Re: usb_buffer_alloc

[Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx>]

On Tue, Mar 10, 2009 at 10:16:16AM -0700, Paul Thomas wrote:
> >I dug into this a further, and confirmed that the call to
> >usb_buffer_alloc() is indeed returning the faulty address.  Here is
> >the allocation stack:
> 
> >vm_region_alloc
> >__dma_alloc
> >dma_alloc_coherent
> >hcd_buffer_alloc
> >usb_buffer_alloc
> 
> >Now, vm_region_alloc makes use of CONSISTENT_BASE, which is where the
> >address 0xffc08000 comes from.

Okay, so you got _coherent_ memory.

>  However, when the URB is submitted, a
> >call to dma_cache_maint, and the virt_addr_valid() check triggers the
> >BUG().

And you've passed it via a function which wants to _remap_ the coherent
memory using the dma_map_* calls.  That's totally invalid.  For
dma_map_single, the DMA API document says:

| Notes:  Not all memory regions in a machine can be mapped by this
| API.  Further, regions that appear to be physically contiguous in
| kernel virtual space may not be contiguous as physical memory.  Since
| this API does not provide any scatter/gather capability, it will fail
| if the user tries to map a non-physically contiguous piece of memory.
| For this reason, it is recommended that memory mapped by this API be
| obtained only from sources which guarantee it to be physically contiguous
| (like kmalloc).

And, since ARM is a non-coherent architecture unlike x86, the DMA
supporting functions are a lot more complex.  (x86 they're basically
no-ops.)  The result is that we _have_ the restriction that any
remapped memory (such as via dma_alloc_coherent) can _never_ be passed
via the streaming DMA API.  Only memory from kmalloc or get_free_page
can.

I can't say where the problem lies, but essentially what's happening in
the USB stack (passing a coherent DMA buffer to the streaming DMA API)
is a bug in the USB stack somewhere.

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