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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. The format depicted as XXXXXXXX above is 8 hexadecimal digits
+  with 0 padding and lower case hexadecimal digitis.
+
+  - /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 user identifier (UID) may be defined as part of the machine
+    configuration or the z/VM or KVM guest configuration. If the accompanying
+    uid_is_unique attribute is 1 the platform guarantees that the UID is unique
+    within that instance and no devices with the same UID can be attached
+    during the lifetime of the system.
+
+  - uid_is_unique
+    Indicates whether the user identifier (UID) is guaranteed to be and remain
+    unique within this Linux instance.
+
+  - 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.