[PATCH v3 0/4] report the controller capabilities through the netlink interface

[PATCH v3 0/4] report the controller capabilities through the netlink interface

[Vincent Mailhol]

The main purpose of this series is to report the CAN controller
capabilities. The proposed method reuses the existing struct
can_ctrlmode and thus do not need a new IFLA_CAN_* entry.

While doing so, I also realized that can_priv::ctrlmode_static could
actually be derived from the other ctrlmode fields. So I added three
extra patches to the series: one to replace that field with a
function, one to add a safeguard on can_set_static_ctrlmode() and one
to repack struct can_priv and fill the hole created after removing
can_priv::ctrlmode_priv.

Please note that the first three patches are not required by the
fourth one. I am just grouping everything in the same series because
the patches all revolve around the controller modes.


** Changelog **

v2 -> v3:

  - Make can_set_static_ctrlmode() return an error and adjust the
    drivers which use this helper function accordingly.

v1 -> v2:

  - Add a first patch to replace can_priv::ctrlmode_static by the
    inline function can_get_static_ctrlmode()

  - Add a second patch to reorder the fields of struct can_priv for
    better packing (save eight bytes on x86_64 \o/)

  - Rewrite the comments of the third patch "can: netlink: report the
    CAN controller mode supported flags" (no changes on the code
    itself).

Vincent Mailhol (4):
  can: dev: replace can_priv::ctrlmode_static by
    can_get_static_ctrlmode()
  can: dev: add sanity check in can_set_static_ctrlmode()
  can: dev: reorder struct can_priv members for better packing
  can: netlink: report the CAN controller mode supported flags

 drivers/net/can/dev/dev.c         |  5 +++--
 drivers/net/can/dev/netlink.c     |  7 +++++--
 drivers/net/can/m_can/m_can.c     | 10 +++++++---
 drivers/net/can/rcar/rcar_canfd.c |  4 +++-
 include/linux/can/dev.h           | 24 +++++++++++++++++-------
 include/uapi/linux/can/netlink.h  |  5 ++++-
 6 files changed, 39 insertions(+), 16 deletions(-)

-- 
2.32.0

[PATCH v3 1/4] can: dev: replace can_priv::ctrlmode_static by can_get_static_ctrlmode()

[Vincent Mailhol]

The statically enabled features of a CAN controller can be retrieved
using below formula:

| u32 ctrlmode_static = priv->ctrlmode & ~priv->ctrlmode_supported;

As such, there is no need to store this information. This patch
removes the field ctrlmode_static of struct can_priv and provides, in
replacement, the inline function can_get_static_ctrlmode() which
returns the same value.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 drivers/net/can/dev/dev.c     | 5 +++--
 drivers/net/can/dev/netlink.c | 2 +-
 include/linux/can/dev.h       | 7 +++++--
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/net/can/dev/dev.c b/drivers/net/can/dev/dev.c
index e3d840b81357..59c79f92fccc 100644
--- a/drivers/net/can/dev/dev.c
+++ b/drivers/net/can/dev/dev.c
@@ -300,6 +300,7 @@ EXPORT_SYMBOL_GPL(free_candev);
 int can_change_mtu(struct net_device *dev, int new_mtu)
 {
 	struct can_priv *priv = netdev_priv(dev);
+	u32 ctrlmode_static = can_get_static_ctrlmode(priv);
 
 	/* Do not allow changing the MTU while running */
 	if (dev->flags & IFF_UP)
@@ -309,7 +310,7 @@ int can_change_mtu(struct net_device *dev, int new_mtu)
 	switch (new_mtu) {
 	case CAN_MTU:
 		/* 'CANFD-only' controllers can not switch to CAN_MTU */
-		if (priv->ctrlmode_static & CAN_CTRLMODE_FD)
+		if (ctrlmode_static & CAN_CTRLMODE_FD)
 			return -EINVAL;
 
 		priv->ctrlmode &= ~CAN_CTRLMODE_FD;
@@ -318,7 +319,7 @@ int can_change_mtu(struct net_device *dev, int new_mtu)
 	case CANFD_MTU:
 		/* check for potential CANFD ability */
 		if (!(priv->ctrlmode_supported & CAN_CTRLMODE_FD) &&
-		    !(priv->ctrlmode_static & CAN_CTRLMODE_FD))
+		    !(ctrlmode_static & CAN_CTRLMODE_FD))
 			return -EINVAL;
 
