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SubjectRe: [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()?

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