infiniband_mad

Field	Value
Subsystem	RDMA userspace MAD access, drivers/infiniband/core/user_mad.c (class `umad_class`, name “infiniband_mad”)
Config gate	CONFIG_INFINIBAND_USER_MAD, “InfiniBand userspace MAD support”, depends on INFINIBAND (drivers/infiniband/Kconfig)
ABI documentation	Documentation/ABI/stable/sysfs-class-infiniband (umad/issm section, 5 What: entries)
Since	v2.6.12 (all entries, dated Apr 2005)
Live (5.10.134 host)	none observed (no RDMA hardware in the container)
Ledger ID	SYSC-infiniband_mad

What it represents

One device per port of each InfiniBand device for management-datagram access: umad<N> for MAD traffic and issm<N> for the subnet-manager port bit. A two-port HCA gets two umad and two issm devices; a switch gets one of each for port 0 (ABI description). The class-level abi_version counts user-visible ABI breaks.

Relationship to the device model

Class devices are named umad%d and issm%d (user_mad.c dev_set_name) and carry the devnode callback “infiniband/%s”, so the char device nodes land in /dev/infiniband/. Each umad/issm device is associated with one port of the HCA named by its ibdev attribute.

Attributes (source: ABI stable/sysfs-class-infiniband; class code user_mad.c)

Attribute	R/W	Meaning
`abi_version`	RO	Class attr; incremented when umad/issm userspace ABI compatibility breaks (user_mad.c returns IB_USER_MAD_ABI_VERSION)
`umad<N>/ibdev`	RO	InfiniBand device name this port belongs to
`umad<N>/port`	RO	Port number
`issm<N>/ibdev`	RO	InfiniBand device name
`issm<N>/port`	RO	Port number

Safety and writes

All five attributes are read-only. Opening the issm device sets the IsSM bit in the port capability mask (documented in the infiniband.md cap_mask row); MAD traffic itself flows through /dev/infiniband/umadN. None observed on the container host.

References

Documentation/ABI/stable/sysfs-class-infiniband
drivers/infiniband/core/user_mad.c (fetched from torvalds/linux master into source-cache)
sysfs overview, ledger, InfiniBand storage page, infiniband, infiniband_verbs