[Bug 215906] DMAR fault when connected usb hub (xhci_hcd)

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=215906

--- Comment #17 from Mathias Nyman (mathias.nyman@linux.intel.com) ---
If I understand correctly this is caused by the VIA VL805 xHC controller with
bad firmware accessing some DMA address outside the allowed range.

With IOMMU enabled the IOMMU will prevent this access, and the controller
fails.

I'm speculating here, but it could be possible the controller accesses past one
of the DMA ranges wile trying to read-ahead.

If we can figure out past which area, then its possible to make a driver
workaround for this controller that allocates a bit larger DMA chunk for that
specific purpose. 

DMA memory allocated for xHC use before any USB device is connected:

- dcbaa device context base address array.
  arrays of pointers to device contexts
  dma_alloc_coherent(dev)

- command ring
  dma_pool_zalloc(segment_pool)

- event ring
  dma_pool_zalloc(segment_pool)

- event ring segment table
  info about event ring, segents, size and location
  dma_alloc_coherent(dev)

- Scratchpad,
  only touched (RW) by xHC controller, not driver.
  dma_alloc_coheret()

DMA memory allocated for each connected USB device.

- device contexts
  dma_pool_zalloc(device_pool)

- transfer rings,
  contains TRBs, metadata about transfers.
  dma_pool_zalloc(segment_pool)

- stream contexts,
  dma_alloc_coherent() or dma_pool_alloc(*_streams_pool)

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