On 05/03/15 17:28, Mitchel Humpherys wrote:
On Thu, Mar 05 2015 at 02:38:45 AM, Robin Murphy <[email protected]> wrote:
Hi Mitch,
On 05/03/15 00:18, Mitchel Humpherys wrote:
We're currently mapping a page in arm_smmu_flush_pgtable without ever
unmapping it. Fix this by calling dma_unmap_page on the returned dma
address. Since the only reason we're calling dma_map_page is to make
sure it actually gets flushed out to RAM, we can just call
dma_unmap_page immediately following the map.
Without this, eventually swiotlb runs out of memory and starts printing
things like:
[ 35.545076] arm-smmu d00000.arm,smmu: swiotlb buffer is full (sz: 128
bytes)
So, you have non-coherent SMMUs too ;) The real problem is that the SMMU's
DMA mask is wrong (as it happens I've just given Will a patch to fix that)
- this is really just doing a whole bunch of unnecessary work (two memory
copies and two cache flushes, one of which isn't even flushing the right
area) to hide the problem. With an appropriate DMA mask set,
swiotlb_map_page becomes a no-op and we fall through to the cache flush
without ever allocating anything.
Yeah I noticed that as well... But isn't this still incorrect usage of
the API (DMA-API-HOWTO.txt seems to indicate that calls to map should
always be balanced with calls to unmap)? What we really want to do here
is just call __dma_map_area directly, but the comment on that guy
expressly forbids it... Not sure what's worse, abusing the DMA API or
disobeying that comment?
Actually, I think we've figured out a fairly nice way to make the core
io-pgtbl code more DMA-API aware, so we can make all the appropriate
calls for the way we're actually using the page tables - I'm having a go
at a patch now...
Robin.
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