linux-kernel - Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ) Array support"

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Message-ID: <1424408500.27448.25.camel@kernel.crashing.org>
Date:	Fri, 20 Feb 2015 16:01:40 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Sreekanth Reddy <Sreekanth.Reddy@...gotech.com>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	scsi <linux-scsi@...r.kernel.org>, Christoph Hellwig <hch@....de>
Subject: Concerns about "mpt2sas: Added Reply Descriptor Post Queue (RDPQ)
 Array support"

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 ?

Cheers,
Ben.

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