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SubjectRe: [PATCH 5/5] dma-mapping: remove ARCH_NO_COHERENT_DMA_MMAP
On Sat, 03 Aug 2019 12:30:24 +0200,
Christoph Hellwig wrote:
>
> 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.

Yes, this would fit perfect.

> But how
> do I get at a suitable struct device in hw_support_mmap()?

substream->dma_buffer.dev.dev can be that, which is used in the mmap
helper side, snd_pcm_lib_default_mmap().


Thanks!

Takashi

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