[PATCH v8 0/4] vfio/dma-buf: add TPH support for peer-to-peer access

[PATCH v8 0/4] vfio/dma-buf: add TPH support for peer-to-peer access

[Zhiping Zhang]

This series adds TLP Processing Hints (TPH) support to the VFIO dma-buf
export path, allowing importing drivers (e.g. mlx5) to use the
exporter's steering tag when performing peer-to-peer DMA into a
VFIO-owned device.

There is no separate in-tree vendor kernel driver for the target device:
vfio-pci is the in-tree driver and the targeted device is managed
from userspace via VFIO passthrough. That is why the ST has to flow
through a uAPI: userspace owns the device and its ST table, so it is the
entity that can publish a meaningful value for a given dma-buf. The
kernel-visible participants are still in-tree: vfio-pci exports the
dma-buf and mlx5 imports it.

On the effect: the endpoint's PCIe ingress block uses the ST as
an in-band instruction for the incoming P2P TLP -- selecting a target
cache partition and, on writes, an in-flight operation on the data
before it lands. The dma-buf callback keeps this opaque to the
framework -- only the producer (userspace owner of the VFIO device)
and the consumer (endpoint block) need to interpret the value. The
dma-buf get_pci_tph callback itself is optional for workloads that depend
on the endpoint's in-flight operation that fallback does not produce
the same result.

The dma-buf hook is intentionally generic and discoverable rather than
a private side channel. The exporter owns the completing address
space for the dma-buf and decides whether it can provide a meaningful
ST/PH tuple for that completer; the dma-buf core keeps the tuple opaque,
and importers merely request the namespace they support and place the
returned value on generated TLPs. Exporters that cannot derive a
meaningful tuple simply return -EOPNOTSUPP.

Patch 1 adds small PCI/TPH type helpers so drivers can query the enabled
TPH requester mode and the device's TPH Completer Supported field
without reaching into pci_dev internals (and so callers in
CONFIG_PCIE_TPH=n builds get a clean fallback).
Patch 2 adds the optional dma_buf_ops::get_pci_tph callback plus the
dma_buf_get_pci_tph() importer wrapper so importers can fetch TPH
metadata from an exporter under dmabuf->resv.
Patch 3 implements get_pci_tph in vfio-pci and adds the new uAPI
(VFIO_DEVICE_FEATURE_DMA_BUF_TPH) for userspace to attach the metadata.
Patch 4 wires up the mlx5 RDMA driver as a consumer.

Build-tested with both CONFIG_PCIE_TPH=y and CONFIG_PCIE_TPH=n.
Functional validation on the target topology: PCIe analyzer captures
on the P2P TLPs confirm the ST emitted by mlx5 matches the value
published through VFIO_DEVICE_FEATURE_DMA_BUF_TPH, and the end-to-end
P2P workload only produces results consistent with the endpoint's
ST-selected in-flight operation. For example, with userspace
publishing 8-bit ST=0xf0 and PH=2, an analyzer capture of a peer-to-
peer MWr64 shows "STP MWr64 TC=0 OHC=2 ..." followed by "OHC-B
ST=F0h PH=2 HV=1":
(TLP Captures)
08000260 -> STP MWr64 TC=0 OHC=2 TS=0 Attr=0 L=8
F0000004 -> RID=4h:0h.0h EP- Tag=F0h
E0200000 -> AddrH=000020E0h
00080006 -> AddrL=06000800h
90F00000 -> OHC-B ST=F0h PH=2 HV=1 AMA=0 AV-

Depends on (submitted separately):
  net/mlx5: free mlx5_st_idx_data on final dealloc
    https://lore.kernel.org/linux-rdma/20260612170406.3339093-1-zhipingz@meta.com
  RDMA/mlx5: Fix TPH extraction in FRMR pool key (Michael Guralnik)
    https://lore.kernel.org/linux-rdma/20260610000145.820592-3-michaelgur@nvidia.com/

Previous link:
v7: https://lore.kernel.org/dri-devel/20260611161546.4075580-1-zhipingz@meta.com/
v6: https://lore.kernel.org/dri-devel/20260608185646.4085127-1-zhipingz@meta.com/
v5: https://lore.kernel.org/dri-devel/20260526144401.1485788-1-zhipingz@meta.com/
v4: https://lore.kernel.org/linux-pci/20260519201401.1558410-1-zhipingz@meta.com/
v3: https://lore.kernel.org/linux-pci/20260512184755.4137227-1-zhipingz@meta.com/
v2: https://lore.kernel.org/linux-pci/20260430200704.352228-1-zhipingz@meta.com/

Zhiping Zhang (4):
  PCI/TPH: Add requester/completer type helpers
  dma-buf: add optional get_pci_tph() callback
  vfio/pci: implement get_pci_tph and DMA_BUF_TPH feature
  RDMA/mlx5: get tph for p2p access when registering dma-buf mr

 drivers/dma-buf/dma-buf.c                     |  25 ++++
 drivers/infiniband/hw/mlx5/main.c             |   1 +
 drivers/infiniband/hw/mlx5/mr.c               | 105 ++++++++++++++++++-
 .../net/ethernet/mellanox/mlx5/core/lib/st.c  |  49 ++++++--
 drivers/pci/tph.c                             |  45 ++++++++
 drivers/vfio/pci/vfio_pci_core.c              |   3 +
 drivers/vfio/pci/vfio_pci_dmabuf.c            |  97 +++++++++++++++-
 drivers/vfio/pci/vfio_pci_priv.h              |  12 ++
 include/linux/dma-buf.h                       |  16 +++
 include/linux/mlx5/driver.h                   |  12 ++
 include/linux/pci-tph.h                       |   8 ++
 include/uapi/linux/vfio.h                     |  37 ++++++
 12 files changed, 399 insertions(+), 11 deletions(-)

--
2.53.0-Meta

[PATCH v8 1/4] PCI/TPH: Add requester/completer type helpers

[Zhiping Zhang]

Add pcie_tph_enabled_req_type() so drivers can query the enabled TPH
requester mode without reaching into pci_dev internals.

