[PATCH bpf-next V2] xdp: bpf_xdp_metadata use NODEV for no device support

[Jesper Dangaard Brouer <brouer@xxxxxxxxxx>]

With our XDP-hints kfunc approach, where individual drivers overload the
default implementation, it can be hard for API users to determine
whether or not the current device driver have this kfunc available.

Change the default implementations to use an errno (ENODEV), that
drivers shouldn't return, to make it possible for BPF runtime to
determine if bpf kfunc for xdp metadata isn't implemented by driver.

This is intended to ease supporting and troubleshooting setups. E.g.
when users on mailing list report -19 (ENODEV) as an error, then we can
immediately tell them their device driver is too old.

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
 Documentation/networking/xdp-rx-metadata.rst |    3 ++-
 net/core/xdp.c                               |    8 ++++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/Documentation/networking/xdp-rx-metadata.rst b/Documentation/networking/xdp-rx-metadata.rst
index aac63fc2d08b..89f6a7d1be38 100644
--- a/Documentation/networking/xdp-rx-metadata.rst
+++ b/Documentation/networking/xdp-rx-metadata.rst
@@ -26,7 +26,8 @@ consumers, an XDP program can store it into the metadata area carried
 ahead of the packet.
 
 Not all kfuncs have to be implemented by the device driver; when not
-implemented, the default ones that return ``-EOPNOTSUPP`` will be used.
+implemented, the default ones that return ``-ENODEV`` will be used to
+indicate the device driver have not implemented this kfunc.
 
 Within an XDP frame, the metadata layout (accessed via ``xdp_buff``) is
 as follows::
diff --git a/net/core/xdp.c b/net/core/xdp.c
index 26483935b7a4..7bb5984ae4f7 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -722,10 +722,12 @@ __diag_ignore_all("-Wmissing-prototypes",
  * @timestamp: Return value pointer.
  *
  * Returns 0 on success or ``-errno`` on error.
+ *
+ *  -ENODEV (19): means device driver doesn't implement kfunc
  */
 __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
 {
-	return -EOPNOTSUPP;
+	return -ENODEV;
 }
 
 /**
@@ -734,10 +736,12 @@ __bpf_kfunc int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *tim
  * @hash: Return value pointer.
  *
  * Returns 0 on success or ``-errno`` on error.
+ *
+ *  -ENODEV (19): means device driver doesn't implement kfunc
  */
 __bpf_kfunc int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
 {
-	return -EOPNOTSUPP;
+	return -ENODEV;
 }
 
 __diag_pop();