[RFC PATCH 1/4] sunrpc: add supports_p2pdma to rpc_xprt_ops

[Pranjal Shrivastava <praan@google.com>]

Add a new transport op, supports_p2pdma, to allow upper layers (such as
NFS) to query whether the underlying RPC transport supports PCI
Peer-to-Peer DMA (P2PDMA).

Since the capability is hardware-dependent. For the RDMA transport,
implement this by querying the underlying InfiniBand device via
ib_dma_pci_p2p_dma_supported() to ensures that P2PDMA is only
attempted when both the transport and the HCA drivers support it.

Signed-off-by: Pranjal Shrivastava <praan@google.com>
---
 include/linux/sunrpc/xprt.h     | 1 +
 net/sunrpc/xprtrdma/transport.c | 9 +++++++++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
index f46d1fb8f71a..e451acd2e047 100644
--- a/include/linux/sunrpc/xprt.h
+++ b/include/linux/sunrpc/xprt.h
@@ -187,6 +187,7 @@ struct rpc_xprt_ops {
 	void		(*bc_free_rqst)(struct rpc_rqst *rqst);
 	void		(*bc_destroy)(struct rpc_xprt *xprt,
 				      unsigned int max_reqs);
+	bool		(*supports_p2pdma)(struct rpc_xprt *xprt);
 };
 
 /*
diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
index 9a8ce5df83ca..1c1714189a29 100644
--- a/net/sunrpc/xprtrdma/transport.c
+++ b/net/sunrpc/xprtrdma/transport.c
@@ -717,6 +717,14 @@ xprt_rdma_disable_swap(struct rpc_xprt *xprt)
 {
 }
 
+static bool
+xprt_rdma_supports_p2pdma(struct rpc_xprt *xprt)
+{
+	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
+
+	return ib_dma_pci_p2p_dma_supported(r_xprt->rx_ep->re_id->device);
+}
+
 /*
  * Plumbing for rpc transport switch and kernel module
  */
@@ -742,6 +750,7 @@ static const struct rpc_xprt_ops xprt_rdma_procs = {
 	.enable_swap		= xprt_rdma_enable_swap,
 	.disable_swap		= xprt_rdma_disable_swap,
 	.inject_disconnect	= xprt_rdma_inject_disconnect,
+	.supports_p2pdma	= xprt_rdma_supports_p2pdma,
 #if defined(CONFIG_SUNRPC_BACKCHANNEL)
 	.bc_setup		= xprt_rdma_bc_setup,
 	.bc_maxpayload		= xprt_rdma_bc_maxpayload,
-- 
2.53.0.1185.g05d4b7b318-goog