linux-kernel - Re: NVMe vs DMA addressing limitations

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Date:   Thu, 12 Jan 2017 14:07:52 +0100
From:   Christoph Hellwig <hch@....de>
To:     Arnd Bergmann <arnd@...aro.org>
Cc:     Sagi Grimberg <sagi@...mberg.me>, Christoph Hellwig <hch@....de>,
        Nikita Yushchenko <nikita.yoush@...entembedded.com>,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        Simon Horman <horms@...ge.net.au>, linux-pci@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        artemi.ivanov@...entembedded.com,
        Keith Busch <keith.busch@...el.com>, Jens Axboe <axboe@...com>,
        linux-nvme@...ts.infradead.org
Subject: Re: NVMe vs DMA addressing limitations

On Thu, Jan 12, 2017 at 12:56:07PM +0100, Arnd Bergmann wrote:
> That is an interesting question: We actually have the
> "DMA_ATTR_NO_KERNEL_MAPPING" for this case, and ARM implements
> it in the coherent interface, so that might be a good fit.

Yes. my WIP HMB patch uses DMA_ATTR_NO_KERNEL_MAPPING, although I'm
workin on x86 at the moment where it's a no-op.

> Implementing it in the streaming API makes no sense since we
> already have a kernel mapping here, but using a normal allocation
> (possibly with DMA_ATTR_NON_CONSISTENT or DMA_ATTR_SKIP_CPU_SYNC,
> need to check) might help on other architectures that have
> limited amounts of coherent memory and no CMA.

Though about that - but in the end DMA_ATTR_NO_KERNEL_MAPPING implies
those, so instead of using lots of flags in driver I'd rather fix
up more dma_ops implementations to take advantage of
DMA_ATTR_NO_KERNEL_MAPPING.