[PATCH v2 3/3] can: netlink: report the CAN controller mode supported flags

[Vincent Mailhol <mailhol.vincent@wanadoo.fr>]

This patch introduces a method for the user to check both the
supported and the static capabilities. The proposed method reuses the
existing struct can_ctrlmode and thus do not need a new IFLA_CAN_*
entry.

Currently, the CAN netlink interface provides no easy ways to check
the capabilities of a given controller. The only method from the
command line is to try each CAN_CTRLMODE_ individually to check
whether the netlink interface returns an -EOPNOTSUPP error or not
(alternatively, one may find it easier to directly check the source
code of the driver instead...)

It appears that, can_ctrlmode::mask is only used in one direction:
from the userland to the kernel. So we can just reuse this field in
the other direction (from the kernel to userland). But, because the
semantic is different, we use a union to give this field a proper
name: supported.

Below table explains how the two fields can_ctrlmode::supported and
can_ctrlmode::flags, when masked with any of the CAN_CTRLMODE_* bit
flags, allow us to identify both the supported and the static
capabilities:

 supported &	flags &		Controller capabilities
 CAN_CTRLMODE_*	CAN_CTRLMODE_*
 -----------------------------------------------------------------------
 false		false		Feature not supported (always disabled)
 false		true		Static feature (always enabled)
 true		false		Feature supported but disabled
 true		true		Feature supported and enabled

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Please refer to below link for the iproute2-next counterpart of this
patch:

https://lore.kernel.org/linux-can/20211003050147.569044-1-mailhol.vincent@wanadoo.fr/T/#t
---
 drivers/net/can/dev/netlink.c    | 5 ++++-
 include/uapi/linux/can/netlink.h | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
index 6c9906e8040c..86521836fd6d 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/drivers/net/can/dev/netlink.c
@@ -264,7 +264,10 @@ static size_t can_get_size(const struct net_device *dev)
 static int can_fill_info(struct sk_buff *skb, const struct net_device *dev)
 {
 	struct can_priv *priv = netdev_priv(dev);
-	struct can_ctrlmode cm = {.flags = priv->ctrlmode};
+	struct can_ctrlmode cm = {
+		.supported = priv->ctrlmode_supported,
+		.flags = priv->ctrlmode
+	};
 	struct can_berr_counter bec = { };
 	enum can_state state = priv->state;
 
diff --git a/include/uapi/linux/can/netlink.h b/include/uapi/linux/can/netlink.h
index f730d443b918..2847ed0dcac3 100644
--- a/include/uapi/linux/can/netlink.h
+++ b/include/uapi/linux/can/netlink.h
@@ -88,7 +88,10 @@ struct can_berr_counter {
  * CAN controller mode
  */
 struct can_ctrlmode {
-	__u32 mask;
+	union {
+		__u32 mask;		/* Userland to kernel */
+		__u32 supported;	/* Kernel to userland */
+	};
 	__u32 flags;
 };
 
-- 
2.32.0