Add pcie_tph_completer_type() so drivers that publish TPH metadata for
a device acting as a completer can gate on the "TPH Completer
Supported" field of Device Capabilities 2 (bits 13:12,
PCI_EXP_DEVCAP2_TPH_COMP_MASK) rather than reusing requester-side
state. Fold the reserved 0b10 encoding into NONE so callers only see
the defined values.

This keeps pci_dev::tph_req_type and the completer-capability decode
inside the PCI/TPH code and provides !CONFIG_PCIE_TPH stubs for
callers.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/pci/tph.c       | 45 +++++++++++++++++++++++++++++++++++++++++
 include/linux/pci-tph.h |  8 ++++++++
 2 files changed, 53 insertions(+)

diff --git a/drivers/pci/tph.c b/drivers/pci/tph.c
index 91145e8d9d95..717b47d1d9ca 100644
--- a/drivers/pci/tph.c
+++ b/drivers/pci/tph.c
@@ -174,6 +174,51 @@ u32 pcie_tph_get_st_table_loc(struct pci_dev *pdev)
 }
 EXPORT_SYMBOL(pcie_tph_get_st_table_loc);
 
+/**
+ * pcie_tph_enabled_req_type - Return the device's enabled TPH requester type
+ * @pdev: PCI device to query
+ *
+ * Return: PCI_TPH_REQ_DISABLE, PCI_TPH_REQ_TPH_ONLY or PCI_TPH_REQ_EXT_TPH.
+ */
+u8 pcie_tph_enabled_req_type(struct pci_dev *pdev)
+{
+	return pdev->tph_req_type;
+}
+EXPORT_SYMBOL(pcie_tph_enabled_req_type);
+
+/**
+ * pcie_tph_completer_type - Return the device's TPH Completer support
+ * @pdev: PCI device to query
+ *
+ * Reads the "TPH Completer Supported" field (bits 13:12) of Device
+ * Capabilities 2. The reserved 0b10 encoding is folded into
+ * "not supported" so callers only need to compare against the three
+ * defined values.
+ *
+ * Return: one of %PCI_EXP_DEVCAP2_TPH_COMP_NONE,
+ *         %PCI_EXP_DEVCAP2_TPH_COMP_TPH_ONLY or
+ *         %PCI_EXP_DEVCAP2_TPH_COMP_EXT_TPH.
+ */
+u8 pcie_tph_completer_type(struct pci_dev *pdev)
+{
+	u32 reg;
+
+	if (pcie_capability_read_dword(pdev, PCI_EXP_DEVCAP2, &reg))
+		return PCI_EXP_DEVCAP2_TPH_COMP_NONE;
+	if (PCI_POSSIBLE_ERROR(reg))
+		return PCI_EXP_DEVCAP2_TPH_COMP_NONE;
+
+	switch (FIELD_GET(PCI_EXP_DEVCAP2_TPH_COMP_MASK, reg)) {
+	case PCI_EXP_DEVCAP2_TPH_COMP_TPH_ONLY:
+		return PCI_EXP_DEVCAP2_TPH_COMP_TPH_ONLY;
+	case PCI_EXP_DEVCAP2_TPH_COMP_EXT_TPH:
+		return PCI_EXP_DEVCAP2_TPH_COMP_EXT_TPH;
+	default:
+		return PCI_EXP_DEVCAP2_TPH_COMP_NONE;
+	}
+}
+EXPORT_SYMBOL(pcie_tph_completer_type);
+
 /*
  * Return the size of ST table. If ST table is not in TPH Requester Extended
  * Capability space, return 0. Otherwise return the ST Table Size + 1.
diff --git a/include/linux/pci-tph.h b/include/linux/pci-tph.h
index be68cd17f2f8..7743af6fe432 100644
--- a/include/linux/pci-tph.h
+++ b/include/linux/pci-tph.h
@@ -9,6 +9,8 @@
 #ifndef LINUX_PCI_TPH_H
 #define LINUX_PCI_TPH_H
 
+#include <linux/pci_regs.h>
+
 /*
  * According to the ECN for PCI Firmware Spec, Steering Tag can be different
  * depending on the memory type: Volatile Memory or Persistent Memory. When a
@@ -30,6 +32,8 @@ void pcie_disable_tph(struct pci_dev *pdev);
 int pcie_enable_tph(struct pci_dev *pdev, int mode);
 u16 pcie_tph_get_st_table_size(struct pci_dev *pdev);
 u32 pcie_tph_get_st_table_loc(struct pci_dev *pdev);
+u8 pcie_tph_enabled_req_type(struct pci_dev *pdev);
+u8 pcie_tph_completer_type(struct pci_dev *pdev);
 #else
 static inline int pcie_tph_set_st_entry(struct pci_dev *pdev,
 					unsigned int index, u16 tag)
@@ -41,6 +45,10 @@ static inline int pcie_tph_get_cpu_st(struct pci_dev *dev,
 static inline void pcie_disable_tph(struct pci_dev *pdev) { }
 static inline int pcie_enable_tph(struct pci_dev *pdev, int mode)
 { return -EINVAL; }
+static inline u8 pcie_tph_enabled_req_type(struct pci_dev *pdev)
+{ return PCI_TPH_REQ_DISABLE; }
+static inline u8 pcie_tph_completer_type(struct pci_dev *pdev)
+{ return PCI_EXP_DEVCAP2_TPH_COMP_NONE; }
 #endif
 
