Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support"
From: Benjamin Herrenschmidt
Date: Fri Feb 20 2015 - 00:02:03 EST
Hi Sreekanth !
While looking at some (unrelated) issue where mtp2sas seems to be using
32-bit DMA instead of 64-bit DMA on some POWER platforms, I noticed this
patch which was merged as 5fb1bf8aaa832e1e9ca3198de7bbecb8eff7db9c.
Can you confirm my understanding that you are:
- Setting the DMA mask to 32-bit
- Mapping pages for DMA
- Changing the DMA mask to 64-bit
If yes, then I don't think this is a legal thing to do and definitely
not something supported by all architectures. It might work by accident,
but there is no telling that any translation/DMA mapping provided before
a call to set_dma_mask() is still valid after that call.
The architecture might have to completely reconfigure the iommu, for
example on some PowerPC platforms, we switch from a remapped mapping to
a direct linear map of all memory, all translations established before
the switch might be lost (it depends on the specific implementation).
How does it work on x86 with DMAR ?
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