amdgpu/TTM oopses since merging swiotlb_dma_ops into the dma_direct code

[Christoph Hellwig]

On Thu, Jan 10, 2019 at 10:59:02AM +0100, Michel Dänzer wrote:
> 
> Hi Christoph,
> 
> 
> https://bugs.freedesktop.org/109234 (please ignore comments #6-#9) was
> bisected to your commit 55897af63091 "dma-direct: merge swiotlb_dma_ops
> into the dma_direct code". Any ideas?

>From the trace it looks like we git the case where swiotlb tries
to copy back data from a bounce buffer, but hits a dangling or NULL
pointer.  So a couple questions for the submitter:

 - does the system have more than 4GB memory and thus use swiotlb?
   (check /proc/meminfo, and if something SWIOTLB appears in dmesg)
 - does the device this happens on have a DMA mask smaller than
   the available memory, that is should swiotlb be used here to start
   with?