linux-kernel - Re: revert dma direct internals abuse

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Date:   Tue, 9 Apr 2019 11:57:40 +0200
From:   "hch@....de" <hch@....de>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     "hch@....de" <hch@....de>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Deepak Singh Rawat <drawat@...are.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: revert dma direct internals abuse

On Mon, Apr 08, 2019 at 06:47:52PM +0000, Thomas Hellstrom wrote:
> We HAVE discussed our needs, although admittedly some of my emails
> ended up unanswered.

And than you haven't followed up, and instead ignored the layering
instructions and just commited a broken patch?

> We've as you're well aware of had a discussion with the other
> subsystems doing user-space DMA-buffers wanting this functionality from
> the dma api (AMD graphics and RDMA people IIRC). that is a bool that
> tells us whether streaming dma mappings are coherent, and I described
> in detail why we couldn't use the dma_sync_* API and
> dma_alloc_coherent().

And that is not at all what you either check or claim to do in the
changelog (which btw, are two different things).

> The other option we have is to just fail miserably without messages if
> streaming DMA is not coherent, which I think the other drivers might
> do... That's all I'm trying to avoid here. I'd much prefer to have the
> dma API export this bool.

Both DMA direct and non-DMA direct streaming mappings can be either
coherent or incoherent, so your patch doesn't archive that.  The
commit log claims the following:

"drm/vmwgfx: Improve on IOMMU detection
    
 instead of relying on intel_iommu_enabled, use the fact that the
 dma_map_ops::map_page != dma_direct_map_page"

which has nothing to do with the fact that streaming mappings are
coherent.  It also is incorrect as there are direct mapping that
do not use dma_direct_map_page (e.g. on ARM, or x86 with VMD).