 #endif /* LINUX_PCI_TPH_H */
-- 
2.53.0-Meta

[PATCH v8 2/4] dma-buf: add optional get_pci_tph() callback

[Zhiping Zhang]

Add an optional dma_buf_ops.get_pci_tph callback and a
DMA-buf importer wrapper, dma_buf_get_pci_tph().

TPH is PCIe TLP Processing Hint. 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_pci_tph() is the importer entry point. It requires
&dmabuf->resv to be held while the callback runs and returns
-EOPNOTSUPP when the exporter does not provide PCI TPH metadata.

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   | 16 ++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index d504c636dc29..7a4c9b0d5dab 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_pci_tph - Retrieve PCIe TLP Processing Hint (TPH) metadata
+ * @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_pci_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_pci_tph(struct dma_buf *dmabuf, bool extended,
+			u16 *steering_tag, u8 *ph)
+{
+	dma_resv_assert_held(dmabuf->resv);
+
+	if (!dmabuf->ops->get_pci_tph)
+		return -EOPNOTSUPP;
+
+	return dmabuf->ops->get_pci_tph(dmabuf, extended, steering_tag, ph);
+}
+EXPORT_SYMBOL_NS_GPL(dma_buf_get_pci_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..5e7b69a40f3d 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -113,6 +113,20 @@ struct dma_buf_ops {
 	 */
 	void (*unpin)(struct dma_buf_attachment *attach);
 
+	/**
+	 * @get_pci_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)
+	 *
+	 * Optional callback for dma_buf_get_pci_tph(). Called with
+	 * &dma_buf.resv held.
+	 */
+	int (*get_pci_tph)(struct dma_buf *dmabuf, bool extended,
+			   u16 *steering_tag, u8 *ph);
+
 	/**
 	 * @map_dma_buf:
 	 *
@@ -563,6 +577,8 @@ 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_pci_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

Re: [PATCH v8 2/4] dma-buf: add optional get_pci_tph() callback

[Leon Romanovsky]

On Sun, Jun 14, 2026 at 11:58:59PM -0700, Zhiping Zhang wrote:
> Add an optional dma_buf_ops.get_pci_tph callback and a
> DMA-buf importer wrapper, dma_buf_get_pci_tph().
> 
> TPH is PCIe TLP Processing Hint. 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_pci_tph() is the importer entry point. It requires
> &dmabuf->resv to be held while the callback runs and returns
> -EOPNOTSUPP when the exporter does not provide PCI TPH metadata.
> 
> 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   | 16 ++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d504c636dc29..7a4c9b0d5dab 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_pci_tph - Retrieve PCIe TLP Processing Hint (TPH) metadata
> + * @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_pci_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_pci_tph(struct dma_buf *dmabuf, bool extended,
> +			u16 *steering_tag, u8 *ph)
> +{
> +	dma_resv_assert_held(dmabuf->resv);
> +
> +	if (!dmabuf->ops->get_pci_tph)
> +		return -EOPNOTSUPP;
> +
> +	return dmabuf->ops->get_pci_tph(dmabuf, extended, steering_tag, ph);
> +}
> +EXPORT_SYMBOL_NS_GPL(dma_buf_get_pci_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..5e7b69a40f3d 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -113,6 +113,20 @@ struct dma_buf_ops {
>  	 */
>  	void (*unpin)(struct dma_buf_attachment *attach);
>  
> +	/**
> +	 * @get_pci_tph:

There should be a blank line after the line above, along with a clear and
concise description of what this callback does.

Thanks

> +	 * @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)
> +	 *
> +	 * Optional callback for dma_buf_get_pci_tph(). Called with
> +	 * &dma_buf.resv held.
> +	 */
> +	int (*get_pci_tph)(struct dma_buf *dmabuf, bool extended,
> +			   u16 *steering_tag, u8 *ph);
> +
>  	/**
>  	 * @map_dma_buf:
>  	 *
> @@ -563,6 +577,8 @@ 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_pci_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
> 

[PATCH v8 3/4] vfio/pci: implement get_pci_tph and DMA_BUF_TPH feature

[Zhiping Zhang]

Implement dma-buf get_pci_tph for vfio-pci exported dma-bufs and add
VFIO_DEVICE_FEATURE_DMA_BUF_TPH so userspace can publish TPH metadata
for a VFIO-owned device.

8-bit ST and 16-bit Extended ST are distinct PCIe TPH namespaces; the
uAPI carries both with explicit validity flags, and get_pci_tph()
returns the value matching the importer's requested namespace or
-EOPNOTSUPP.

Publish and read the TPH descriptor under dmabuf->resv, matching the
locking used for other importer-visible dma-buf state. The SET ioctl
takes dma_resv_lock_interruptible(), while the callback runs under
DMA-buf's asserted resv lock.

