linux-kernel - generic DMA bypass flag

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Message-Id: <20191113133731.20870-1-hch@lst.de>
Date:   Wed, 13 Nov 2019 14:37:29 +0100
From:   Christoph Hellwig <hch@....de>
To:     iommu@...ts.linux-foundation.org,
        Alexey Kardashevskiy <aik@...abs.ru>
Cc:     linuxppc-dev@...ts.ozlabs.org, Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org
Subject: generic DMA bypass flag

Hi all,

I've recently beeing chatting with Lu about using dma-iommu and
per-device DMA ops in the intel IOMMU driver, and one missing feature
in dma-iommu is a bypass mode where the direct mapping is used even
when an iommu is attached to improve performance.  The powerpc
code already has a similar mode, so I'd like to move it to the core
DMA mapping code.  As part of that I noticed that the current
powerpc code has a little bug in that it used the wrong check in the
dma_sync_* routines to see if the direct mapping code is used.

These two patches just add the generic code and move powerpc over,
the intel IOMMU bits will require a separate discussion.

The x86 AMD Gart code also has a bypass mode, but it is a lot
strange, so I'm not going to touch it for now.