Re: [RFC PATCH v3 13/35] NTB: ntb_transport: Introduce get_dma_dev() helper

[Frank Li <Frank.li@nxp.com>]

On Thu, Dec 18, 2025 at 12:15:47AM +0900, Koichiro Den wrote:
> When ntb_transport is used on top of an endpoint function (EPF) NTB
> implementation, DMA mappings should be associated with the underlying
> PCIe controller device rather than the virtual NTB PCI function. This
> matters for IOMMU configuration and DMA mask validation.
>
> Add a small helper, get_dma_dev(), that returns the appropriate struct
> device for DMA mapping, i.e. &pdev->dev for a regular NTB host bridge
> and the EPC parent device for EPF-based NTB endpoints. Use it in the
> places where we set up DMA mappings or log DMA-related errors.
>
> Signed-off-by: Koichiro Den <den@valinux.co.jp>
> ---
>  drivers/ntb/ntb_transport.c | 35 ++++++++++++++++++++++++++++-------
>  1 file changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/ntb/ntb_transport.c b/drivers/ntb/ntb_transport.c
> index bac842177b55..78d0469edbcc 100644
> --- a/drivers/ntb/ntb_transport.c
> +++ b/drivers/ntb/ntb_transport.c
> @@ -63,6 +63,7 @@
>  #include <linux/mutex.h>
>  #include "linux/ntb.h"
>  #include "linux/ntb_transport.h"
> +#include <linux/pci-epc.h>
>
>  #define NTB_TRANSPORT_VERSION	4
>  #define NTB_TRANSPORT_VER	"4"
> @@ -259,6 +260,26 @@ struct ntb_payload_header {
>  	unsigned int flags;
>  };
>
> +/*
> + * Return the device that should be used for DMA mapping.
> + *
> + * On RC, this is simply &pdev->dev.
> + * On EPF-backed NTB endpoints, use the EPC parent device so that
> + * DMA capabilities and IOMMU configuration are taken from the
> + * controller rather than the virtual NTB PCI function.
> + */
> +static struct device *get_dma_dev(struct ntb_dev *ndev)
> +{
> +	struct device *dev = &ndev->pdev->dev;
> +	struct pci_epc *epc;
> +
> +	epc = (struct pci_epc *)ntb_get_private_data(ndev);
> +	if (epc)
> +		dev = epc->dev.parent;
> +
> +	return dev;
> +}
> +

I think add callback .get_dma_dev() directly. So vntb epf driver to provide
a implement. The file is common for all ntb transfer, should not include
ntb lower driver's specific implmentatin.

Frank

>  enum {
>  	VERSION = 0,
>  	QP_LINKS,
> @@ -771,13 +792,13 @@ static void ntb_transport_msi_desc_changed(void *data)
>  static void ntb_free_mw(struct ntb_transport_ctx *nt, int num_mw)
>  {
>  	struct ntb_transport_mw *mw = &nt->mw_vec[num_mw];
> -	struct pci_dev *pdev = nt->ndev->pdev;
> +	struct device *dev = get_dma_dev(nt->ndev);
>
>  	if (!mw->virt_addr)
>  		return;
>
>  	ntb_mw_clear_trans(nt->ndev, PIDX, num_mw);
> -	dma_free_coherent(&pdev->dev, mw->alloc_size,
> +	dma_free_coherent(dev, mw->alloc_size,
>  			  mw->alloc_addr, mw->dma_addr);
>  	mw->xlat_size = 0;
>  	mw->buff_size = 0;
> @@ -847,7 +868,7 @@ static int ntb_set_mw(struct ntb_transport_ctx *nt, int num_mw,
>  		      resource_size_t size)
>  {
>  	struct ntb_transport_mw *mw = &nt->mw_vec[num_mw];
> -	struct pci_dev *pdev = nt->ndev->pdev;
> +	struct device *dev = get_dma_dev(nt->ndev);
>  	size_t xlat_size, buff_size;
>  	resource_size_t xlat_align;
>  	resource_size_t xlat_align_size;
> @@ -877,12 +898,12 @@ static int ntb_set_mw(struct ntb_transport_ctx *nt, int num_mw,
>  	mw->buff_size = buff_size;
>  	mw->alloc_size = buff_size;
>
> -	rc = ntb_alloc_mw_buffer(mw, &pdev->dev, xlat_align);
> +	rc = ntb_alloc_mw_buffer(mw, dev, xlat_align);
>  	if (rc) {
>  		mw->alloc_size *= 2;
> -		rc = ntb_alloc_mw_buffer(mw, &pdev->dev, xlat_align);
> +		rc = ntb_alloc_mw_buffer(mw, dev, xlat_align);
>  		if (rc) {
> -			dev_err(&pdev->dev,
> +			dev_err(dev,
>  				"Unable to alloc aligned MW buff\n");
>  			mw->xlat_size = 0;
>  			mw->buff_size = 0;
> @@ -895,7 +916,7 @@ static int ntb_set_mw(struct ntb_transport_ctx *nt, int num_mw,
>  	rc = ntb_mw_set_trans(nt->ndev, PIDX, num_mw, mw->dma_addr,
>  			      mw->xlat_size, offset);
>  	if (rc) {
> -		dev_err(&pdev->dev, "Unable to set mw%d translation", num_mw);
> +		dev_err(dev, "Unable to set mw%d translation", num_mw);
>  		ntb_free_mw(nt, num_mw);
>  		return -EIO;
>  	}
> --
> 2.51.0
>