Reject requests the device cannot consume as a completer:
pcie_tph_completer_type() must report at least
PCI_EXP_DEVCAP2_TPH_COMP_TPH_ONLY, and Extended ST requires
PCI_EXP_DEVCAP2_TPH_COMP_EXT_TPH. Make PROBE follow the same hardware
gate so the feature only probes as supported when the device can really
consume it.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/vfio/pci/vfio_pci_core.c   |  3 +
 drivers/vfio/pci/vfio_pci_dmabuf.c | 97 +++++++++++++++++++++++++++++-
 drivers/vfio/pci/vfio_pci_priv.h   | 12 ++++
 include/uapi/linux/vfio.h          | 37 ++++++++++++
 4 files changed, 148 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 050e7542952e..4fa36f2f7555 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1569,6 +1569,9 @@ int vfio_pci_core_ioctl_feature(struct vfio_device *device, u32 flags,
 		return vfio_pci_core_feature_token(vdev, flags, arg, argsz);
 	case VFIO_DEVICE_FEATURE_DMA_BUF:
 		return vfio_pci_core_feature_dma_buf(vdev, flags, arg, argsz);
+	case VFIO_DEVICE_FEATURE_DMA_BUF_TPH:
+		return vfio_pci_core_feature_dma_buf_tph(vdev, flags, arg,
+							 argsz);
 	default:
 		return -ENOTTY;
 	}
diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
index 1a177ce7de54..6284ce208a6a 100644
--- a/drivers/vfio/pci/vfio_pci_dmabuf.c
+++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
@@ -3,6 +3,7 @@
  */
 #include <linux/dma-buf-mapping.h>
 #include <linux/pci-p2pdma.h>
+#include <linux/pci-tph.h>
 #include <linux/dma-resv.h>
 
 #include "vfio_pci_priv.h"
@@ -19,7 +20,14 @@ struct vfio_pci_dma_buf {
 	u32 nr_ranges;
 	struct kref kref;
 	struct completion comp;
-	u8 revoked : 1;
+
+	/* Protected by dmabuf->resv. */
+	u16 tph_st_ext;
+	u8 tph_st;
+	u8 revoked:1;
+	u8 tph_st_valid:1;
+	u8 tph_st_ext_valid:1;
+	u8 tph_ph:2;
 };
 
 static int vfio_pci_dma_buf_attach(struct dma_buf *dmabuf,
@@ -69,6 +77,26 @@ vfio_pci_dma_buf_map(struct dma_buf_attachment *attachment,
 	return ret;
 }
 
+static int vfio_pci_dma_buf_get_pci_tph(struct dma_buf *dmabuf, bool extended,
+					u16 *steering_tag, u8 *ph)
+{
+	struct vfio_pci_dma_buf *priv = dmabuf->priv;
+
+	dma_resv_assert_held(dmabuf->resv);
+
+	if (extended) {
+		if (!priv->tph_st_ext_valid)
+			return -EOPNOTSUPP;
+		*steering_tag = priv->tph_st_ext;
+	} else {
+		if (!priv->tph_st_valid)
+			return -EOPNOTSUPP;
+		*steering_tag = priv->tph_st;
+	}
+	*ph = priv->tph_ph;
+	return 0;
+}
+
 static void vfio_pci_dma_buf_unmap(struct dma_buf_attachment *attachment,
 				   struct sg_table *sgt,
 				   enum dma_data_direction dir)
@@ -101,6 +129,7 @@ static void vfio_pci_dma_buf_release(struct dma_buf *dmabuf)
 
 static const struct dma_buf_ops vfio_pci_dmabuf_ops = {
 	.attach = vfio_pci_dma_buf_attach,
+	.get_pci_tph = vfio_pci_dma_buf_get_pci_tph,
 	.map_dma_buf = vfio_pci_dma_buf_map,
 	.unmap_dma_buf = vfio_pci_dma_buf_unmap,
 	.release = vfio_pci_dma_buf_release,
@@ -333,6 +362,72 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
 	return ret;
 }
 
