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

[Sumit Semwal]

Hi Russell, everyone,

First up, sincere apologies for being awol for sometime; had some
personal / medical things to take care of, and then I thought I'd wait
for the merge window to get over before beginning to discuss this
again.

On 11 February 2015 at 21:53, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> 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.
>

So, for basic constraint-sharing, we'll just go with the flag based
approach, with a flag (best place for it is still dev->dma_params I
suppose) for denoting contiguous or scatterlist. Is that agreed, then?
Also, with this idea, of course, there won't be any helpers for trying
to calculate constraints; it would be totally the exporter's
responsibility to handle it via the attach() dma_buf_op if it wishes
to.

>> 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.
>
+1 for Keep it simple, and the idea overall. Though I suspect more
dma-buf users (dri / v4l friends?) should comment if this doesn't help
solve things on some platforms / subsystems that they care about.

> 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.
>
Could I please send a patchset first for the constraint-flags
addition, and then take this up in a following patch-set, once we have
agreement from other stake holders as well?

Russell: would it be ok if we discuss it offline more? I am afraid I
am not as knowledgeable on this topic, and would probably request your
help / guidance here.

> 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.)
>
> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.

Best regards,
~Sumit.