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

From: Christoph Hellwig <[email-masked]>
Date: Sat, 3 Aug 2019 08:48:12 +0200

On Fri, Aug 02, 2019 at 11:38:03AM +0100, Will Deacon wrote:
> 
> So this boils down to a terminology mismatch. The Arm architecture doesn't have
> anything called "write combine", so in Linux we instead provide what the Arm
> architecture calls "Normal non-cacheable" memory for pgprot_writecombine().
> Amongst other things, this memory type permits speculation, unaligned accesses
> and merging of writes. I found something in the architecture spec about
> non-cachable memory, but it's written in Armglish[1].
> 
> pgprot_noncached(), on the other hand, provides what the architecture calls
> Strongly Ordered or Device-nGnRnE memory. This is intended for mapping MMIO
> (i.e. PCI config space) and therefore forbids speculation, preserves access
> size, requires strict alignment and also forces write responses to come from
> the endpoint.
> 
> I think the naming mismatch is historical, but on arm64 we wanted to use the
> same names as arm32 so that any drivers using these things directly would get
> the same behaviour.

That all makes sense, but it totally needs a comment.  I'll try to draft
one based on this.  I've also looked at the arm32 code a bit more, and
it seems arm always (?) supported Normal non-cacheable attribute, but
Linux only optionally uses it for arm v6+ because of fears of drivers
missing barriers.  The other really weird things is that in arm32
pgprot_dmacoherent incudes the L_PTE_XN bit, which from my understanding
is the no-execture bit, but pgprot_writecombine does not.  This seems to
not very unintentional.  So minus that the whole DMA_ATTR_WRITE_COMBІNE
seems to be about flagging old arm specific drivers as having the proper
barriers in places and otherwise is a no-op.

Here is my tentative plan:

 - respin this patch with a small fix to handle the
   DMA_ATTR_NON_CONSISTENT (as in ignore it unless actually supported),
   but keep the name as-is to avoid churn.  This should allow 5.3
   inclusion and backports
 - remove DMA_ATTR_WRITE_COMBINE support from mips, probably also 5.3
   material.
 - move all architectures but arm over to just define
   pgprot_dmacoherent, including a comment with the above explanation
   for arm64.
 - make DMA_ATTR_WRITE_COMBINE a no-op and schedule it for removal,
   thus removing the last instances of arch_dma_mmap_pgprot