It's not weird hardware, as far as I know. It's just a 64-bit system
with a 32-bit USB host controller.
(And remember, while there are 64-bit EHCI controllers, there are not
any 64-bit OHCI or UHCI controllers. So whenever somebody plugs a
full-speed or low-speed device into a 64-bit machine, they will face
this problem. It's like the old problem of ISA devices that could
only do DMA to addresses in the first 16 MB of memory -- what the
I don't know what the overhead is. But usb_buffer_alloc() requires the
caller to keep track of the buffer's DMA address, so it's not a simple
plug-in replacement. In addition, the consistent memory that
usb_buffer_alloc() provides is a scarce resource on some platforms.
Writing new functions is the way to go.
Ok, I'll write some dummies for usb_malloc() and usb_zalloc() which
will just call kmalloc() with GFP_DMA32 for now. And while at it,
usb_alloc_buffer() will be renamed to usb_alloc_consistent(). Then I'll
try to clean up all existing drivers to use this new interface and
follow the changes.
In a next step, we should fine-tune when GFP_DMA32 is really needed.
And I'll leave all occurances of usb_alloc_consistent() as they are now.
Does that sound ok?
I agree to both points, will do so unless anyone has a harsh opinion
Another thing: I guess we don't need a corresponding free() function
that just calls kfree(), right? Or should we introduce it now to be
flexible for future extensions?
At Wed, 7 Apr 2010 11:55:19 -0400 (EDT),
Yeah, also the area is aligned to kernel pages, and it may be much
bigger than the requested (power-of-two). If not needed, we should