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 15:51:06 +0100
From:   Christoph Hellwig <hch@....de>
To:     Nikita Yushchenko <nikita.yoush@...entembedded.com>
Cc:     Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>,
        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, robin.murphy@....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 03:47:25PM +0300, Nikita Yushchenko wrote:
> With this direction, semantics of dma mask becomes even more
> questionable. I'd say dma_mask is candidate for removal (or to move to
> swiotlb's or iommu's local area)

We need the dma mask so that the device can advertise what addresses
the device supports.  Many old devices only support 32-bit DMA addressing,
and some less common ones just 24-bit or other weird ones.