# 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: (Aspeed family boards (ast2500-evb, ast2600-evb, ast2700-evb, bletchley-bmc, fuji-bmc, fby35-bmc, fp5280g2-bmc, g220a-bmc, palmetto-bmc, qcom-dc-scm-v1-bmc, qcom-firework-bmc, quanta-q71l-bmc, rainier-bmc, romulus-bmc, sonorapass-bmc, supermicrox11-bmc, supermicrox11spi-bmc, tiogapass-bmc, tacoma-bmc, witherspoon-bmc, yosemitev2-bmc))

    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:

    (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)

    root@p10bmc:~# pdbg -a getcfam 0x0
    p0: 0x0 = 0xc0022d15