can: netlink: report the CAN controller mode supported flags

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...)

This patch introduces a method for the user to check both the
supported and the static capabilities. The proposed method introduces
a new IFLA nest: IFLA_CAN_CTRLMODE_EXT which extends the current
IFLA_CAN_CTRLMODE. This is done to guaranty a full forward and
backward compatibility between the kernel and the user land
applications.

The IFLA_CAN_CTRLMODE_EXT nest contains one single entry:
IFLA_CAN_CTRLMODE_SUPPORTED. Because this entry is only used in one
direction: kernel to userland, no new struct nla_policy are
introduced.

Below table explains how IFLA_CAN_CTRLMODE_SUPPORTED (hereafter:
"supported") and can_ctrlmode::flags (hereafter: "flags") allow us to
identify both the supported and the static capabilities, when masked
with any of the CAN_CTRLMODE_* bit flags:

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

Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Vincent Mailhol <[e-mail]>
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/dev/netlink.c

···
 	[IFLA_CAN_DATA_BITTIMING_CONST]	= { .len = sizeof(struct can_bittiming_const) },
 	[IFLA_CAN_TERMINATION] = { .type = NLA_U16 },
 	[IFLA_CAN_TDC] = { .type = NLA_NESTED },
+	[IFLA_CAN_CTRLMODE_EXT] = { .type = NLA_NESTED },
 };
 
 static const struct nla_policy can_tdc_policy[IFLA_CAN_TDC_MAX + 1] = {
···
 	return size;
 }
 
+static size_t can_ctrlmode_ext_get_size(void)
+{
+	return nla_total_size(0) +		/* nest IFLA_CAN_CTRLMODE_EXT */
+		nla_total_size(sizeof(u32));	/* IFLA_CAN_CTRLMODE_SUPPORTED */
+}
+
 static size_t can_get_size(const struct net_device *dev)
 {
 	struct can_priv *priv = netdev_priv(dev);
···
 				       priv->data_bitrate_const_cnt);
 	size += sizeof(priv->bitrate_max);			/* IFLA_CAN_BITRATE_MAX */
 	size += can_tdc_get_size(dev);				/* IFLA_CAN_TDC */
+	size += can_ctrlmode_ext_get_size();			/* IFLA_CAN_CTRLMODE_EXT */
 
 	return size;
 }
···
 err_cancel:
 	nla_nest_cancel(skb, nest);
 	return -EMSGSIZE;
+}
+
+static int can_ctrlmode_ext_fill_info(struct sk_buff *skb,
+				      const struct can_priv *priv)
+{
+	struct nlattr *nest;
+
+	nest = nla_nest_start(skb, IFLA_CAN_CTRLMODE_EXT);
+	if (!nest)
+		return -EMSGSIZE;
+
+	if (nla_put_u32(skb, IFLA_CAN_CTRLMODE_SUPPORTED,
+			priv->ctrlmode_supported)) {
+		nla_nest_cancel(skb, nest);
+		return -EMSGSIZE;
+	}
+
+	nla_nest_end(skb, nest);
+	return 0;
 }
 
 static int can_fill_info(struct sk_buff *skb, const struct net_device *dev)
···
 		     sizeof(priv->bitrate_max),
 		     &priv->bitrate_max)) ||
 
-	    (can_tdc_fill_info(skb, dev))
+	    can_tdc_fill_info(skb, dev) ||
+
+	    can_ctrlmode_ext_fill_info(skb, priv)
 	    )
 
 		return -EMSGSIZE;

include/uapi/linux/can/netlink.h

···
 	IFLA_CAN_DATA_BITRATE_CONST,
 	IFLA_CAN_BITRATE_MAX,
 	IFLA_CAN_TDC,
+	IFLA_CAN_CTRLMODE_EXT,
 
 	/* add new constants above here */
 	__IFLA_CAN_MAX,
···
 	/* add new constants above here */
 	__IFLA_CAN_TDC,
 	IFLA_CAN_TDC_MAX = __IFLA_CAN_TDC - 1
+};
+
+/*
+ * IFLA_CAN_CTRLMODE_EXT nest: controller mode extended parameters
+ */
+enum {
+	IFLA_CAN_CTRLMODE_UNSPEC,
+	IFLA_CAN_CTRLMODE_SUPPORTED,	/* u32 */
+
+	/* add new constants above here */
+	__IFLA_CAN_CTRLMODE,
+	IFLA_CAN_CTRLMODE_MAX = __IFLA_CAN_CTRLMODE - 1
 };
 
 /* u16 termination range: 1..65535 Ohms */