1===================================
2Writing s390 channel device drivers
3===================================
4
5:Author: Cornelia Huck
6
7Introduction
8============
9
10This document describes the interfaces available for device drivers that
11drive s390 based channel attached I/O devices. This includes interfaces
12for interaction with the hardware and interfaces for interacting with
13the common driver core. Those interfaces are provided by the s390 common
14I/O layer.
15
16The document assumes a familarity with the technical terms associated
17with the s390 channel I/O architecture. For a description of this
18architecture, please refer to the "z/Architecture: Principles of
19Operation", IBM publication no. SA22-7832.
20
21While most I/O devices on a s390 system are typically driven through the
22channel I/O mechanism described here, there are various other methods
23(like the diag interface). These are out of the scope of this document.
24
25The s390 common I/O layer also provides access to some devices that are
26not strictly considered I/O devices. They are considered here as well,
27although they are not the focus of this document.
28
29Some additional information can also be found in the kernel source under
30Documentation/arch/s390/driver-model.rst.
31
32The css bus
33===========
34
35The css bus contains the subchannels available on the system. They fall
36into several categories:
37
38* Standard I/O subchannels, for use by the system. They have a child
39  device on the ccw bus and are described below.
40* I/O subchannels bound to the vfio-ccw driver. See
41  Documentation/arch/s390/vfio-ccw.rst.
42* Message subchannels. No Linux driver currently exists.
43* CHSC subchannels (at most one). The chsc subchannel driver can be used
44  to send asynchronous chsc commands.
45* eADM subchannels. Used for talking to storage class memory.
46
47The ccw bus
48===========
49
50The ccw bus typically contains the majority of devices available to a
51s390 system. Named after the channel command word (ccw), the basic
52command structure used to address its devices, the ccw bus contains
53so-called channel attached devices. They are addressed via I/O
54subchannels, visible on the css bus. A device driver for
55channel-attached devices, however, will never interact with the
56subchannel directly, but only via the I/O device on the ccw bus, the ccw
57device.
58
59I/O functions for channel-attached devices
60------------------------------------------
61
62Some hardware structures have been translated into C structures for use
63by the common I/O layer and device drivers. For more information on the
64hardware structures represented here, please consult the Principles of
65Operation.
66
67.. kernel-doc:: arch/s390/include/asm/cio.h
68   :internal:
69
70ccw devices
71-----------
72
73Devices that want to initiate channel I/O need to attach to the ccw bus.
74Interaction with the driver core is done via the common I/O layer, which
75provides the abstractions of ccw devices and ccw device drivers.
76
77The functions that initiate or terminate channel I/O all act upon a ccw
78device structure. Device drivers must not bypass those functions or
79strange side effects may happen.
80
81.. kernel-doc:: arch/s390/include/asm/ccwdev.h
82   :internal:
83
84.. kernel-doc:: drivers/s390/cio/device.c
85   :export:
86
87.. kernel-doc:: drivers/s390/cio/device_ops.c
88   :export:
89
90The channel-measurement facility
91--------------------------------
92
93The channel-measurement facility provides a means to collect measurement
94data which is made available by the channel subsystem for each channel
95attached device.
96
97.. kernel-doc:: arch/s390/include/uapi/asm/cmb.h
98   :internal:
99
100.. kernel-doc:: drivers/s390/cio/cmf.c
101   :export:
102
103The ccwgroup bus
104================
105
106The ccwgroup bus only contains artificial devices, created by the user.
107Many networking devices (e.g. qeth) are in fact composed of several ccw
108devices (like read, write and data channel for qeth). The ccwgroup bus
109provides a mechanism to create a meta-device which contains those ccw
110devices as slave devices and can be associated with the netdevice.
111
112ccw group devices
113-----------------
114
115.. kernel-doc:: arch/s390/include/asm/ccwgroup.h
116   :internal:
117
118.. kernel-doc:: drivers/s390/cio/ccwgroup.c
119   :export:
120
121Generic interfaces
122==================
123
124The following section contains interfaces in use not only by drivers
125dealing with ccw devices, but drivers for various other s390 hardware
126as well.
127
128Adapter interrupts
129------------------
130
131The common I/O layer provides helper functions for dealing with adapter
132interrupts and interrupt vectors.
133
134.. kernel-doc:: drivers/s390/cio/airq.c
135   :export:
136