Re: [PATCH] arm: dma: fix sharing of coherent DMA memory without struct page

[Russell King - ARM Linux]

On Sun, Apr 16, 2017 at 07:10:21PM -0600, Shuah Khan wrote:
> On 04/14/2017 03:46 AM, Russell King - ARM Linux wrote:
> > On Fri, Apr 14, 2017 at 09:56:07AM +0200, Marek Szyprowski wrote:
> >>>> This would be however quite large task, especially taking into account
> >>>> all current users of DMA-buf framework...
> >>> Yeah it will be a large task.
> >>
> >> Maybe once scatterlist are switched to pfns, changing dmabuf internal
> >> memory representation to pfn array might be much easier.
> > 
> > Switching to a PFN array won't work either as we have no cross-arch
> > way to translate PFNs to a DMA address and vice versa.  Yes, we have
> > them in ARM, but they are an _implementation detail_ of ARM's
> > DMA API support, they are not for use by drivers.
> > 
> > So, the very first problem that needs solving is this:
> > 
> >   How do we go from a coherent DMA allocation for device X to a set
> >   of DMA addresses for device Y.
> > 
> > Essentially, we need a way of remapping the DMA buffer for use with
> > another device, and returning a DMA address suitable for that device.
> > This could well mean that we need to deal with setting up an IOMMU
> > mapping.  My guess is that this needs to happen at the DMA coherent
> > API level - the DMA coherent API needs to be augmented with support
> > for this.  I'll call this "DMA coherent remap".
> > 
> > We then need to think about how to pass this through the dma-buf API.
> > dma_map_sg() is done by the exporter, who should know what kind of
> > memory is being exported.  The exporter can avoid calling dma_map_sg()
> > if it knows in advance that it is exporting DMA coherent memory.
> > Instead, the exporter can simply create a scatterlist with the DMA
> > address and DMA length prepopulated with the results of the DMA
> > coherent remap operation above.
> 
> The only way to conclusively say that it is coming from coherent area
> is at the time it is getting allocated in dma_alloc_from_coherent().
> Since dma_alloc_attrs() will go on to find memory from other areas if
> dma_alloc_from_coherent() doesn't allocate memory.

Sorry, I disagree.

The only thing that matters is "did this memory come from
dma_alloc_coherent()".  It doesn't matter where dma_alloc_coherent()
ultimately got the memory from, it's memory from the coherent allocator
interface, and it should not be passed back into the streaming APIs.
It is, after all, DMA _coherent_ memory, passing it into the streaming
APIs which is for DMA _noncoherent_ memory is insane - the streaming APIs
can bring extra expensive cache flushes, which are not required for
DMA _coherent_ memory.

The exporter should know where it got the memory from.  It's really not
sane for anyone except the _original_ allocator to be exporting memory
through a DMA buffer - only the original allocator knows the properties
of that memory, and how to map it, whether that be for DMA, kmap or
mmap.

If a dmabuf is imported into a driver and then re-exported, the original
dmabuf should be what is re-exported, not some creation of the driver -
the re-exporting driver can't know what the properties of the memory
backing the dmabuf are, so anything else is just insane.

> How about adding get_sgtable, map_sg, unmap_sg to dma_buf_ops. The right
> ops need to be installed based on buffer type. As I mentioned before, we
> don't know which memory we got until dma_alloc_from_coherent() finds
> memory in dev->mem area. So how about using the dma_check_dev_coherent()
> to determine which ops we need. These could be set based on buffer type.
> vb2_dc_get_dmabuf() can do that.

Given my statement above, I don't believe any of that is necessary.  All
memory allocated from dma_alloc_coherent() is DMA coherent.  So, if
memory was obtained from dma_alloc_coherent() or similar, then it must
not be passed to the streaming DMA API.  It doesn't matter where it
ultimately came from.

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