Add an optional dma_buf_ops.get_tph callback and a dma_buf_get_tph()
wrapper for importers.

8-bit ST and 16-bit Extended ST are distinct PCIe TPH namespaces, so
the importer requests the namespace it can emit and the exporter
returns the matching ST/PH tuple or -EOPNOTSUPP.

dma_buf_get_tph() is the importer entry point. It returns -EOPNOTSUPP
when the exporter lacks the callback and requires dmabuf->resv to be
held while the callback runs.

The first user is VFIO_DEVICE_FEATURE_DMA_BUF_TPH in vfio-pci, with
mlx5 as the first importer.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/dma-buf/dma-buf.c | 25 +++++++++++++++++++++++++
 include/linux/dma-buf.h   | 21 +++++++++++++++++++++
 2 files changed, 46 insertions(+)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d504c636dc29..aff79ea12e43 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1144,6 +1144,31 @@ void dma_buf_unpin(struct dma_buf_attachment *attach)
 }
 EXPORT_SYMBOL_NS_GPL(dma_buf_unpin, "DMA_BUF");
 
+/**
+ * dma_buf_get_tph - Retrieve TPH metadata from an exporter
+ * @dmabuf: DMA buffer to query
+ * @extended: false for 8-bit ST, true for 16-bit Extended ST
+ * @steering_tag: returns the raw steering tag for the requested namespace
+ * @ph: returns the TPH processing hint
+ *
+ * Wrapper for the optional &dma_buf_ops.get_tph callback.
+ *
+ * Must be called with &dma_buf.resv held. Returns -EOPNOTSUPP if the
+ * exporter does not implement the callback or has no metadata for the
+ * requested namespace.
+ */
+int dma_buf_get_tph(struct dma_buf *dmabuf, bool extended,
+		    u16 *steering_tag, u8 *ph)
+{
+	dma_resv_assert_held(dmabuf->resv);
+
+	if (!dmabuf->ops->get_tph)
+		return -EOPNOTSUPP;
+
+	return dmabuf->ops->get_tph(dmabuf, extended, steering_tag, ph);
+}
+EXPORT_SYMBOL_NS_GPL(dma_buf_get_tph, "DMA_BUF");
+
 /**
  * dma_buf_map_attachment - Returns the scatterlist table of the attachment;
  * mapped into _device_ address space. Is a wrapper for map_dma_buf() of the
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index d1203da56fc5..6a54e0f251a2 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -113,6 +113,25 @@ struct dma_buf_ops {
 	 */
 	void (*unpin)(struct dma_buf_attachment *attach);
 
+	/**
+	 * @get_tph:
+	 * @dmabuf: DMA buffer for which to retrieve TPH metadata
+	 * @extended: false for 8-bit ST, true for 16-bit Extended ST
+	 * @steering_tag: Returns the raw TPH steering tag for the requested
+	 *                namespace
+	 * @ph: Returns the TPH processing hint (2-bit value)
+	 *
+	 * Return TPH metadata for the namespace selected by @extended. Return
+	 * 0 on success, or -EOPNOTSUPP if no metadata is available.
+	 *
+	 * This callback is optional. Importers must not call it directly;
+	 * the dma_buf_get_tph() wrapper is the only entry point and handles
+	 * the NULL-callback case. The callback is invoked with
+	 * &dma_buf.resv held.
+	 */
+	int (*get_tph)(struct dma_buf *dmabuf, bool extended,
+		       u16 *steering_tag, u8 *ph);
+
 	/**
 	 * @map_dma_buf:
 	 *
@@ -563,6 +582,8 @@ void dma_buf_detach(struct dma_buf *dmabuf,
 		    struct dma_buf_attachment *attach);
 int dma_buf_pin(struct dma_buf_attachment *attach);
 void dma_buf_unpin(struct dma_buf_attachment *attach);
+int dma_buf_get_tph(struct dma_buf *dmabuf, bool extended,
+		    u16 *steering_tag, u8 *ph);
 
 struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info);
 
-- 
2.53.0-Meta