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+======================
+S/390 common I/O-Layer
+======================
+
+command line parameters, procfs and debugfs entries
+===================================================
+
+Command line parameters
+-----------------------
+
+* ccw_timeout_log
+
+  Enable logging of debug information in case of ccw device timeouts.
+
+* cio_ignore = device[,device[,..]]
+
+	device := {all | [!]ipldev | [!]condev | [!]<devno> | [!]<devno>-<devno>}
+
+  The given devices will be ignored by the common I/O-layer; no detection
+  and device sensing will be done on any of those devices. The subchannel to
+  which the device in question is attached will be treated as if no device was
+  attached.
+
+  An ignored device can be un-ignored later; see the "/proc entries"-section for
+  details.
+
+  The devices must be given either as bus ids (0.x.abcd) or as hexadecimal
+  device numbers (0xabcd or abcd, for 2.4 backward compatibility). If you
+  give a device number 0xabcd, it will be interpreted as 0.0.abcd.
+
+  You can use the 'all' keyword to ignore all devices. The 'ipldev' and 'condev'
+  keywords can be used to refer to the CCW based boot device and CCW console
+  device respectively (these are probably useful only when combined with the '!'
+  operator). The '!' operator will cause the I/O-layer to _not_ ignore a device.
+  The command line
+  is parsed from left to right.
+
+  For example::
+
+	cio_ignore=0.0.0023-0.0.0042,0.0.4711
+
+  will ignore all devices ranging from 0.0.0023 to 0.0.0042 and the device
+  0.0.4711, if detected.
+
+  As another example::
+
+	cio_ignore=all,!0.0.4711,!0.0.fd00-0.0.fd02
+
+  will ignore all devices but 0.0.4711, 0.0.fd00, 0.0.fd01, 0.0.fd02.
+
+  By default, no devices are ignored.
+
+
+/proc entries
+-------------
+
+* /proc/cio_ignore
+
+  Lists the ranges of devices (by bus id) which are ignored by common I/O.
+
+  You can un-ignore certain or all devices by piping to /proc/cio_ignore.
+  "free all" will un-ignore all ignored devices,
+  "free <device range>, <device range>, ..." will un-ignore the specified
+  devices.
+
+  For example, if devices 0.0.0023 to 0.0.0042 and 0.0.4711 are ignored,
+
+  - echo free 0.0.0030-0.0.0032 > /proc/cio_ignore
+    will un-ignore devices 0.0.0030 to 0.0.0032 and will leave devices 0.0.0023
+    to 0.0.002f, 0.0.0033 to 0.0.0042 and 0.0.4711 ignored;
+  - echo free 0.0.0041 > /proc/cio_ignore will furthermore un-ignore device
+    0.0.0041;
+  - echo free all > /proc/cio_ignore will un-ignore all remaining ignored
+    devices.
+
+  When a device is un-ignored, device recognition and sensing is performed and
+  the device driver will be notified if possible, so the device will become
+  available to the system. Note that un-ignoring is performed asynchronously.
+
+  You can also add ranges of devices to be ignored by piping to
+  /proc/cio_ignore; "add <device range>, <device range>, ..." will ignore the
+  specified devices.
+
+  Note: While already known devices can be added to the list of devices to be
+	ignored, there will be no effect on then. However, if such a device
+	disappears and then reappears, it will then be ignored. To make
+	known devices go away, you need the "purge" command (see below).
+
+  For example::
+
+	"echo add 0.0.a000-0.0.accc, 0.0.af00-0.0.afff > /proc/cio_ignore"
+
+  will add 0.0.a000-0.0.accc and 0.0.af00-0.0.afff to the list of ignored
+  devices.
+
+  You can remove already known but now ignored devices via::
+
+	"echo purge > /proc/cio_ignore"
+
+  All devices ignored but still registered and not online (= not in use)
+  will be deregistered and thus removed from the system.
+
+  The devices can be specified either by bus id (0.x.abcd) or, for 2.4 backward
+  compatibility, by the device number in hexadecimal (0xabcd or abcd). Device
+  numbers given as 0xabcd will be interpreted as 0.0.abcd.
+
+* /proc/cio_settle
+
+  A write request to this file is blocked until all queued cio actions are
+  handled. This will allow userspace to wait for pending work affecting
+  device availability after changing cio_ignore or the hardware configuration.
+
+* For some of the information present in the /proc filesystem in 2.4 (namely,
+  /proc/subchannels and /proc/chpids), see driver-model.txt.
+  Information formerly in /proc/irq_count is now in /proc/interrupts.
+
+
+debugfs entries
+---------------
+
+* /sys/kernel/debug/s390dbf/cio_*/ (S/390 debug feature)
+
+  Some views generated by the debug feature to hold various debug outputs.
+
+  - /sys/kernel/debug/s390dbf/cio_crw/sprintf
+    Messages from the processing of pending channel report words (machine check
+    handling).
+
+  - /sys/kernel/debug/s390dbf/cio_msg/sprintf
+    Various debug messages from the common I/O-layer.
+
+  - /sys/kernel/debug/s390dbf/cio_trace/hex_ascii
+    Logs the calling of functions in the common I/O-layer and, if applicable,
+    which subchannel they were called for, as well as dumps of some data
+    structures (like irb in an error case).
+
+  The level of logging can be changed to be more or less verbose by piping to
+  /sys/kernel/debug/s390dbf/cio_*/level a number between 0 and 6; see the
+  documentation on the S/390 debug feature (Documentation/s390/s390dbf.rst)
+  for details.