linux-kernel - Re: [PATCH v5 2/7] dma: Add simple dma_noop

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Message-ID: <20170626094535.GD21570@infradead.org>
Date:   Mon, 26 Jun 2017 02:45:35 -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,
        Joerg Roedel <jroedel@...e.de>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v5 2/7] dma: Add simple dma_noop_mmap

On Thu, Jun 22, 2017 at 01:46:09PM +0100, Vladimir Murzin wrote:
> On 20/06/17 14:23, Christoph Hellwig wrote:
> > On Wed, May 24, 2017 at 11:24:27AM +0100, Vladimir Murzin wrote:
> >> This patch adds a simple implementation of mmap to dma_noop_ops.
> > 
> > Currently we use dma_common_mmap as the generic fallback if a dma_ops
> > instance doesn't implement a mmap method.  Can you just fix up
> > dma_common_mmap for your nommu case - it looks like yours is basically
> > a subset.
> 
> dma_common_map() is guarded with CONFIG_MMU and I'm not dare to change that
> since I have no idea 1) why it was done 2) how it affects other arches :(

That's still not a good reason for duplicating it.  Please instead
make all the existing nommu architecture select
CONFIG_ARCH_NO_COHERENT_DMA_MMAP as a prep patch, then provide an
arch helper to abstract away the pgprot_noncached, then lift
CONFIG_ARCH_NO_COHERENT_DMA_MMAP for arm nommu.  Bonus points
for lifting it elsewhere or consolidating various copy & paste
versions of the function.