+int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
+				      u32 flags,
+				      struct vfio_device_feature_dma_buf_tph __user *arg,
+				      size_t argsz)
+{
+	struct vfio_device_feature_dma_buf_tph set_tph;
+	struct vfio_pci_dma_buf *priv;
+	struct dma_buf *dmabuf;
+	u8 comp;
+	int ret;
+
+	comp = pcie_tph_completer_type(vdev->pdev);
+	if (comp == PCI_EXP_DEVCAP2_TPH_COMP_NONE)
+		return -EOPNOTSUPP;
+
+	ret = vfio_check_feature(flags, argsz, VFIO_DEVICE_FEATURE_SET,
+				 sizeof(set_tph));
+	if (ret != 1)
+		return ret;
+
+	if (copy_from_user(&set_tph, arg, sizeof(set_tph)))
+		return -EFAULT;
+
+	if (set_tph.flags & ~(VFIO_DMA_BUF_TPH_ST | VFIO_DMA_BUF_TPH_ST_EXT))
+		return -EINVAL;
+
+	if (set_tph.ph & ~0x3)
+		return -EINVAL;
+
+	if ((set_tph.flags & VFIO_DMA_BUF_TPH_ST_EXT) &&
+	    comp != PCI_EXP_DEVCAP2_TPH_COMP_EXT_TPH)
+		return -EOPNOTSUPP;
+
+	dmabuf = dma_buf_get(set_tph.dmabuf_fd);
+	if (IS_ERR(dmabuf))
+		return PTR_ERR(dmabuf);
+
+	if (dmabuf->ops != &vfio_pci_dmabuf_ops) {
+		ret = -EINVAL;
+		goto out_put;
+	}
+
+	priv = dmabuf->priv;
+	if (priv->vdev != vdev) {
+		ret = -EINVAL;
+		goto out_put;
+	}
+
+	ret = dma_resv_lock_interruptible(dmabuf->resv, NULL);
+	if (ret)
+		goto out_put;
+
+	priv->tph_st         = set_tph.steering_tag;
+	priv->tph_st_ext     = set_tph.steering_tag_ext;
+	priv->tph_ph         = set_tph.ph;
+	priv->tph_st_valid   = !!(set_tph.flags & VFIO_DMA_BUF_TPH_ST);
+	priv->tph_st_ext_valid =
+		!!(set_tph.flags & VFIO_DMA_BUF_TPH_ST_EXT);
+	dma_resv_unlock(dmabuf->resv);
+	ret = 0;
+
+out_put:
+	dma_buf_put(dmabuf);
+	return ret;
+}
+
 void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked)
 {
 	struct vfio_pci_dma_buf *priv;
diff --git a/drivers/vfio/pci/vfio_pci_priv.h b/drivers/vfio/pci/vfio_pci_priv.h
index fca9d0dfac90..c58f369be4b3 100644
--- a/drivers/vfio/pci/vfio_pci_priv.h
+++ b/drivers/vfio/pci/vfio_pci_priv.h
@@ -118,6 +118,10 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
 int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
 				  struct vfio_device_feature_dma_buf __user *arg,
 				  size_t argsz);
+int vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev,
+				      u32 flags,
+				      struct vfio_device_feature_dma_buf_tph __user *arg,
+				      size_t argsz);
 void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev);
 void vfio_pci_dma_buf_move(struct vfio_pci_core_device *vdev, bool revoked);
 #else
@@ -128,6 +132,14 @@ vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
 {
 	return -ENOTTY;
 }
+
+static inline int
+vfio_pci_core_feature_dma_buf_tph(struct vfio_pci_core_device *vdev, u32 flags,
+				  struct vfio_device_feature_dma_buf_tph __user *arg,
+				  size_t argsz)
+{
+	return -ENOTTY;
+}
 static inline void vfio_pci_dma_buf_cleanup(struct vfio_pci_core_device *vdev)
 {
 }
diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 5de618a3a5ee..2d30ba43e2cf 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h
@@ -1534,6 +1534,43 @@ struct vfio_device_feature_dma_buf {
  */
 #define VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2  12
 
+/**
+ * Upon VFIO_DEVICE_FEATURE_SET associate TPH (TLP Processing Hints) metadata
+ * with a vfio-exported dma-buf. The dma-buf must have been created by
+ * VFIO_DEVICE_FEATURE_DMA_BUF on this device, and the device must report
+ * TPH Completer support in Device Capabilities 2 (bits 13:12); requests
+ * carrying VFIO_DMA_BUF_TPH_ST_EXT additionally require the device to
+ * report the Extended TPH Completer encoding. Otherwise the ioctl
+ * returns -EOPNOTSUPP.
+ *
+ * dmabuf_fd is the file descriptor returned by VFIO_DEVICE_FEATURE_DMA_BUF.
+ *
+ * 8-bit ST (steering_tag) and 16-bit Extended ST (steering_tag_ext) are
+ * distinct namespaces. Userspace supplies whichever values are valid and sets
+ * the matching VFIO_DMA_BUF_TPH_ST / VFIO_DMA_BUF_TPH_ST_EXT bits in @flags;
+ * an importer requests one namespace and receives the matching value.
+ *
+ * @flags == 0 marks any previously published ST / Extended-ST as invalid
+ * for future PCI TPH queries on this dma-buf.
+ *
+ * ph is the 2-bit TLP Processing Hint and must be in the range [0, 3].
+ *
+ * Userspace must publish TPH before handing the dma-buf fd to an importer.
+ * Calling SET again replaces the published values.
+ */
+#define VFIO_DEVICE_FEATURE_DMA_BUF_TPH 13
+
+#define VFIO_DMA_BUF_TPH_ST		(1 << 0)
+#define VFIO_DMA_BUF_TPH_ST_EXT		(1 << 1)
+
+struct vfio_device_feature_dma_buf_tph {
+	__s32	dmabuf_fd;
+	__u32	flags;
+	__u16	steering_tag_ext;
+	__u8	steering_tag;
+	__u8	ph;
+};
+
 /* -------- API for Type1 VFIO IOMMU -------- */
 
 /**
-- 
2.53.0-Meta

[PATCH v8 4/4] RDMA/mlx5: get tph for p2p access when registering dma-buf mr

[Zhiping Zhang]

Query dma-buf PCI TPH metadata when registering a dma-buf MR for
peer-to-peer access to a PCIe endpoint and use it to program
requester-side TPH on the outbound mkey. If the exporter has no
metadata, fall back to the existing no-TPH path.

Use mlx5_st_alloc_index_by_tag() to translate exporter-provided
steering tags into local ST entries when table mode is active, and add
mlx5_st_get_index() for DMAH-backed flows that already carry an ST
index.

