Introduction
============

The FPGA subsystem supports reprogramming FPGAs dynamically under
Linux.  Some of the core intentions of the FPGA subsystems are:

* The FPGA subsystem is vendor agnostic.

* The FPGA subsystem separates upper layers (userspace interfaces and
  enumeration) from lower layers that know how to program a specific
  FPGA.

* Code should not be shared between upper and lower layers.  This
  should go without saying.  If that seems necessary, there's probably
  framework functionality that that can be added that will benefit
  other users.  Write the linux-fpga mailing list and maintainers and
  seek out a solution that expands the framework for broad reuse.

* Generally, when adding code, think of the future.  Plan for re-use.

The framework in the kernel is divided into:

FPGA Manager
------------

If you are adding a new FPGA or a new method of programming a FPGA,
this is the subsystem for you.  Low level FPGA manager drivers contain
the knowledge of how to program a specific device.  This subsystem
includes the framework in fpga-mgr.c and the low level drivers that
are registered with it.

FPGA Bridge
-----------

FPGA Bridges prevent spurious signals from going out of a FPGA or a
region of a FPGA during programming.  They are disabled before
programming begins and re-enabled afterwards.  An FPGA bridge may be
actual hard hardware that gates a bus to a cpu or a soft ("freeze")
bridge in FPGA fabric that surrounds a partial reconfiguration region
of an FPGA.  This subsystem includes fpga-bridge.c and the low level
drivers that are registered with it.

FPGA Region
-----------

If you are adding a new interface to the FPGA framework, add it on top
of a FPGA region to allow the most reuse of your interface.

The FPGA Region framework (fpga-region.c) associates managers and
bridges as reconfigurable regions.  A region may refer to the whole
FPGA in full reconfiguration or to a partial reconfiguration region.

The Device Tree FPGA Region support (of-fpga-region.c) handles
reprogramming FPGAs when device tree overlays are applied.