On Tue. 26 Oct 2021 at 02:22, Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> 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@xxxxxxxxxx> > --- > Please refer to below link for the iproute2-next counterpart of this > patch: > > https://lore.kernel.org/linux-can/20211003050147.569044-1-mailhol.vincent@xxxxxxxxxx/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@xxxxxxxxxxxx/T/#u [2] https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html > __u32 flags; > }; Yours sincerely, Vincent Mailhol