For TPH-backed FRMRs, keep the extra ST-table reference tied to MR
lifetime rather than pooled mkey lifetime. Acquire the ref before MR
creation and release it again when the MR is returned to the pool or
the backing mkey is destroyed, while leaving the generic FRMR pool core
unchanged.

Import the DMA_BUF namespace for the new dma_buf_get_pci_tph() call so
modular mlx5_ib builds link cleanly.

Signed-off-by: Zhiping Zhang <zhipingz@meta.com>
---
 drivers/infiniband/hw/mlx5/main.c             |   1 +
 drivers/infiniband/hw/mlx5/mr.c               | 105 +++++++++++++++++-
 .../net/ethernet/mellanox/mlx5/core/lib/st.c  |  49 ++++++--
 include/linux/mlx5/driver.h                   |  12 ++
 4 files changed, 157 insertions(+), 10 deletions(-)

diff --git a/drivers/infiniband/hw/mlx5/main.c b/drivers/infiniband/hw/mlx5/main.c
index 61078281953d..6d14909c8a2c 100644
--- a/drivers/infiniband/hw/mlx5/main.c
+++ b/drivers/infiniband/hw/mlx5/main.c
@@ -60,6 +60,7 @@
 MODULE_AUTHOR("Eli Cohen <eli@mellanox.com>");
 MODULE_DESCRIPTION("Mellanox 5th generation network adapters (ConnectX series) IB driver");
 MODULE_LICENSE("Dual BSD/GPL");
+MODULE_IMPORT_NS("DMA_BUF");
 
 struct mlx5_ib_event_work {
 	struct work_struct	work;
diff --git a/drivers/infiniband/hw/mlx5/mr.c b/drivers/infiniband/hw/mlx5/mr.c
index 8984b31b9429..d73005bd32a6 100644
--- a/drivers/infiniband/hw/mlx5/mr.c
+++ b/drivers/infiniband/hw/mlx5/mr.c
@@ -39,6 +39,7 @@
 #include <linux/delay.h>
 #include <linux/dma-buf.h>
 #include <linux/dma-resv.h>
+#include <linux/pci-tph.h>
 #include <rdma/frmr_pools.h>
 #include <rdma/ib_umem_odp.h>
 #include "dm.h"
@@ -167,6 +168,32 @@ static int get_unchangeable_access_flags(struct mlx5_ib_dev *dev,
 #define MLX5_FRMR_POOLS_KERNEL_KEY_PH_MASK GENMASK_ULL(23, 16)
 #define MLX5_FRMR_POOLS_KERNEL_KEY_ST_INDEX_MASK GENMASK_ULL(15, 0)
 
+static int mlx5_ib_get_frmr_st_handle_ref(struct mlx5_ib_dev *dev,
+					  u16 st_index)
+{
+	if (st_index == MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX)
+		return 0;
+
+	return mlx5_st_get_index(dev->mdev, st_index);
+}
+
+static void mlx5_ib_put_st_index_ref(struct mlx5_ib_dev *dev, u16 st_index)
+{
+	if (st_index == MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX)
+		return;
+
+	mlx5_st_dealloc_index(dev->mdev, st_index);
+}
+
+static void mlx5_ib_put_frmr_st_handle_ref(struct mlx5_ib_dev *dev,
+					   u64 kernel_vendor_key)
+{
+	u16 st_index = FIELD_GET(MLX5_FRMR_POOLS_KERNEL_KEY_ST_INDEX_MASK,
+				 kernel_vendor_key);
+
+	mlx5_ib_put_st_index_ref(dev, st_index);
+}
+
 static struct mlx5_ib_mr *
 _mlx5_frmr_pool_alloc(struct mlx5_ib_dev *dev, struct ib_umem *umem,
 		      int access_flags, int access_mode,
@@ -218,7 +245,9 @@ struct mlx5_ib_mr *mlx5_mr_cache_alloc(struct mlx5_ib_dev *dev,
 				      0 :
 				      MLX5_FRMR_POOLS_KEY_ACCESS_MODE_KSM_MASK,
 		.num_dma_blocks = ndescs,
-		.kernel_vendor_key = 0, /* no PH and no ST index */
+		.kernel_vendor_key =
+			FIELD_PREP(MLX5_FRMR_POOLS_KERNEL_KEY_ST_INDEX_MASK,
+				   MLX5_MKC_PCIE_TPH_NO_STEERING_TAG_INDEX),
 	};
 	struct mlx5_ib_mr *mr;
 	int ret;
@@ -335,6 +364,7 @@ static int mlx5r_build_frmr_key(struct ib_device *device,
 		get_unchangeable_access_flags(dev, in->access_flags);
 	out->vendor_key = in->vendor_key;
 	out->num_dma_blocks = in->num_dma_blocks;
+	out->kernel_vendor_key = in->kernel_vendor_key;
 
 	return 0;
 }
@@ -557,6 +587,9 @@ static struct mlx5_ib_mr *reg_create(struct ib_pd *pd, struct ib_umem *umem,
 	mr->ibmr.pd = pd;
 	mr->access_flags = access_flags;
 	mr->page_shift = order_base_2(page_size);
+	mr->ibmr.frmr.key.kernel_vendor_key =
+		FIELD_PREP(MLX5_FRMR_POOLS_KERNEL_KEY_ST_INDEX_MASK, st_index) |
+		FIELD_PREP(MLX5_FRMR_POOLS_KERNEL_KEY_PH_MASK, ph);
 