 		priv->ctrlmode |= CAN_CTRLMODE_FD;
diff --git a/drivers/net/can/dev/netlink.c b/drivers/net/can/dev/netlink.c
index 95cca4e5251f..26c336808be5 100644
--- a/drivers/net/can/dev/netlink.c
+++ b/drivers/net/can/dev/netlink.c
@@ -211,7 +211,7 @@ static int can_changelink(struct net_device *dev, struct nlattr *tb[],
 		if (dev->flags & IFF_UP)
 			return -EBUSY;
 		cm = nla_data(data[IFLA_CAN_CTRLMODE]);
-		ctrlstatic = priv->ctrlmode_static;
+		ctrlstatic = can_get_static_ctrlmode(priv);
 		maskedflags = cm->flags & cm->mask;
 
 		/* check whether provided bits are allowed to be passed */
diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 45f19d9db5ca..92e2d69462f0 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -69,7 +69,6 @@ struct can_priv {
 	/* CAN controller features - see include/uapi/linux/can/netlink.h */
 	u32 ctrlmode;		/* current options setting */
 	u32 ctrlmode_supported;	/* options that can be modified by netlink */
-	u32 ctrlmode_static;	/* static enabled options for driver/hardware */
 
 	int restart_ms;
 	struct delayed_work restart_work;
@@ -139,13 +138,17 @@ static inline void can_set_static_ctrlmode(struct net_device *dev,
 
 	/* alloc_candev() succeeded => netdev_priv() is valid at this point */
 	priv->ctrlmode = static_mode;
-	priv->ctrlmode_static = static_mode;
 
 	/* override MTU which was set by default in can_setup()? */
 	if (static_mode & CAN_CTRLMODE_FD)
 		dev->mtu = CANFD_MTU;
 }
 
+static inline u32 can_get_static_ctrlmode(struct can_priv *priv)
+{
+	return priv->ctrlmode & ~priv->ctrlmode_supported;
+}
+
 void can_setup(struct net_device *dev);
 
 struct net_device *alloc_candev_mqs(int sizeof_priv, unsigned int echo_skb_max,
-- 
2.32.0

[PATCH v3 2/4] can: dev: add sanity check in can_set_static_ctrlmode()

[Vincent Mailhol]

Previous patch removed can_priv::ctrlmode_static to replace it with
can_get_static_ctrlmode().

A condition sine qua non for this to work is that the controller
static modes should never be set in can_priv::ctrlmode_supported
(c.f. the comment on can_priv::ctrlmode_supported which states that it
is for "options that can be *modified* by netlink"). Also, this
condition is already correctly fulfilled by all existing drivers
which rely on the ctrlmode_static feature.

Nonetheless, we added an extra safeguard in can_set_static_ctrlmode()
to return an error value and to warn the developer who would be
adventurous enough to set to static a given feature that is already
set to supported.

The drivers which rely on the static controller mode are then updated
to check the return value of can_set_static_ctrlmode().

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
Some few comments on how the rcar_canfd and m_can drivers free their
allocated resources when an error occurs during probing.

The function rcar_canfd_channel_probe() is quite inconsistent with the
way it handles errors. After the call to alloc_candev, there are
several "goto fail" statements that would directly exit without
calling free_candev()!

Nonetheless, later on the driver will check the return value of
rcar_canfd_channel_probe() and call rcar_canfd_channel_remove() which
will correctly call free_candev(). Even if this is inconsistent, there
is no sign of a memory leak. So I just applied the change the
can_set_static_ctrlmode() without bothering more (N.B. I do not own
that device so I am not willing to take the risk of making bigger
changes because I can not test).

On the other hand, m_can_dev_setup() is fine: the return value is
checked by the caller and necessary actions are taken.

