[RFCv3 2/2] dma-buf: add helpers for sharing attacher constraints with dma-parms

[Russell King - ARM Linux]

On Wed, Feb 11, 2015 at 01:20:24PM +0100, Marek Szyprowski wrote:
> Hello,
> 
> On 2015-02-11 12:12, Russell King - ARM Linux wrote:
> >Which is a damn good reason to NAK it - by that admission, it's a half-baked
> >idea.
> >
> >If all we want to know is whether the importer can accept only contiguous
> >memory or not, make a flag to do that, and allow the exporter to test this
> >flag.  Don't over-engineer this to make it _seem_ like it can do something
> >that it actually totally fails with.
> >
> >As I've already pointed out, there's a major problem if you have already
> >had a less restrictive attachment which has an active mapping, and a new
> >more restrictive attachment comes along later.
> >
> >It seems from Rob's descriptions that we also need another flag in the
> >importer to indicate whether it wants to have a valid struct page in the
> >scatter list, or whether it (correctly) uses the DMA accessors on the
> >scatter list - so that exporters can reject importers which are buggy.
> 
> Okay, but flag-based approach also have limitations.

Yes, the flag-based approach doesn't let you describe in detail what
the importer can accept - which, given the issues that I've raised
is a *good* thing.  We won't be misleading anyone into thinking that
we can do something that's really half-baked, and which we have no
present requirement for.

This is precisely what Linus talks about when he says "don't over-
engineer" - if we over-engineer this, we end up with something that
sort-of works, and that's a bad thing.

The Keep It Simple approach here makes total sense - what are our
current requirements - to be able to say that an importer can only accept:
  - contiguous memory rather than a scatterlist
  - scatterlists with struct page pointers

Does solving that need us to compare all the constraints of each and
every importer, possibly ending up with constraints which can't be
satisfied?  No.  Does the flag approach satisfy the requirements?  Yes.

> Frankly, if we want to make it really portable and sharable between devices,
> then IMO we should get rid of struct scatterlist and replace it with simple
> array of pfns in dma_buf. This way at least the problem of missing struct
> page will be solved and the buffer representation will be also a bit more
> compact.

... and move the mapping and unmapping of the PFNs to the importer,
which IMHO is where it should already be (so the importer can decide
when it should map the buffer itself independently of getting the
details of the buffer.)

> Such solution however also requires extending dma-mapping API to handle
> mapping and unmapping of such pfn arrays. The advantage of this approach
> is the fact that it will be completely new API, so it can be designed
> well from the beginning.

As far as I'm aware, there was no big discussion of the dma_buf API -
it's something that just appeared one day (I don't remember seeing it
discussed.)  So, that may well be a good thing if it means we can get
these kinds of details better hammered out.

However, I don't share your view of "completely new API" - that would
be far too disruptive.  I think we can modify the existing API, to
achieve our goals.

I don't think changing the dma-mapping API just for this case is really
on though - if we're passed a list of PFNs, they either must be all
associated with a struct page - iow, pfn_valid(pfn) returns true for
all of them or none of them.  If none of them, then we need to be able
to convert those PFNs to a dma_addr_t for the target device (yes, that
may need the dma-mapping API augmenting.)

However, if they are associated with a struct page, then we should use
the established APIs and use a scatterlist, otherwise we're looking
at rewriting all IOMMUs and all DMA mapping implementations to handle
what would become a special case for dma_buf.

I'd rather... Keep It Simple.

So, maybe, as a first step, let's augment dma_buf with a pair of
functions which get the "raw" unmapped scatterlist:

struct sg_table *dma_buf_get_scatterlist(struct dma_buf_attachment *attach)
{
	struct sg_table *sg_table;

	might_sleep();

	if (!attach->dmabuf->ops->get_scatterlist)
		return ERR_PTR(-EINVAL);

	sg_table = attach->dmabuf->ops->get_scatterlist(attach);
	if (!sg_table)
		sg_table = ERR_PTR(-ENOMEM);

	return sg_table;
}

void dma_buf_put_scatterlist(struct dma_buf_attachment *attach,
			     struct sg_table *sg_table)
{
	might_sleep();

	attach->dmabuf->ops->put_scatterlist(attach, sg_table);
}

Implementations should arrange for dma_buf_get_scatterlist() to return
the EINVAL error pointer if they are unable to provide an unmapped
scatterlist (in other words, they are exporting a set of PFNs or
already-mapped dma_addr_t's.)  This can be done by either not
implementing the get_scatterlist method, or by implementing it and
returning that forementioned error pointer value.

Where these two are implemented and used, the importer is responsible
for calling dma_map_sg() and dma_unmap_sg() on the returned scatterlist
table.

unsigned long *dma_buf_get_pfns(struct dma_buf_attachment *attach)
{
	unsigned long *pfns;

	might_sleep();

	if (!attach->dmabuf->ops->get_pfns)
		return ERR_PTR(-EINVAL);

	return attach->dmabuf->ops->get_pfns(attach);
}

void dma_buf_put_pfns(struct dma_buf_attachment *attach, unsigned long *pfns)
{
	might_sleep();

	attach->dmabuf->ops->put_pfns(attach, pfns);
}

Similar to the above, but this gets a list of PFNs.  Each PFN entry prior
to the last describes a page starting at offset 0 extending to the end of
the page.  The last PFN entry describes a page starting at offset 0 and
extending to the offset of the attachment size within the page.  Again,
if not implemented or it is not possible to represent the buffer as PFNs,
it returns -EINVAL.

For the time being, we keep the existing dma_buf_map_attachment() and
dma_buf_unmap_attachment() while we transition users over to the new
interfaces.

We may wish to augment struct dma_buf_attachment with a couple of flags
to indicate which of these the attached dma_buf supports, so that drivers
can deal with these themselves.

We may also wish in the longer term to keep dma_buf_map_attachment() but
implement it as a wrapper around get_scatterlist() as a helper to provide
its existing functionality - providing a mapped scatterlist.  Possibly
also including get_pfns() in that as well if we need to.

However, I would still like more thought put into Rob's issue to see
whether we can solve his problem with using the dma_addr_t in a more
elegant way.  (I wish I had some hardware to experiment with for that.)

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