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 ?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at