Nouveau - Jul 2025 - [PATCH v2 4/5] RDMA/mlx5: Enable P2P DMA with fallback mechanism

From: Yonatan Maman <Ymaman at Nvidia.com>

Add support for P2P for MLX5 NIC devices with automatic fallback to
standard DMA when P2P mapping fails.

The change introduces P2P DMA requests by default using the
HMM_PFN_ALLOW_P2P flag. If P2P mapping fails with -EFAULT error, the
operation is retried without the P2P flag, ensuring a fallback to
standard DMA flow (using host memory).

Signed-off-by: Yonatan Maman <Ymaman at Nvidia.com>
Signed-off-by: Gal Shalom <GalShalom at Nvidia.com>
---
 drivers/infiniband/hw/mlx5/odp.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/infiniband/hw/mlx5/odp.c b/drivers/infiniband/hw/mlx5/odp.c
index f6abd64f07f7..6a0171117f48 100644
--- a/drivers/infiniband/hw/mlx5/odp.c
+++ b/drivers/infiniband/hw/mlx5/odp.c
@@ -715,6 +715,10 @@ static int pagefault_real_mr(struct mlx5_ib_mr *mr, struct
ib_umem_odp *odp,
 	if (odp->umem.writable && !downgrade)
 		access_mask |= HMM_PFN_WRITE;
 
+	/*
+	 * try fault with HMM_PFN_ALLOW_P2P flag
+	 */
+	access_mask |= HMM_PFN_ALLOW_P2P;
 	np = ib_umem_odp_map_dma_and_lock(odp, user_va, bcnt, access_mask, fault);
 	if (np < 0)
 		return np;
@@ -724,6 +728,18 @@ static int pagefault_real_mr(struct mlx5_ib_mr *mr, struct
ib_umem_odp *odp,
 	 * ib_umem_odp_map_dma_and_lock already checks this.
 	 */
 	ret = mlx5r_umr_update_xlt(mr, start_idx, np, page_shift, xlt_flags);
+	if (ret == -EFAULT) {
+		/*
+		 * Indicate P2P Mapping Error, retry with no HMM_PFN_ALLOW_P2P
+		 */
+		mutex_unlock(&odp->umem_mutex);
+		access_mask &= ~(HMM_PFN_ALLOW_P2P);
+		np = ib_umem_odp_map_dma_and_lock(odp, user_va, bcnt, access_mask, fault);
+		if (np < 0)
+			return np;
+		ret = mlx5r_umr_update_xlt(mr, start_idx, np, page_shift, xlt_flags);
+	}
+
 	mutex_unlock(&odp->umem_mutex);
 
 	if (ret < 0) {
-- 
2.34.1

On Fri, Jul 18, 2025 at 02:51:11PM +0300, Yonatan Maman
wrote:
> From: Yonatan Maman <Ymaman at Nvidia.com>
> 
> Add support for P2P for MLX5 NIC devices with automatic fallback to
> standard DMA when P2P mapping fails.
That's now how the P2P API works.  You need to check the P2P availability
higher up.