linux-kernel - Re: [PATCH] dma-mapping: fix page attributes for dma_mmap

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Date:   Thu, 1 Aug 2019 17:44:12 +0100
From:   Will Deacon <will@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     iommu@...ts.linux-foundation.org,
        Shawn Anastasio <shawn@...stas.io>,
        Michael Ellerman <mpe@...erman.id.au>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Robin Murphy <robin.murphy@....com>,
        linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_*

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 <will@...nel.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