bit addresses.
> But not all, and other controller types aren't. In principle we could
> create a new allocation routine, which would take a pointer to the USB
> bus as an additional argument and use it to decide whether the memory
> needs to lie below 4 GB. I'm not sure adding this extra complexity
> would be worthwhile.

What about XHCI? Do you really want to limit it to 32bits?

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