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+.. SPDX-License-Identifier: GPL-2.0
+
+=========
+S/390 PCI
+=========
+
+Authors:
+ - Pierre Morel
+
+Copyright, IBM Corp. 2020
+
+
+Command line parameters and debugfs entries
+===========================================
+
+Command line parameters
+-----------------------
+
+* nomio
+
+ Do not use PCI Mapped I/O (MIO) instructions.
+
+* norid
+
+ Ignore the RID field and force use of one PCI domain per PCI function.
+
+debugfs entries
+---------------
+
+The S/390 debug feature (s390dbf) generates views to hold various debug results in sysfs directories of the form:
+
+ * /sys/kernel/debug/s390dbf/pci_*/
+
+For example:
+
+ - /sys/kernel/debug/s390dbf/pci_msg/sprintf
+ Holds messages from the processing of PCI events, like machine check handling
+ and setting of global functionality, like UID checking.
+
+ Change the level of logging to be more or less verbose by piping
+ a number between 0 and 6 to /sys/kernel/debug/s390dbf/pci_*/level. For
+ details, see the documentation on the S/390 debug feature at
+ Documentation/s390/s390dbf.rst.
+
+Sysfs entries
+=============
+
+Entries specific to zPCI functions and entries that hold zPCI information.
+
+* /sys/bus/pci/slots/XXXXXXXX
+
+ The slot entries are set up using the function identifier (FID) of the
+ PCI function.
+
+ - /sys/bus/pci/slots/XXXXXXXX/power
+
+ A physical function that currently supports a virtual function cannot be
+ powered off until all virtual functions are removed with:
+ echo 0 > /sys/bus/pci/devices/XXXX:XX:XX.X/sriov_numvf
+
+* /sys/bus/pci/devices/XXXX:XX:XX.X/
+
+ - function_id
+ A zPCI function identifier that uniquely identifies the function in the Z server.
+
+ - function_handle
+ Low-level identifier used for a configured PCI function.
+ It might be useful for debuging.
+
+ - pchid
+ Model-dependent location of the I/O adapter.
+
+ - pfgid
+ PCI function group ID, functions that share identical functionality
+ use a common identifier.
+ A PCI group defines interrupts, IOMMU, IOTLB, and DMA specifics.
+
+ - vfn
+ The virtual function number, from 1 to N for virtual functions,
+ 0 for physical functions.
+
+ - pft
+ The PCI function type
+
+ - port
+ The port corresponds to the physical port the function is attached to.
+ It also gives an indication of the physical function a virtual function
+ is attached to.
+
+ - uid
+ The unique identifier (UID) is defined when configuring an LPAR and is
+ unique in the LPAR.
+
+ - pfip/segmentX
+ The segments determine the isolation of a function.
+ They correspond to the physical path to the function.
+ The more the segments are different, the more the functions are isolated.
+
+Enumeration and hotplug
+=======================
+
+The PCI address consists of four parts: domain, bus, device and function,
+and is of this form: DDDD:BB:dd.f
+
+* When not using multi-functions (norid is set, or the firmware does not
+ support multi-functions):
+
+ - There is only one function per domain.
+
+ - The domain is set from the zPCI function's UID as defined during the
+ LPAR creation.
+
+* When using multi-functions (norid parameter is not set),
+ zPCI functions are addressed differently:
+
+ - There is still only one bus per domain.
+
+ - There can be up to 256 functions per bus.
+
+ - The domain part of the address of all functions for
+ a multi-Function device is set from the zPCI function's UID as defined
+ in the LPAR creation for the function zero.
+
+ - New functions will only be ready for use after the function zero
+ (the function with devfn 0) has been enumerated.