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