linux-kernel - Re: [RFC PATCH v4 11/38] NTB: core: Add .get_dma_dev() to ntb_dev

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Message-ID: <mfvy2vvs2272mq5qsstpluiir4ww4sdp22mj34viizr4dgwjt5@qhy2cr4rrjm3>
Date: Wed, 21 Jan 2026 10:44:07 +0900
From: Koichiro Den <den@...inux.co.jp>
To: Frank Li <Frank.li@....com>
Cc: dave.jiang@...el.com, cassel@...nel.org, mani@...nel.org, 
	kwilczynski@...nel.org, kishon@...nel.org, bhelgaas@...gle.com, geert+renesas@...der.be, 
	robh@...nel.org, vkoul@...nel.org, jdmason@...zu.us, allenbh@...il.com, 
	jingoohan1@...il.com, lpieralisi@...nel.org, linux-pci@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org, 
	devicetree@...r.kernel.org, dmaengine@...r.kernel.org, iommu@...ts.linux.dev, 
	ntb@...ts.linux.dev, netdev@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	arnd@...db.de, gregkh@...uxfoundation.org, joro@...tes.org, will@...nel.org, 
	robin.murphy@....com, magnus.damm@...il.com, krzk+dt@...nel.org, conor+dt@...nel.org, 
	corbet@....net, skhan@...uxfoundation.org, andriy.shevchenko@...ux.intel.com, 
	jbrunet@...libre.com, utkarsh02t@...il.com
Subject: Re: [RFC PATCH v4 11/38] NTB: core: Add .get_dma_dev() to ntb_dev_ops

On Mon, Jan 19, 2026 at 03:09:18PM -0500, Frank Li wrote:
> On Sun, Jan 18, 2026 at 10:54:13PM +0900, Koichiro Den wrote:
> > Not all NTB implementations are able to naturally do DMA mapping through
> > the NTB PCI device itself (e.g. due to IOMMU topology or non-PCI backing
> > devices).
> >
> > Add an optional .get_dma_dev() callback and helper so clients can use
> > the appropriate struct device for DMA API allocations and mappings.
> >
> > Signed-off-by: Koichiro Den <den@...inux.co.jp>
> > ---
> >  include/linux/ntb.h | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/include/linux/ntb.h b/include/linux/ntb.h
> > index aa888219732a..7ac8cb13e90d 100644
> > --- a/include/linux/ntb.h
> > +++ b/include/linux/ntb.h
> > @@ -262,6 +262,7 @@ struct ntb_mw_subrange {
> >   * @msg_clear_mask:	See ntb_msg_clear_mask().
> >   * @msg_read:		See ntb_msg_read().
> >   * @peer_msg_write:	See ntb_peer_msg_write().
> > + * @get_dma_dev:	See ntb_get_dma_dev().
> >   * @get_private_data:	See ntb_get_private_data().
> >   */
> >  struct ntb_dev_ops {
> > @@ -339,6 +340,7 @@ struct ntb_dev_ops {
> >  	int (*msg_clear_mask)(struct ntb_dev *ntb, u64 mask_bits);
> >  	u32 (*msg_read)(struct ntb_dev *ntb, int *pidx, int midx);
> >  	int (*peer_msg_write)(struct ntb_dev *ntb, int pidx, int midx, u32 msg);
> > +	struct device *(*get_dma_dev)(struct ntb_dev *ntb);
> >  	void *(*get_private_data)(struct ntb_dev *ntb);
> >  };
> >
> > @@ -405,6 +407,7 @@ static inline int ntb_dev_ops_is_valid(const struct ntb_dev_ops *ops)
> >  		!ops->peer_msg_write == !ops->msg_count		&&
> >
> >  		/* Miscellaneous optional callbacks */
> > +		/* ops->get_dma_dev				&& */
> >  		/* ops->get_private_data			&& */
> >  		1;
> >  }
> > @@ -1614,6 +1617,21 @@ static inline int ntb_peer_msg_write(struct ntb_dev *ntb, int pidx, int midx,
> >  	return ntb->ops->peer_msg_write(ntb, pidx, midx, msg);
> >  }
> >
> > +/**
> > + * ntb_get_dma_dev() - get the device suitable for DMA mapping
> > + * @ntb:	NTB device context.
> > + *
> > + * Retrieve a struct device which is suitable for DMA mapping.
> > + *
> > + * Return: Pointer to struct device.
> > + */
> > +static inline struct device __maybe_unused *ntb_get_dma_dev(struct ntb_dev *ntb)
> 
> I remember if there are inline,  needn't __maybe_unused.

My bad, I'll drop it.

Thanks,
Koichiro

> 
> Reviewed-by: Frank Li <Frank.Li@....com>
> > +{
> > +	if (!ntb->ops->get_dma_dev)
> > +		return ntb->dev.parent;
> > +	return ntb->ops->get_dma_dev(ntb);
> > +}
> > +
> >  /**
> >   * ntb_get_private_data() - get private data specific to the hardware driver
> >   * @ntb:	NTB device context.
> > --
> > 2.51.0
> >