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and fix all in-tree references. Architecture-specific documentation is being moved into Documentation/arch/ as a way of cleaning up the top-level documentation directory and making the docs hierarchy more closely match the source hierarchy. Signed-off-by: Costa Shulyupin <costa.shul@redhat.com> Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com> Acked-by: Jonathan Corbet <corbet@lwn.net> Acked-by: Heiko Carstens <hca@linux.ibm.com> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Reviewed-by: Randy Dunlap <rdunlap@infradead.org> Link: https://lore.kernel.org/r/20230718045550.495428-1-costa.shul@redhat.com Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
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===================================
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Writing s390 channel device drivers
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===================================
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:Author: Cornelia Huck
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Introduction
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============
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This document describes the interfaces available for device drivers that
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drive s390 based channel attached I/O devices. This includes interfaces
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for interaction with the hardware and interfaces for interacting with
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the common driver core. Those interfaces are provided by the s390 common
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I/O layer.
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The document assumes a familarity with the technical terms associated
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with the s390 channel I/O architecture. For a description of this
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architecture, please refer to the "z/Architecture: Principles of
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Operation", IBM publication no. SA22-7832.
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While most I/O devices on a s390 system are typically driven through the
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channel I/O mechanism described here, there are various other methods
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(like the diag interface). These are out of the scope of this document.
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The s390 common I/O layer also provides access to some devices that are
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not strictly considered I/O devices. They are considered here as well,
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although they are not the focus of this document.
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Some additional information can also be found in the kernel source under
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Documentation/arch/s390/driver-model.rst.
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The css bus
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===========
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The css bus contains the subchannels available on the system. They fall
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into several categories:
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* Standard I/O subchannels, for use by the system. They have a child
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device on the ccw bus and are described below.
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* I/O subchannels bound to the vfio-ccw driver. See
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Documentation/arch/s390/vfio-ccw.rst.
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* Message subchannels. No Linux driver currently exists.
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* CHSC subchannels (at most one). The chsc subchannel driver can be used
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to send asynchronous chsc commands.
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* eADM subchannels. Used for talking to storage class memory.
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The ccw bus
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===========
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The ccw bus typically contains the majority of devices available to a
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s390 system. Named after the channel command word (ccw), the basic
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command structure used to address its devices, the ccw bus contains
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so-called channel attached devices. They are addressed via I/O
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subchannels, visible on the css bus. A device driver for
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channel-attached devices, however, will never interact with the
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subchannel directly, but only via the I/O device on the ccw bus, the ccw
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device.
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I/O functions for channel-attached devices
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------------------------------------------
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Some hardware structures have been translated into C structures for use
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by the common I/O layer and device drivers. For more information on the
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hardware structures represented here, please consult the Principles of
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Operation.
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.. kernel-doc:: arch/s390/include/asm/cio.h
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:internal:
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ccw devices
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-----------
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Devices that want to initiate channel I/O need to attach to the ccw bus.
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Interaction with the driver core is done via the common I/O layer, which
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provides the abstractions of ccw devices and ccw device drivers.
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The functions that initiate or terminate channel I/O all act upon a ccw
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device structure. Device drivers must not bypass those functions or
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strange side effects may happen.
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.. kernel-doc:: arch/s390/include/asm/ccwdev.h
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:internal:
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.. kernel-doc:: drivers/s390/cio/device.c
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:export:
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.. kernel-doc:: drivers/s390/cio/device_ops.c
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:export:
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The channel-measurement facility
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--------------------------------
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The channel-measurement facility provides a means to collect measurement
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data which is made available by the channel subsystem for each channel
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attached device.
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.. kernel-doc:: arch/s390/include/uapi/asm/cmb.h
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:internal:
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.. kernel-doc:: drivers/s390/cio/cmf.c
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:export:
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The ccwgroup bus
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================
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The ccwgroup bus only contains artificial devices, created by the user.
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Many networking devices (e.g. qeth) are in fact composed of several ccw
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devices (like read, write and data channel for qeth). The ccwgroup bus
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provides a mechanism to create a meta-device which contains those ccw
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devices as slave devices and can be associated with the netdevice.
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ccw group devices
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-----------------
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.. kernel-doc:: arch/s390/include/asm/ccwgroup.h
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:internal:
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.. kernel-doc:: drivers/s390/cio/ccwgroup.c
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:export:
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Generic interfaces
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==================
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The following section contains interfaces in use not only by drivers
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dealing with ccw devices, but drivers for various other s390 hardware
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as well.
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Adapter interrupts
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------------------
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The common I/O layer provides helper functions for dealing with adapter
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interrupts and interrupt vectors.
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.. kernel-doc:: drivers/s390/cio/airq.c
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:export:
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