Documentation/driver-api/s390-drivers.rst

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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

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