As such, for both driver, we did a minimal change.
---
 drivers/net/can/m_can/m_can.c     | 10 +++++++---
 drivers/net/can/rcar/rcar_canfd.c |  4 +++-
 include/linux/can/dev.h           | 11 +++++++++--
 3 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/drivers/net/can/m_can/m_can.c b/drivers/net/can/m_can/m_can.c
index 2470c47b2e31..ab4371aa4ff1 100644
--- a/drivers/net/can/m_can/m_can.c
+++ b/drivers/net/can/m_can/m_can.c
@@ -1463,7 +1463,7 @@ static bool m_can_niso_supported(struct m_can_classdev *cdev)
 static int m_can_dev_setup(struct m_can_classdev *cdev)
 {
 	struct net_device *dev = cdev->net;
-	int m_can_version;
+	int m_can_version, err;
 
 	m_can_version = m_can_check_core_release(cdev);
 	/* return if unsupported version */
@@ -1493,13 +1493,17 @@ static int m_can_dev_setup(struct m_can_classdev *cdev)
 	switch (cdev->version) {
 	case 30:
 		/* CAN_CTRLMODE_FD_NON_ISO is fixed with M_CAN IP v3.0.x */
-		can_set_static_ctrlmode(dev, CAN_CTRLMODE_FD_NON_ISO);
+		err = can_set_static_ctrlmode(dev, CAN_CTRLMODE_FD_NON_ISO);
+		if (err)
+			return err;
 		cdev->can.bittiming_const = &m_can_bittiming_const_30X;
 		cdev->can.data_bittiming_const = &m_can_data_bittiming_const_30X;
 		break;
 	case 31:
 		/* CAN_CTRLMODE_FD_NON_ISO is fixed with M_CAN IP v3.1.x */
-		can_set_static_ctrlmode(dev, CAN_CTRLMODE_FD_NON_ISO);
+		err = can_set_static_ctrlmode(dev, CAN_CTRLMODE_FD_NON_ISO);
+		if (err)
+			return err;
 		cdev->can.bittiming_const = &m_can_bittiming_const_31X;
 		cdev->can.data_bittiming_const = &m_can_data_bittiming_const_31X;
 		break;
diff --git a/drivers/net/can/rcar/rcar_canfd.c b/drivers/net/can/rcar/rcar_canfd.c
index ff9d0f5ae0dd..e225c1c03812 100644
--- a/drivers/net/can/rcar/rcar_canfd.c
+++ b/drivers/net/can/rcar/rcar_canfd.c
@@ -1706,7 +1706,9 @@ static int rcar_canfd_channel_probe(struct rcar_canfd_global *gpriv, u32 ch,
 			&rcar_canfd_data_bittiming_const;
 
 		/* Controller starts in CAN FD only mode */
-		can_set_static_ctrlmode(ndev, CAN_CTRLMODE_FD);
+		err = can_set_static_ctrlmode(ndev, CAN_CTRLMODE_FD);
+		if (err)
+			goto fail;
 		priv->can.ctrlmode_supported = CAN_CTRLMODE_BERR_REPORTING;
 	} else {
 		/* Controller starts in Classical CAN only mode */
diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index 92e2d69462f0..fff3f70df697 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -131,17 +131,24 @@ static inline s32 can_get_relative_tdco(const struct can_priv *priv)
 }
 
 /* helper to define static CAN controller features at device creation time */
-static inline void can_set_static_ctrlmode(struct net_device *dev,
-					   u32 static_mode)
+static inline int __must_check can_set_static_ctrlmode(struct net_device *dev,
+						       u32 static_mode)
 {
 	struct can_priv *priv = netdev_priv(dev);
 
 	/* alloc_candev() succeeded => netdev_priv() is valid at this point */
+	if (priv->ctrlmode_supported & static_mode) {
+		netdev_warn(dev,
+			    "Controller features can not be supported and static at the same time\n");
+		return -EINVAL;
+	}
 	priv->ctrlmode = static_mode;
 
 	/* override MTU which was set by default in can_setup()? */
 	if (static_mode & CAN_CTRLMODE_FD)
 		dev->mtu = CANFD_MTU;
+
+	return 0;
 }
 
 static inline u32 can_get_static_ctrlmode(struct can_priv *priv)
-- 
2.32.0

[PATCH v3 3/4] can: dev: reorder struct can_priv members for better packing

[Vincent Mailhol]

Save eight bytes of holes on x86-64 architectures by reordering the
members of struct can_priv.

