FMC (FPGA Mezzanine Card) is the standard we use for our I/O devices,
in the context of White Rabbit and related hardware.

In our I/O environments we need to write drivers for each mezzanine
card, and such drivers must work regardless of the carrier being used.
To achieve this, we abstract the FMC interface.

We have a carrier for PCI-E called SPEC and one for VME called SVEC,
but more are planned. Also, we support stand-alone devices (usually
plugged on a SPEC card), controlled through Etherbone, developed by GSI.

Code and documentation for the FMC bus was born as part of the spec-sw
project, but now it lives in its own project. Other projects, i.e.
software support for the various carriers, should include this as a

The most up to date version of code and documentation is always
available from the repository you can clone from:

    git:// (read-only) (read-write for developers)

Selected versions of the documentation, as well as complete tar
archives for selected revisions are placed to the Files section of the
project: `'

What is FMC

FMC, as said, stands for “FPGA Mezzanine Card”. It is a standard
developed by the VME consortium called VITA (VMEbus International Trade
Association and ratified by ANSI, the American National Standard
Institute. The official documentation is called “ANSI-VITA 57.1”.

The FMC card is an almost square PCB, around 70x75 millimeters, that is
called mezzanine in this document. It usually lives plugged into
another PCB for power supply and control; such bigger circuit board is
called carrier from now on, and a single carrier may host more than one

In the typical application the mezzanine is mostly analog while the
carrier is mostly digital, and hosts an FPGA that must be configured to
match the specific mezzanine and the desired application. Thus, you may
need to load different FPGA images to drive different instances of the
same mezzanine.

FMC, as such, is not a bus in the usual meaning of the term, because
most carriers have only one connector, and carriers with several
connectors have completely separate electrical connections to them.
This package, however, implements a bus as a software abstraction.

What is SDB

SDB (Self Describing Bus) is a set of data structures that we use for
enumerating the internal structure of an FPGA image. We also use it as
a filesystem inside the FMC EEPROM.

SDB is not mandatory for use of this FMC kernel bus, but if you have SDB
this package can make good use of it. SDB itself is developed in the
fpga-config-space OHWR project. The link to the repository is
`git://’ and what is used in
this project lives in the sdbfs subdirectory in there.

SDB support for FMC is described in *note FMC Identification:: and
*note SDB Support::

SDB Support

The fmc.ko bus driver exports a few functions to help drivers taking
advantage of the SDB information that may be present in your own FPGA
memory image.

The module exports the following functions, in the special header
<linux/fmc-sdb.h>. The linux/ prefix in the name is there because we
plan to submit it upstream in the future, and don’t want to force
changes on our drivers if that happens.

     int fmc_scan_sdb_tree(struct fmc_device *fmc, unsigned long address);
     void fmc_show_sdb_tree(struct fmc_device *fmc);
     signed long fmc_find_sdb_device(struct sdb_array *tree, uint64_t vendor,
                                     uint32_t device, unsigned long *sz);
     int fmc_free_sdb_tree(struct fmc_device *fmc);