 	inlen = MLX5_ST_SZ_BYTES(create_mkey_in);
 	if (populate)
@@ -755,6 +788,12 @@ static struct ib_mr *create_real_mr(struct ib_pd *pd, struct ib_umem *umem,
 
 	xlt_with_umr = mlx5r_umr_can_load_pas(dev, umem->length);
 	if (xlt_with_umr) {
+		err = mlx5_ib_get_frmr_st_handle_ref(dev, st_index);
+		if (err) {
+			ib_umem_release(umem);
+			return ERR_PTR(err);
+		}
+
 		mr = alloc_cacheable_mr(pd, umem, iova, access_flags,
 					MLX5_MKC_ACCESS_MODE_MTT,
 					st_index, ph);
@@ -769,6 +808,8 @@ static struct ib_mr *create_real_mr(struct ib_pd *pd, struct ib_umem *umem,
 		mutex_unlock(&dev->slow_path_mutex);
 	}
 	if (IS_ERR(mr)) {
+		if (xlt_with_umr)
+			mlx5_ib_put_st_index_ref(dev, st_index);
 		ib_umem_release(umem);
 		return ERR_CAST(mr);
 	}
@@ -901,6 +942,52 @@ static struct dma_buf_attach_ops mlx5_ib_dmabuf_attach_ops = {
 	.invalidate_mappings = mlx5_ib_dmabuf_invalidate_cb,
 };
 
+static void get_pci_tph_mr_dmabuf(struct mlx5_ib_dev *dev, struct dma_buf *dmabuf,
+				  u16 *st_index, u8 *ph)
+{
+	u16 local_st_index;
+	u16 steering_tag;
+	u8 local_ph;
+	bool extended;
+	int ret;
+
+	switch (pcie_tph_enabled_req_type(dev->mdev->pdev)) {
+	case PCI_TPH_REQ_TPH_ONLY:
+		extended = false;
+		break;
+	case PCI_TPH_REQ_EXT_TPH:
+		extended = true;
+		break;
+	default:
+		return;
+	}
+
+	dma_resv_lock(dmabuf->resv, NULL);
+	ret = dma_buf_get_pci_tph(dmabuf, extended, &steering_tag, &local_ph);
+	dma_resv_unlock(dmabuf->resv);
+	if (ret) {
+		if (ret != -EOPNOTSUPP)
+			mlx5_ib_dbg(dev, "get_pci_tph failed (%d)\n", ret);
+		return;
+	}
+
+	ret = mlx5_st_alloc_index_by_tag(dev->mdev, steering_tag,
+					 &local_st_index);
+	if (ret) {
+		mlx5_ib_dbg(dev, "st_alloc_index_by_tag failed (%d)\n", ret);
+		return;
+	}
+
+	*st_index = local_st_index;
+	*ph = local_ph;
+}
+
+static void mlx5_ib_mr_put_frmr_st_handle_ref(struct mlx5_ib_mr *mr)
+{
+	mlx5_ib_put_frmr_st_handle_ref(mr_to_mdev(mr),
+				       mr->ibmr.frmr.key.kernel_vendor_key);
+}
+
 static struct ib_mr *
 reg_user_mr_dmabuf(struct ib_pd *pd, struct device *dma_device,
 		   u64 offset, u64 length, u64 virt_addr,
@@ -943,12 +1030,22 @@ reg_user_mr_dmabuf(struct ib_pd *pd, struct device *dma_device,
 		ph = dmah->ph;
 		if (dmah->valid_fields & BIT(IB_DMAH_CPU_ID_EXISTS))
 			st_index = mdmah->st_index;
+
+		err = mlx5_ib_get_frmr_st_handle_ref(dev, st_index);
+		if (err) {
+			ib_umem_release(&umem_dmabuf->umem);
+			return ERR_PTR(err);
+		}
+	} else {
+		get_pci_tph_mr_dmabuf(dev, umem_dmabuf->attach->dmabuf,
+				      &st_index, &ph);
 	}
 
 	mr = alloc_cacheable_mr(pd, &umem_dmabuf->umem, virt_addr,
 				access_flags, access_mode,
 				st_index, ph);
 	if (IS_ERR(mr)) {
+		mlx5_ib_put_st_index_ref(dev, st_index);
 		ib_umem_release(&umem_dmabuf->umem);
 		return ERR_CAST(mr);
 	}
@@ -1384,8 +1481,10 @@ static int mlx5r_handle_mkey_cleanup(struct mlx5_ib_mr *mr)
 	int ret;
 
 	if (mr->ibmr.frmr.pool && !mlx5_umr_revoke_mr_with_lock(mr) &&
-	    !ib_frmr_pool_push(mr->ibmr.device, &mr->ibmr))
+	    !ib_frmr_pool_push(mr->ibmr.device, &mr->ibmr)) {
+		mlx5_ib_mr_put_frmr_st_handle_ref(mr);
 		return 0;
+	}
 
 	if (is_odp)
 		mutex_lock(&to_ib_umem_odp(mr->umem)->umem_mutex);
@@ -1406,6 +1505,8 @@ static int mlx5r_handle_mkey_cleanup(struct mlx5_ib_mr *mr)
 		dma_resv_unlock(
 			to_ib_umem_dmabuf(mr->umem)->attach->dmabuf->resv);
 	}
+	if (!ret)
+		mlx5_ib_mr_put_frmr_st_handle_ref(mr);
 	return ret;
 }
 
diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
index 7cedc348790d..877b37b4e639 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/st.c
@@ -92,23 +92,18 @@ void mlx5_st_destroy(struct mlx5_core_dev *dev)
 	kfree(st);
 }
 
