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 <redacted>
---
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 (local)
---
 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 26c336808be5..32e1eb63ee7d 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/drivers/net/can/dev/netlink.c

@@ -475,7 +475,10 @@ static int can_tdc_fill_info(struct sk_buff *skb, 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 75b85c60efb2..b846922ac18f 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 */
+       };