Subject: Re: [PATCH] dma-mapping: fix page attributes for dma_mmap_*

From: Christoph Hellwig <[email-masked]>
Date: Thu, 1 Aug 2019 18:34:57 +0200

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 <[email-masked]>
> 
> 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.  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);
}

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.