On Thu, Mar 05, 2015 at 11:16:28AM +0000, Robin Murphy wrote:

Hi Laura,

On 05/03/15 00:19, Laura Abbott wrote:
[...]

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Consider that the IOMMU's page table walker is a DMA master in its own

right, and that is the device you're mapping the page tables for.
Therefore your IOMMU driver needs to have access to the struct device
of the IOMMU itself to use for the page-table-related mappings. Also,
be sure to set the IOMMU's DMA mask correctly to prevent SWIOTLB bounce
buffers being created in the process (which as I've found generally ends
in disaster).

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         And normally, we always need do cache maintenance only for some
bytes in the pagetable but not whole a page. Then is there a easy way to
do the cache maintenance?

For a noncoherent device, dma_map_single() will end up calling

__dma_map_area() with the page offset and size of the original request, so
the updated part gets flushed by VA, and the rest of the page isn't touched
if it doesn't need to be. On the other hand if the page tables were
allocated with dma_alloc_coherent() in the first place, then just calling
dma_sync_single_for_device() for the updated region should suffice.

That's wrong.  dma_sync_single_*() is not permitted to be called on
coherently allocated memory.  Where coherent memory needs to be remapped,
dma_sync_single_*() will panic the kernel.

If it's in coherent memory, all you should need is the appropriate
memory barrier to ensure that the DMA agent can see the writes.

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Where exactly would you call the dma_unmap? It seems a bit strange to
be repeatedly calling dma_map and never calling dma_unmap. I don't see it
explicitly forbidden in the docs anywhere to do this but it seems like
it would be violating the implicit handoff of dma_map/dma_unmap.

I think ideally you'd call dma_map_page when you first create the page
table, dma_sync_single_for_device on any update, and dma_unmap_page when you
tear it down, and you'd also use the appropriate DMA addresses everywhere
instead of physical addresses.

No.

dma_map_page()			ownership changes CPU->DMA
dma_sync_single_for_cpu()	ownership changes DMA->CPU
dma_sync_single_for_device()	ownership changes CPU->DMA
dma_unmap_page()		ownership changes DMA->CPU

It's invalid to miss out the pairing that give those ownership changes.

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