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+===================================
+Writing s390 channel device drivers
+===================================
+
+:Author: Cornelia Huck
+
+Introduction
+============
+
+This document describes the interfaces available for device drivers that
+drive s390 based channel attached I/O devices. This includes interfaces
+for interaction with the hardware and interfaces for interacting with
+the common driver core. Those interfaces are provided by the s390 common
+I/O layer.
+
+The document assumes a familarity with the technical terms associated
+with the s390 channel I/O architecture. For a description of this
+architecture, please refer to the "z/Architecture: Principles of
+Operation", IBM publication no. SA22-7832.
+
+While most I/O devices on a s390 system are typically driven through the
+channel I/O mechanism described here, there are various other methods
+(like the diag interface). These are out of the scope of this document.
+
+The s390 common I/O layer also provides access to some devices that are
+not strictly considered I/O devices. They are considered here as well,
+although they are not the focus of this document.
+
+Some additional information can also be found in the kernel source under
+Documentation/arch/s390/driver-model.rst.
+
+The css bus
+===========
+
+The css bus contains the subchannels available on the system. They fall
+into several categories:
+
+* Standard I/O subchannels, for use by the system. They have a child
+  device on the ccw bus and are described below.
+* I/O subchannels bound to the vfio-ccw driver. See
+  Documentation/arch/s390/vfio-ccw.rst.
+* Message subchannels. No Linux driver currently exists.
+* CHSC subchannels (at most one). The chsc subchannel driver can be used
+  to send asynchronous chsc commands.
+* eADM subchannels. Used for talking to storage class memory.
+
+The ccw bus
+===========
+
+The ccw bus typically contains the majority of devices available to a
+s390 system. Named after the channel command word (ccw), the basic
+command structure used to address its devices, the ccw bus contains
+so-called channel attached devices. They are addressed via I/O
+subchannels, visible on the css bus. A device driver for
+channel-attached devices, however, will never interact with the
+subchannel directly, but only via the I/O device on the ccw bus, the ccw
+device.
+
+I/O functions for channel-attached devices
+------------------------------------------
+
+Some hardware structures have been translated into C structures for use
+by the common I/O layer and device drivers. For more information on the
+hardware structures represented here, please consult the Principles of
+Operation.
+
+.. kernel-doc:: arch/s390/include/asm/cio.h
+   :internal:
+
+ccw devices
+-----------
+
+Devices that want to initiate channel I/O need to attach to the ccw bus.
+Interaction with the driver core is done via the common I/O layer, which
+provides the abstractions of ccw devices and ccw device drivers.
+
+The functions that initiate or terminate channel I/O all act upon a ccw
+device structure. Device drivers must not bypass those functions or
+strange side effects may happen.
+
+.. kernel-doc:: arch/s390/include/asm/ccwdev.h
+   :internal:
+
+.. kernel-doc:: drivers/s390/cio/device.c
+   :export:
+
+.. kernel-doc:: drivers/s390/cio/device_ops.c
+   :export:
+
+The channel-measurement facility
+--------------------------------
+
+The channel-measurement facility provides a means to collect measurement
+data which is made available by the channel subsystem for each channel
+attached device.
+
+.. kernel-doc:: arch/s390/include/uapi/asm/cmb.h
+   :internal:
+
+.. kernel-doc:: drivers/s390/cio/cmf.c
+   :export:
+
+The ccwgroup bus
+================
+
+The ccwgroup bus only contains artificial devices, created by the user.
+Many networking devices (e.g. qeth) are in fact composed of several ccw
+devices (like read, write and data channel for qeth). The ccwgroup bus
+provides a mechanism to create a meta-device which contains those ccw
+devices as slave devices and can be associated with the netdevice.
+
+ccw group devices
+-----------------
+
+.. kernel-doc:: arch/s390/include/asm/ccwgroup.h
+   :internal:
+
+.. kernel-doc:: drivers/s390/cio/ccwgroup.c
+   :export:
+
+Generic interfaces
+==================
+
+The following section contains interfaces in use not only by drivers
+dealing with ccw devices, but drivers for various other s390 hardware
+as well.
+
+Adapter interrupts
+------------------
+
+The common I/O layer provides helper functions for dealing with adapter
+interrupts and interrupt vectors.
+
+.. kernel-doc:: drivers/s390/cio/airq.c
+   :export: