linux-kernel - Re: [PATCH v5 4/7] drivers: dma-coherent: Introduce default DMA pool

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Date:   Mon, 26 Jun 2017 02:44:10 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux@...linux.org.uk, sza@....hu, arnd@...db.de,
        gregkh@...uxfoundation.org, akpm@...ux-foundation.org,
        alexandre.torgue@...com, robin.murphy@....com,
        benjamin.gaignard@...aro.org, kbuild-all@...org,
        Michal Nazarewicz <mina86@...a86.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 4/7] drivers: dma-coherent: Introduce default DMA pool

On Thu, Jun 22, 2017 at 02:18:48PM +0100, Vladimir Murzin wrote:
> It is how it has been started [1] - defining memory which is not cacheable
> (i.e. suitable for coherent allocations) and building custom allocator on top
> of it, like it was done for c6x and blackfin. The annoying thing was that we
> needed to advertise such memory via command line parameter plus some "mem="
> adjustment to hide coherent memory from buddy allocator. So it was suggested
> to use reserved memory and this makes things look much better, but on the
> other hand require changes on dts side to "bind" devices with reserved memory
> - default DMA pool removes such drawback.

I like the idea in general, I'm just worried about the overlap with the
per-device coherent memory, especially when we have slight semantic
mismatches like the one about the physical (or rather dma) address
earlier.