linux-kernel - Re: [PATCH v2] arm64: do not set dma masks that device connection can't handle

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Date:   Tue, 10 Jan 2017 15:59:14 +0100
From:   Christoph Hellwig <hch@....de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Robin Murphy <robin.murphy@....com>,
        Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        Will Deacon <will.deacon@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Simon Horman <horms@...ge.net.au>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        artemi.ivanov@...entembedded.com, fkan@....com,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] arm64: do not set dma masks that device connection
        can't handle

On Tue, Jan 10, 2017 at 02:42:23PM +0100, Arnd Bergmann wrote:
> It's a much rarer problem for the IOMMU case though, because it only
> impacts devices that are restricted to addressing of less than 32-bits.
> 
> If you have an IOMMU enabled, the dma-mapping interface does not care
> if the device can do wider than 32 bit addressing, as it will never
> hand out IOVAs above 0xffffffff.

That's absolutely not the case.  IOMMUs can and do generate addresses
larger than 32-bit.  Also various platforms have modes where an IOMMU
can be used when <= 32-bit addresses are used and bypassed if full 64-bit
addressing is supported and I/O isolation is not explicitly requested.