-int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
-			unsigned int cpu_uid, u16 *st_index)
+int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev, u16 tag,
+			       u16 *st_index)
 {
 	struct mlx5_st_idx_data *idx_data;
 	struct mlx5_st *st = dev->st;
 	unsigned long index;
 	u32 xa_id;
-	u16 tag;
-	int ret;
+	int ret = 0;
 
 	if (!st)
 		return -EOPNOTSUPP;
 
-	ret = pcie_tph_get_cpu_st(dev->pdev, mem_type, cpu_uid, &tag);
-	if (ret)
-		return ret;
-
 	if (st->direct_mode) {
 		*st_index = tag;
 		return 0;
@@ -152,8 +147,46 @@ int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
 	mutex_unlock(&st->lock);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(mlx5_st_alloc_index_by_tag);
+
+int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
+			unsigned int cpu_uid, u16 *st_index)
+{
+	u16 tag;
+	int ret;
+
+	ret = pcie_tph_get_cpu_st(dev->pdev, mem_type, cpu_uid, &tag);
+	if (ret)
+		return ret;
+
+	return mlx5_st_alloc_index_by_tag(dev, tag, st_index);
+}
 EXPORT_SYMBOL_GPL(mlx5_st_alloc_index);
 
+int mlx5_st_get_index(struct mlx5_core_dev *dev, u16 st_index)
+{
+	struct mlx5_st_idx_data *idx_data;
+	struct mlx5_st *st = dev->st;
+	int ret = 0;
+
+	if (!st)
+		return -EOPNOTSUPP;
+
+	if (st->direct_mode)
+		return 0;
+
+	mutex_lock(&st->lock);
+	idx_data = xa_load(&st->idx_xa, st_index);
+	if (WARN_ON_ONCE(!idx_data))
+		ret = -EINVAL;
+	else
+		refcount_inc(&idx_data->usecount);
+	mutex_unlock(&st->lock);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(mlx5_st_get_index);
+
 int mlx5_st_dealloc_index(struct mlx5_core_dev *dev, u16 st_index)
 {
 	struct mlx5_st_idx_data *idx_data;
diff --git a/include/linux/mlx5/driver.h b/include/linux/mlx5/driver.h
index 04b96c5abb57..0480b5c4f189 100644
--- a/include/linux/mlx5/driver.h
+++ b/include/linux/mlx5/driver.h
@@ -1166,10 +1166,22 @@ int mlx5_dm_sw_icm_dealloc(struct mlx5_core_dev *dev, enum mlx5_sw_icm_type type
 			   u64 length, u16 uid, phys_addr_t addr, u32 obj_id);
 
 #ifdef CONFIG_PCIE_TPH
+int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev, u16 tag,
+			       u16 *st_index);
+int mlx5_st_get_index(struct mlx5_core_dev *dev, u16 st_index);
 int mlx5_st_alloc_index(struct mlx5_core_dev *dev, enum tph_mem_type mem_type,
 			unsigned int cpu_uid, u16 *st_index);
 int mlx5_st_dealloc_index(struct mlx5_core_dev *dev, u16 st_index);
 #else
+static inline int mlx5_st_alloc_index_by_tag(struct mlx5_core_dev *dev,
+					     u16 tag, u16 *st_index)
+{
+	return -EOPNOTSUPP;
+}
+static inline int mlx5_st_get_index(struct mlx5_core_dev *dev, u16 st_index)
+{
+	return -EOPNOTSUPP;
+}
 static inline int mlx5_st_alloc_index(struct mlx5_core_dev *dev,
 				      enum tph_mem_type mem_type,
 				      unsigned int cpu_uid, u16 *st_index)
-- 
2.53.0-Meta

Re: [PATCH v8 4/4] RDMA/mlx5: get tph for p2p access when registering dma-buf mr

[Leon Romanovsky]

On Sun, Jun 14, 2026 at 11:59:01PM -0700, Zhiping Zhang wrote:
> Query dma-buf PCI TPH metadata when registering a dma-buf MR for
> peer-to-peer access to a PCIe endpoint and use it to program
> requester-side TPH on the outbound mkey. If the exporter has no
> metadata, fall back to the existing no-TPH path.
> 
> Use mlx5_st_alloc_index_by_tag() to translate exporter-provided
> steering tags into local ST entries when table mode is active, and add
> mlx5_st_get_index() for DMAH-backed flows that already carry an ST
> index.
> 
> For TPH-backed FRMRs, keep the extra ST-table reference tied to MR
> lifetime rather than pooled mkey lifetime. Acquire the ref before MR
> creation and release it again when the MR is returned to the pool or
> the backing mkey is destroyed, while leaving the generic FRMR pool core
> unchanged.
> 
> Import the DMA_BUF namespace for the new dma_buf_get_pci_tph() call so
> modular mlx5_ib builds link cleanly.

The commit message explains *what* the patch does, but it lacks context on
*why* the change is needed. The 'what' is mostly clear from reading the code;
the important part missing here is the rationale behind the change.

Thanks