On Thu, Aug 01, 2019 at 06:34:57PM +0200, Christoph Hellwig wrote:
> On Thu, Aug 01, 2019 at 05:23:06PM +0100, Will Deacon wrote:
> > > - if (!dev_is_dma_coherent(dev) || (attrs & DMA_ATTR_WRITE_COMBINE))
> > > - return pgprot_writecombine(prot);
> > > - return prot;
> > > + return pgprot_writecombine(prot);
> > > }
> >
> > Seems like a sensible cleanup to me:
> >
> > Acked-by: Will Deacon <w...@kernel.org>
> >
> > Although arch_dma_mmap_pgprot() is a bit of a misnomer now that it only
> > gets involved in the non-coherent case.
>
> A better name is welcome.
How about arch_dma_noncoherent_mmap_pgprot() ? Too long?
> My other idea would be to just remove it entirely and do something like:
>
> #ifndef pgprot_dmacoherent
> #define pgprot_dmacoherent pgprot_noncached
> #endif
>
> pgprot_t dma_mmap_pgprot(struct device *dev, pgprot_t prot, unsigned long
> attrs)
> {
> if (dev_is_dma_coherent(dev) || (attrs & DMA_ATTR_NON_CONSISTENT))
> return prot;
> #ifdef pgprot_writecombine
> if (attrs & DMA_ATTR_WRITE_COMBINE)
> return pgprot_writecombine(prot);
> #endif
> return pgprot_dmacoherent(prot);
> }
Oh, I prefer that!
> But my worry is how this interacts with architectures that have an
> uncached segment (mips, nios2, microblaze, extensa) where we'd have
> the kernel access DMA_ATTR_WRITE_COMBINE mappigns using the uncached
> segment, and userspace mmaps using pgprot_writecombine, which could
> lead to aliasing issues. But then again mips already supports
> DMA_ATTR_WRITE_COMBINE, so this must be ok somehow. I guess I'll
> need to field that question to the relevant parties.
Or it's always been busted and happens to work out in practice...
Will
