s390/cio: fix invalid -EBUSY on ccw_device_start

s390/cio: fix invalid -EBUSY on ccw_device_start

The s390 common I/O layer (CIO) returns an unexpected -EBUSY return code
when drivers try to start I/O while a path-verification (PV) process is
pending. This can lead to failed device initialization attempts with
symptoms like broken network connectivity after boot.

Fix this by replacing the -EBUSY return code with a deferred condition
code 1 reply to make path-verification handling consistent from a
driver's point of view.

The problem can be reproduced semi-regularly using the following process,
while repeating steps 2-3 as necessary (example assumes an OSA device
with bus-IDs 0.0.a000-0.0.a002 on CHPID 0.02):

1. echo 0.0.a000,0.0.a001,0.0.a002 >/sys/bus/ccwgroup/drivers/qeth/group
2. echo 0 > /sys/bus/ccwgroup/devices/0.0.a000/online
3. echo 1 > /sys/bus/ccwgroup/devices/0.0.a000/online ; \
   echo on > /sys/devices/css0/chp0.02/status

Background information:

The common I/O layer starts path-verification I/Os when it receives
indications about changes in a device path's availability. This occurs
for example when hardware events indicate a change in channel-path
status, or when a manual operation such as a CHPID vary or configure
operation is performed.

If a driver attempts to start I/O while a PV is running, CIO reports a
successful I/O start (ccw_device_start() return code 0). Then, after
completion of PV, CIO synthesizes an interrupt response that indicates
an asynchronous status condition that prevented the start of the I/O
(deferred condition code 1).

If a PV indication arrives while a device is busy with driver-owned I/O,
PV is delayed until after I/O completion was reported to the driver's
interrupt handler. To ensure that PV can be started eventually, CIO
reports a device busy condition (ccw_device_start() return code -EBUSY)
if a driver tries to start another I/O while PV is pending.

In some cases this -EBUSY return code causes device drivers to consider
a device not operational, resulting in failed device initialization.

Note: The code that introduced the problem was added in 2003. Symptoms
started appearing with the following CIO commit that causes a PV
indication when a device is removed from the cio_ignore list after the
associated parent subchannel device was probed, but before online
processing of the CCW device has started:

2297791c92d0 ("s390/cio: dont unregister subchannel from child-drivers")

During boot, the cio_ignore list is modified by the cio_ignore dracut
module [1] as well as Linux vendor-specific systemd service scripts[2].
When combined, this commit and boot scripts cause a frequent occurrence
of the problem during boot.

[1] https://github.com/dracutdevs/dracut/tree/master/modules.d/81cio_ignore
[2] https://github.com/SUSE/s390-tools/blob/master/cio_ignore.service

Cc: stable@vger.kernel.org # v5.15+
Fixes: 2297791c92d0 ("s390/cio: dont unregister subchannel from child-drivers")
Tested-By: Thorsten Winkler <twinkler@linux.ibm.com>
Reviewed-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>