Before:

| $ pahole -C can_priv drivers/net/can/dev/dev.o
| struct can_priv {
| 	struct net_device *        dev;                  /*     0     8 */
| 	struct can_device_stats    can_stats;            /*     8    24 */
| 	const struct can_bittiming_const  * bittiming_const; /*    32     8 */
| 	const struct can_bittiming_const  * data_bittiming_const; /*    40     8 */
| 	struct can_bittiming       bittiming;            /*    48    32 */
| 	/* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
| 	struct can_bittiming       data_bittiming;       /*    80    32 */
| 	const struct can_tdc_const  * tdc_const;         /*   112     8 */
| 	struct can_tdc             tdc;                  /*   120    12 */
| 	/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
| 	unsigned int               bitrate_const_cnt;    /*   132     4 */
| 	const u32  *               bitrate_const;        /*   136     8 */
| 	const u32  *               data_bitrate_const;   /*   144     8 */
| 	unsigned int               data_bitrate_const_cnt; /*   152     4 */
| 	u32                        bitrate_max;          /*   156     4 */
| 	struct can_clock           clock;                /*   160     4 */
| 	unsigned int               termination_const_cnt; /*   164     4 */
| 	const u16  *               termination_const;    /*   168     8 */
| 	u16                        termination;          /*   176     2 */
|
| 	/* XXX 6 bytes hole, try to pack */
|
| 	struct gpio_desc *         termination_gpio;     /*   184     8 */
| 	/* --- cacheline 3 boundary (192 bytes) --- */
| 	u16                        termination_gpio_ohms[2]; /*   192     4 */
| 	enum can_state             state;                /*   196     4 */
| 	u32                        ctrlmode;             /*   200     4 */
| 	u32                        ctrlmode_supported;   /*   204     4 */
| 	int                        restart_ms;           /*   208     4 */
|
| 	/* XXX 4 bytes hole, try to pack */
|
| 	struct delayed_work        restart_work;         /*   216    88 */
|
| 	/* XXX last struct has 4 bytes of padding */
|
| 	/* --- cacheline 4 boundary (256 bytes) was 48 bytes ago --- */
| 	int                        (*do_set_bittiming)(struct net_device *); /*   304     8 */
| 	int                        (*do_set_data_bittiming)(struct net_device *); /*   312     8 */
| 	/* --- cacheline 5 boundary (320 bytes) --- */
| 	int                        (*do_set_mode)(struct net_device *, enum can_mode); /*   320     8 */
| 	int                        (*do_set_termination)(struct net_device *, u16); /*   328     8 */
| 	int                        (*do_get_state)(const struct net_device  *, enum can_state *); /*   336     8 */
| 	int                        (*do_get_berr_counter)(const struct net_device  *, struct can_berr_counter *); /*   344     8 */
| 	unsigned int               echo_skb_max;         /*   352     4 */
|
| 	/* XXX 4 bytes hole, try to pack */
|
| 	struct sk_buff * *         echo_skb;             /*   360     8 */
|
| 	/* size: 368, cachelines: 6, members: 32 */
| 	/* sum members: 354, holes: 3, sum holes: 14 */
| 	/* paddings: 1, sum paddings: 4 */
| 	/* last cacheline: 48 bytes */
| };

After:

