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

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Message-ID: <20260118135440.1958279-12-den@valinux.co.jp>
Date: Sun, 18 Jan 2026 22:54:13 +0900
From: Koichiro Den <den@...inux.co.jp>
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Subject: [RFC PATCH v4 11/38] NTB: core: Add .get_dma_dev() to ntb_dev_ops

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)
+{
+	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