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The importer > passes its supported steering-tag width and the exporter returns the > matching value, or -EOPNOTSUPP if no metadata is available for that > width. > > The callback is intentionally exporter-owned and optional. The exporter > owns the completing address space for the dma-buf, so only it can decide > whether it has meaningful TPH metadata for that completer. The dma-buf > core keeps the returned ST/PH tuple opaque and simply provides a > discoverable negotiation point between exporter and importer; exporters > that cannot derive a useful tuple just return -EOPNOTSUPP. > > That keeps the kernel API generic rather than VFIO-specific. The first > user is VFIO_DEVICE_FEATURE_DMA_BUF_TPH in vfio-pci, with the mlx5 RDMA > driver as the first importer, but any future exporter that can derive a > TPH tuple for its completing address space can reuse the same callback. > > Signed-off-by: Zhiping Zhang > --- > include/linux/dma-buf.h | 31 +++++++++++++++++++++++++++++++ > 1 file changed, 31 insertions(+) > > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h > index d1203da56fc5..8437dbe4a83e 100644 > --- a/include/linux/dma-buf.h > +++ b/include/linux/dma-buf.h > @@ -113,6 +113,37 @@ struct dma_buf_ops { > */ > void (*unpin)(struct dma_buf_attachment *attach); > > + /** > + * @get_tph: > + * @dmabuf: DMA buffer for which to retrieve TPH metadata > + * @extended: false to request the 8-bit ST namespace, true to request > + * the 16-bit Extended ST namespace > + * @steering_tag: Returns the raw TPH steering tag for the requested > + * namespace > + * @ph: Returns the TPH processing hint (2-bit value) > + * > + * Return the TPH (TLP Processing Hints) metadata associated with this > + * DMA buffer for the requested steering-tag namespace. 8-bit ST and > + * 16-bit Extended ST are distinct namespaces in the PCIe TPH ST table > + * and may both be present with different values, so the exporter must > + * select the value that matches @extended and must not substitute one > + * for the other. > + * > + * The exporter owns the completing address space for @dmabuf and > + * therefore decides whether it can derive meaningful TPH metadata for > + * that completer. The dma-buf core treats the returned ST/PH tuple as > + * opaque transport metadata; importers that support TPH place it on > + * outbound TLPs, while exporters that cannot derive a useful tuple > + * simply return -EOPNOTSUPP. > + * > + * Return 0 on success, or -EOPNOTSUPP if no metadata is available for > + * the requested namespace. > + * > + * This callback is optional. > + */ > + int (*get_tph)(struct dma_buf *dmabuf, bool extended, > + u16 *steering_tag, u8 *ph); > + That needs a wrapper for importers to call which also handles if the callback isn't present. Regards, Christian. > /** > * @map_dma_buf: > *