RFC nvme async-connect and discovery uevents

Hi all,

here's my next attempt to solve the auto-connect issue. The overall idea is to have an 'async_connect' option to the nvme cli, which would return immediately from the ioctl without waiting for any I/O to complete. With that we're safe to call 'nvme connect' or 'nvme discover' from udev rules, and don't run into the risk of accidentally stalling udev as the nvme cli call doesn't return.
As we then can't return the discovery log anymore (as the connect hasn't happened by the time we're returning from the 'connect' ioctl) I've also implemented discovery uevents for each entry in the discovery log page. And while I've been at it I've also implemented a 'discovery' sysfs attribute for the discovery controller, holding all found discovery records.

With these patches we can implement a simple autoconnect:

1) call 'nvme discover --async_connect --trtype ...'.
2) The discovery log is fetched from the kernel, and an uevent for each discovery record is generated:

UDEV [1920.740092] change /devices/virtual/nvme-fabrics/ctl/nvme0 (nvme)
ACTION=change
DEVNAME=/dev/nvme0 DEVPATH=/devices/virtual/nvme-fabrics/ctl/nvme0
MAJOR=244
MINOR=0
NVME_DISCOVERY_ADRFAM=fc NVME_DISCOVERY_HOST_TRADDR=nn-0x20000090fae06325:pn-0x10000090fae06325 NVME_DISCOVERY_SUBNQN=blktests-subsys NVME_DISCOVERY_TRADDR=nn-0x200200a09890f5bf:pn-0x200300a09890f5bf NVME_DISCOVERY_TRSVCID=none
NVME_DISCOVERY_TRTYPE=fc
NVME_EVENT=discovery
SEQNUM=6940
SUBSYSTEM=nvme
USEC_INITIALIZED=1920739942

3) The uevent is evaluated with a simple udev rule, which essentially just calls 'nvme connect --async_connect' with the parameters from the uevent:

SUBSYSTEM=="nvme", ACTION=="change", RUN+="/usr/sbin/nvme connect --async_connect --transport=$env{NVME_DISCOVERY_TRTYPE} --traddr=$env{NVME_DISCOVERY_TRADDR} --host-traddr=$env{NVME_DISCOVERY_HOST_TRADDR} --nqn=$env{NVME_DISCOVERY_SUBNQN}"

4) The connection is established, all namespaces are parsed etc.
5) After two minutes the discovery controller is removed.

As usual, comments and reviews are welcome.

Hannes Reinecke (4):
nvme-rdma: use reconnect_work for initial connect
nvme: implement 'async_connect' cli option
nvme: implement 'discovery' sysfs entry and discovery uevents
nvme: delete discovery controller after 2 minutes

 drivers/nvme/host/core.c    | 168 +++++++++++++++++++++++++++++++++++++++++++-
 drivers/nvme/host/fabrics.c |   9 ++-
 drivers/nvme/host/fabrics.h |   6 ++
 drivers/nvme/host/fc.c      |   3 +-
 drivers/nvme/host/nvme.h    |   2 +
 drivers/nvme/host/rdma.c    |  21 ++++--
 6 files changed, 198 insertions(+), 11 deletions(-)

2.16.4