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

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Message-ID: <aW6PbmKpYVxHrnJE@lizhi-Precision-Tower-5810>
Date: Mon, 19 Jan 2026 15:09:18 -0500
From: Frank Li <Frank.li@....com>
To: Koichiro Den <den@...inux.co.jp>
Cc: dave.jiang@...el.com, cassel@...nel.org, mani@...nel.org,
	kwilczynski@...nel.org, kishon@...nel.org, bhelgaas@...gle.com,
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	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,
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Subject: Re: [RFC PATCH v4 11/38] NTB: core: Add .get_dma_dev() to ntb_dev_ops

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.

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
>