[Linaro-mm-sig] Re: [RFC v2 2/8] dma-buf: Add a helper to match interconnects between exporter/importer

On Sun, Oct 26, 2025 at 09:44:14PM -0700, Vivek Kasireddy wrote:

+/**

• • dma_buf_match_interconnects - determine if there is a specific interconnect
• • that is supported by both exporter and importer.
• • @attach: [in] attachment to populate ic_match field
• • @exp: [in] array of interconnects supported by exporter
• • @exp_ics: [in] number of interconnects supported by exporter
• • @imp: [in] array of interconnects supported by importer
• • @imp_ics: [in] number of interconnects supported by importer
• • This helper function iterates through the list interconnects supported by
• • both exporter and importer to find a match. A successful match means that
• • a common interconnect type is supported by both parties and the exporter's
• • match_interconnect() callback also confirms that the importer is compatible
• • with the exporter for that interconnect type.

Document which of the exporter/importer is supposed to call this

• • If a match is found, the attach->ic_match field is populated with a copy
• • of the exporter's match data.

• • Return: true if a match is found, false otherwise.
• */

+bool dma_buf_match_interconnects(struct dma_buf_attachment *attach,

• 		 const struct dma_buf_interconnect_match *exp,
  

• 		 unsigned int exp_ics,
  

• 		 const struct dma_buf_interconnect_match *imp,
  

• 		 unsigned int imp_ics)
  

+{

• const struct dma_buf_interconnect_ops *ic_ops;
• struct dma_buf_interconnect_match *ic_match;
• struct dma_buf *dmabuf = attach->dmabuf;
• unsigned int i, j;
• if (!exp || !imp)

• return false;
  

• if (!attach->allow_ic)

• return false;
  

Seems redundant with this check for ic_ops == NULL:

• ic_ops = dmabuf->ops->interconnect_ops;
• if (!ic_ops || !ic_ops->match_interconnect)

• return false;
  

This seems like too much of a maze to me..

I think you should structure it like this. First declare an interconnect:

struct dma_buf_interconnect iov_interconnect { .name = "IOV interconnect", .match =.. }

Then the exporters "subclass"

struct dma_buf_interconnect_ops vfio_iov_interconnect { .interconnect = &iov_interconnect, .map = vfio_map, }

I guess no container_of technique..

Then in VFIO's attach trigger the new code:

const struct dma_buf_interconnect_match vfio_exp_ics[] = { {&vfio_iov_interconnect}, };

dma_buf_match_interconnects(attach, &vfio_exp_ics))

Which will callback to the importer:

static const struct dma_buf_attach_ops xe_dma_buf_attach_ops = { .get_importer_interconnects }

dma_buf_match_interconnects() would call aops->get_importer_interconnects and matchs first on .interconnect, then call the interconnect->match function with exp/inpt match structs if not NULL.

+struct dma_buf_interconnect_match {

• const struct dma_buf_interconnect *type;
• struct device *dev;
• unsigned int bar;

+};

This should be more general, dev and bar are unique to the iov importer. Maybe just simple:

struct dma_buf_interconnect_match { struct dma_buf_interconnect *ic; // no need for type const struct dma_buf_interconnct_ops *exporter_ic_ops; u64 match_data[2]; // dev and bar are IOV specific, generalize };

Then some helper

const struct dma_buf_interconnect_match supports_ics[] = { IOV_INTERCONNECT(&vfio_iov_interconnect, dev, bar), }

And it would be nice if interconnect aware drivers could more easially interwork with non-interconnect importers.

So I'd add a exporter type of 'p2p dma mapped scatterlist' that just matches the legacy importer.

Jason