Re: [PATCH v10 2/4] dma-buf: add optional get_pci_tph() callback

["Christian König" <christian.koenig@amd.com>]

On 7/1/26 00:42, 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   | 22 ++++++++++++++++++++++
>  2 files changed, 47 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.

Please add something like this:

* The returned information is only valid till the next invalidate_mappings() callback from the exporter and should be re-queried when a new mapping is created after invalidation.

Apart from that it looks good to me, but I still think we need some kind of example that this works for other DMA-buf users as well.

Just demonstrating that this also works with some simple FPGA or similar PCIe endpoint should be sufficient.

Regards,
Christian.

> + */
> +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..53b2686ad8fc 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -113,6 +113,26 @@ struct dma_buf_ops {
>  	 */
>  	void (*unpin)(struct dma_buf_attachment *attach);
>  
> +	/**
> +	 * @get_pci_tph:
> +	 *
> +	 * Retrieve PCIe TLP Processing Hint (TPH) steering metadata for
> +	 * this buffer so an importer can program a matching ST/PH hint on
> +	 * outbound TLPs targeting the exporter for peer-to-peer DMA.
> +	 *
> +	 * @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. Returns 0 on success or -EOPNOTSUPP when
> +	 * the exporter has no metadata for the requested namespace.
> +	 */
> +	int (*get_pci_tph)(struct dma_buf *dmabuf, bool extended,
> +			   u16 *steering_tag, u8 *ph);
> +
>  	/**
>  	 * @map_dma_buf:
>  	 *
> @@ -563,6 +583,8 @@ 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);
>