diff --git a/docs/specs/fsi.rst b/docs/specs/fsi.rst
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+======================================
+IBM's Flexible Service Interface (FSI)
+======================================
+
+The QEMU FSI emulation implements hardware interfaces between ASPEED SOC, FSI
+master/slave and the end engine.
+
+FSI is a point-to-point two wire interface which is capable of supporting
+distances of up to 4 meters. FSI interfaces have been used successfully for
+many years in IBM servers to attach IBM Flexible Support Processors(FSP) to
+CPUs and IBM ASICs.
+
+FSI allows a service processor access to the internal buses of a host POWER
+processor to perform configuration or debugging. FSI has long existed in POWER
+processes and so comes with some baggage, including how it has been integrated
+into the ASPEED SoC.
+
+Working backwards from the POWER processor, the fundamental pieces of interest
+for the implementation are: (see the `FSI specification`_ for more details)
+
+1. The Common FRU Access Macro (CFAM), an address space containing various
+   "engines" that drive accesses on buses internal and external to the POWER
+   chip. Examples include the SBEFIFO and I2C masters. The engines hang off of
+   an internal Local Bus (LBUS) which is described by the CFAM configuration
+   block.
+
+2. The FSI slave: The slave is the terminal point of the FSI bus for FSI
+   symbols addressed to it. Slaves can be cascaded off of one another. The
+   slave's configuration registers appear in address space of the CFAM to
+   which it is attached.
+
+3. The FSI master: A controller in the platform service processor (e.g. BMC)
+   driving CFAM engine accesses into the POWER chip. At the hardware level
+   FSI is a bit-based protocol supporting synchronous and DMA-driven accesses
+   of engines in a CFAM.
+
+4. The On-Chip Peripheral Bus (OPB): A low-speed bus typically found in POWER
+   processors. This now makes an appearance in the ASPEED SoC due to tight
+   integration of the FSI master IP with the OPB, mainly the existence of an
+   MMIO-mapping of the CFAM address straight onto a sub-region of the OPB
+   address space.
+
+5. An APB-to-OPB bridge enabling access to the OPB from the ARM core in the
+   AST2600. Hardware limitations prevent the OPB from being directly mapped
+   into APB, so all accesses are indirect through the bridge.
+
+The LBUS is modelled to maintain the qdev bus hierarchy and to take advantages
+of the object model to automatically generate the CFAM configuration block.
+The configuration block presents engines in the order they are attached to the
+CFAM's LBUS. Engine implementations should subclass the LBusDevice and set the
+'config' member of LBusDeviceClass to match the engine's type.
+
+CFAM designs offer a lot of flexibility, for instance it is possible for a
+CFAM to be simultaneously driven from multiple FSI links. The modeling is not
+so complete; it's assumed that each CFAM is attached to a single FSI slave (as
+a consequence the CFAM subclasses the FSI slave).
+
+As for FSI, its symbols and wire-protocol are not modelled at all. This is not
+necessary to get FSI off the ground thanks to the mapping of the CFAM address
+space onto the OPB address space - the models follow this directly and map the
+CFAM memory region into the OPB's memory region.
+
+The following commands start the ``rainier-bmc`` machine with built-in FSI
+model. There are no model specific arguments. Please check this document to
+learn more about Aspeed ``rainier-bmc`` machine: (:doc:`../../system/arm/aspeed`)
+
+.. code-block:: console
+
+  qemu-system-arm -M rainier-bmc -nographic \
+  -kernel fitImage-linux.bin \
+  -dtb aspeed-bmc-ibm-rainier.dtb \
+  -initrd obmc-phosphor-initramfs.rootfs.cpio.xz \
+  -drive file=obmc-phosphor-image.rootfs.wic.qcow2,if=sd,index=2 \
+  -append "rootwait console=ttyS4,115200n8 root=PARTLABEL=rofs-a"
+
+The implementation appears as following in the qemu device tree:
+
+.. code-block:: console
+
+  (qemu) info qtree
+  bus: main-system-bus
+    type System
+    ...
+    dev: aspeed.apb2opb, id ""
+      gpio-out "sysbus-irq" 1
+      mmio 000000001e79b000/0000000000001000
+      bus: opb.1
+        type opb
+        dev: fsi.master, id ""
+          bus: fsi.bus.1
+            type fsi.bus
+            dev: cfam.config, id ""
+            dev: cfam, id ""
+              bus: lbus.1
+                type lbus
+                dev: scratchpad, id ""
+                  address = 0 (0x0)
+      bus: opb.0
+        type opb
+        dev: fsi.master, id ""
+          bus: fsi.bus.0
+            type fsi.bus
+            dev: cfam.config, id ""
+            dev: cfam, id ""
+              bus: lbus.0
+                type lbus
+                dev: scratchpad, id ""
+                  address = 0 (0x0)
+
+pdbg is a simple application to allow debugging of the host POWER processors
+from the BMC. (see the `pdbg source repository`_ for more details)
+
+.. code-block:: console
+
+  root@p10bmc:~# pdbg -a getcfam 0x0
+  p0: 0x0 = 0xc0022d15
+
+.. _FSI specification:
+   https://openpowerfoundation.org/specifications/fsi/
+
+.. _pdbg source repository:
+   https://github.com/open-power/pdbg
diff --git a/docs/specs/index.rst b/docs/specs/index.rst
index b3f482b..1484e3e 100644
--- a/docs/specs/index.rst
+++ b/docs/specs/index.rst
@@ -24,6 +24,7 @@
    acpi_erst
    sev-guest-firmware
    fw_cfg
+   fsi
    vmw_pvscsi-spec
    edu
    ivshmem-spec