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

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Date:   Tue, 20 Jun 2017 06:49:32 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Vladimir Murzin <vladimir.murzin@....com>
Cc:     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 Wed, May 24, 2017 at 11:24:29AM +0100, Vladimir Murzin wrote:
> This patch introduces default coherent DMA pool similar to default CMA
> area concept. To keep other users safe code kept under CONFIG_ARM.

I don't see a CONFIG_ARM in the code, although parts of it are added
under CONFIG_OF_RESERVED_MEM.

But overall this code look a bit odd to me.  As far as I can tell
the dma-coherent.c code is for the case where we have a special
piece of coherent memory close to a device.

If you're allocating out of the global allocator the memory should
come from the normal dma_ops ->alloc allocator - and also take
the attrs into account (e.g. for DMA_ATTR_NON_CONSISTENT or
DMA_ATTR_NO_KERNEL_MAPPING requests you don't need coherent memory)