| $ pahole -C can_priv drivers/net/can/dev/dev.o
| struct can_priv {
| 	struct net_device *        dev;                  /*     0     8 */
| 	struct can_device_stats    can_stats;            /*     8    24 */
| 	const struct can_bittiming_const  * bittiming_const; /*    32     8 */
| 	const struct can_bittiming_const  * data_bittiming_const; /*    40     8 */
| 	struct can_bittiming       bittiming;            /*    48    32 */
| 	/* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
| 	struct can_bittiming       data_bittiming;       /*    80    32 */
| 	const struct can_tdc_const  * tdc_const;         /*   112     8 */
| 	struct can_tdc             tdc;                  /*   120    12 */
| 	/* --- cacheline 2 boundary (128 bytes) was 4 bytes ago --- */
| 	unsigned int               bitrate_const_cnt;    /*   132     4 */
| 	const u32  *               bitrate_const;        /*   136     8 */
| 	const u32  *               data_bitrate_const;   /*   144     8 */
| 	unsigned int               data_bitrate_const_cnt; /*   152     4 */
| 	u32                        bitrate_max;          /*   156     4 */
| 	struct can_clock           clock;                /*   160     4 */
| 	unsigned int               termination_const_cnt; /*   164     4 */
| 	const u16  *               termination_const;    /*   168     8 */
| 	u16                        termination;          /*   176     2 */
|
| 	/* XXX 6 bytes hole, try to pack */
|
| 	struct gpio_desc *         termination_gpio;     /*   184     8 */
| 	/* --- cacheline 3 boundary (192 bytes) --- */
| 	u16                        termination_gpio_ohms[2]; /*   192     4 */
| 	unsigned int               echo_skb_max;         /*   196     4 */
| 	struct sk_buff * *         echo_skb;             /*   200     8 */
| 	enum can_state             state;                /*   208     4 */
| 	u32                        ctrlmode;             /*   212     4 */
| 	u32                        ctrlmode_supported;   /*   216     4 */
| 	int                        restart_ms;           /*   220     4 */
| 	struct delayed_work        restart_work;         /*   224    88 */
|
| 	/* XXX last struct has 4 bytes of padding */
|
| 	/* --- cacheline 4 boundary (256 bytes) was 56 bytes ago --- */
| 	int                        (*do_set_bittiming)(struct net_device *); /*   312     8 */
| 	/* --- cacheline 5 boundary (320 bytes) --- */
| 	int                        (*do_set_data_bittiming)(struct net_device *); /*   320     8 */
| 	int                        (*do_set_mode)(struct net_device *, enum can_mode); /*   328     8 */
| 	int                        (*do_set_termination)(struct net_device *, u16); /*   336     8 */
| 	int                        (*do_get_state)(const struct net_device  *, enum can_state *); /*   344     8 */
| 	int                        (*do_get_berr_counter)(const struct net_device  *, struct can_berr_counter *); /*   352     8 */
|
| 	/* size: 360, cachelines: 6, members: 32 */
| 	/* sum members: 354, holes: 1, sum holes: 6 */
| 	/* paddings: 1, sum paddings: 4 */
| 	/* last cacheline: 40 bytes */
| };

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 include/linux/can/dev.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/can/dev.h b/include/linux/can/dev.h
index fff3f70df697..c2ea47f30046 100644
--- a/include/linux/can/dev.h
+++ b/include/linux/can/dev.h
@@ -64,6 +64,9 @@ struct can_priv {
 	struct gpio_desc *termination_gpio;
 	u16 termination_gpio_ohms[CAN_TERMINATION_GPIO_MAX];
 
+	unsigned int echo_skb_max;
+	struct sk_buff **echo_skb;
+
 	enum can_state state;
 
 	/* CAN controller features - see include/uapi/linux/can/netlink.h */
@@ -83,9 +86,6 @@ struct can_priv {
 				   struct can_berr_counter *bec);
 	int (*do_get_auto_tdcv)(const struct net_device *dev, u32 *tdcv);
 
-	unsigned int echo_skb_max;
-	struct sk_buff **echo_skb;
-
 #ifdef CONFIG_CAN_LEDS
 	struct led_trigger *tx_led_trig;
 	char tx_led_trig_name[CAN_LED_NAME_SZ];
-- 
2.32.0

[PATCH v3 4/4] can: netlink: report the CAN controller mode supported flags

[Vincent Mailhol]

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 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 */
+	};
 	__u32 flags;
 };
 
-- 
2.32.0

Re: [PATCH v3 4/4] can: netlink: report the CAN controller mode supported flags

[Vincent MAILHOL]

On Tue. 26 Oct 2021 at 02:22, Vincent Mailhol
<mailhol.vincent@wanadoo.fr> wrote:
> 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 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 */
> +       };

While daydreaming during my lunch break, I suddenly remembered
this thread [1] and was concerned that introducing the union
might break the UAPI.

As a matter of fact, the C standard allows the compiler to add
padding at the end of an union. c.f. ISO/IEC 9899-1999, section
6.7.2.1 "Structure and union specifiers", clause 15: "There may
be unnamed padding at the end of a structure or union."

For example, if the kernel were to be compiled with the
-mstructure-size-boundary=64 ARM option in GCC [2], 32 bits of
padding would be introduced after the union, thus breaking the
alignment of the next field: can_ctrlmode::flags.

As far as my knowledge goes, I am not sure whether or not
-mstructure-size-boundary=64 (or similar options on other
architectures) is actually used. Nonetheless, I think it is safer
to declare the union as __attribute__((packed)) to prevent such
padding from occuring.

I will send a v4 later today to address this.

[1] https://lore.kernel.org/linux-can/212c8bc3-89f9-9c33-ed1b-b50ac04e7532@hartkopp.net/T/#u
[2] https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html

>         __u32 flags;
>  };

Yours sincerely,
Vincent Mailhol