linux-kernel - Re: [PATCH 5/5] dma-mapping: remove ARCH_NO_COHERENT_DMA

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Message-ID: <20190803103024.GA32624@lst.de>
Date:   Sat, 3 Aug 2019 12:30:24 +0200
From:   Christoph Hellwig <hch@....de>
To:     Takashi Iwai <tiwai@...e.de>
Cc:     Christoph Hellwig <hch@....de>, iommu@...ts.linux-foundation.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-xtensa@...ux-xtensa.org, Michal Simek <monstr@...str.eu>,
        linux-parisc@...r.kernel.org, linux-sh@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 5/5] dma-mapping: remove ARCH_NO_COHERENT_DMA_MMAP

On Fri, Aug 02, 2019 at 10:24:02AM +0200, Takashi Iwai wrote:
> I wasn't careful enough to look at that change, sorry.
> 
> The code there tries to check whether dma_mmap_coherent() would always
> fail on some platforms.  Then the driver clears the mmap capability
> flag at the device open time and notifies user-space to fall back to
> the dumb read/write mode.
> 
> So I'm afraid that simply dropping the check would cause the behavior
> regression, e.g. on PARISC.
> 
> Is there any simple way to test whether dma_mmap_coherent() would work
> or not in general on the target platform?  It's not necessarily in an
> ifdef at all.

This isn't really a platform, but a per-device question.  I can add a
"bool dma_can_mmap(struct device *dev)" helper to check that.  But how
do I get at a suitable struct device in hw_support_mmap()?