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+FPGA Region Device Tree Binding
+
+Alan Tull 2016
+
+ CONTENTS
+ - Introduction
+ - Terminology
+ - Sequence
+ - FPGA Region
+ - Supported Use Models
+ - Device Tree Examples
+ - Constraints
+
+
+Introduction
+============
+
+FPGA Regions represent FPGA's and partial reconfiguration regions of FPGA's in
+the Device Tree. FPGA Regions provide a way to program FPGAs under device tree
+control.
+
+This device tree binding document hits some of the high points of FPGA usage and
+attempts to include terminology used by both major FPGA manufacturers. This
+document isn't a replacement for any manufacturers specifications for FPGA
+usage.
+
+
+Terminology
+===========
+
+Full Reconfiguration
+ * The entire FPGA is programmed.
+
+Partial Reconfiguration (PR)
+ * A section of an FPGA is reprogrammed while the rest of the FPGA is not
+ affected.
+ * Not all FPGA's support PR.
+
+Partial Reconfiguration Region (PRR)
+ * Also called a "reconfigurable partition"
+ * A PRR is a specific section of a FPGA reserved for reconfiguration.
+ * A base (or static) FPGA image may create a set of PRR's that later may
+ be independently reprogrammed many times.
+ * The size and specific location of each PRR is fixed.
+ * The connections at the edge of each PRR are fixed. The image that is loaded
+ into a PRR must fit and must use a subset of the region's connections.
+ * The busses within the FPGA are split such that each region gets its own
+ branch that may be gated independently.
+
+Persona
+ * Also called a "partial bit stream"
+ * An FPGA image that is designed to be loaded into a PRR. There may be
+ any number of personas designed to fit into a PRR, but only one at at time
+ may be loaded.
+ * A persona may create more regions.
+
+FPGA Bridge
+ * FPGA Bridges gate bus signals between a host and FPGA.
+ * FPGA Bridges should be disabled while the FPGA is being programmed to
+ prevent spurious signals on the cpu bus and to the soft logic.
+ * FPGA bridges may be actual hardware or soft logic on an FPGA.
+ * During Full Reconfiguration, hardware bridges between the host and FPGA
+ will be disabled.
+ * During Partial Reconfiguration of a specific region, that region's bridge
+ will be used to gate the busses. Traffic to other regions is not affected.
+ * In some implementations, the FPGA Manager transparantly handles gating the
+ buses, eliminating the need to show the hardware FPGA bridges in the
+ device tree.
+ * An FPGA image may create a set of reprogrammable regions, each having its
+ own bridge and its own split of the busses in the FPGA.
+
+FPGA Manager
+ * An FPGA Manager is a hardware block that programs an FPGA under the control
+ of a host processor.
+
+Base Image
+ * Also called the "static image"
+ * An FPGA image that is designed to do full reconfiguration of the FPGA.
+ * A base image may set up a set of partial reconfiguration regions that may
+ later be reprogrammed.
+
+ ---------------- ----------------------------------
+ | Host CPU | | FPGA |
+ | | | |
+ | ----| | ----------- -------- |
+ | | H | | |==>| Bridge0 |<==>| PRR0 | |
+ | | W | | | ----------- -------- |
+ | | | | | |
+ | | B |<=====>|<==| ----------- -------- |
+ | | R | | |==>| Bridge1 |<==>| PRR1 | |
+ | | I | | | ----------- -------- |
+ | | D | | | |
+ | | G | | | ----------- -------- |
+ | | E | | |==>| Bridge2 |<==>| PRR2 | |
+ | ----| | ----------- -------- |
+ | | | |
+ ---------------- ----------------------------------
+
+Figure 1: An FPGA set up with a base image that created three regions. Each
+region (PRR0-2) gets its own split of the busses that is independently gated by
+a soft logic bridge (Bridge0-2) in the FPGA. The contents of each PRR can be
+reprogrammed independently while the rest of the system continues to function.
+
+
+Sequence
+========
+
+When a DT overlay that targets a FPGA Region is applied, the FPGA Region will
+do the following:
+
+ 1. Disable appropriate FPGA bridges.
+ 2. Program the FPGA using the FPGA manager.
+ 3. Enable the FPGA bridges.
+ 4. The Device Tree overlay is accepted into the live tree.
+ 5. Child devices are populated.
+
+When the overlay is removed, the child nodes will be removed and the FPGA Region
+will disable the bridges.
+
+
+FPGA Region
+===========
+
+FPGA Regions represent FPGA's and FPGA PR regions in the device tree. An FPGA
+Region brings together the elements needed to program on a running system and
+add the child devices:
+
+ * FPGA Manager
+ * FPGA Bridges
+ * image-specific information needed to to the programming.
+ * child nodes
+
+The intended use is that a Device Tree overlay (DTO) can be used to reprogram an
+FPGA while an operating system is running.
+
+An FPGA Region that exists in the live Device Tree reflects the current state.
+If the live tree shows a "firmware-name" property or child nodes under a FPGA
+Region, the FPGA already has been programmed. A DTO that targets a FPGA Region
+and adds the "firmware-name" property is taken as a request to reprogram the
+FPGA. After reprogramming is successful, the overlay is accepted into the live
+tree.
+
+The base FPGA Region in the device tree represents the FPGA and supports full
+reconfiguration. It must include a phandle to an FPGA Manager. The base
+FPGA region will be the child of one of the hardware bridges (the bridge that
+allows register access) between the cpu and the FPGA. If there are more than
+one bridge to control during FPGA programming, the region will also contain a
+list of phandles to the additional hardware FPGA Bridges.
+
+For partial reconfiguration (PR), each PR region will have an FPGA Region.
+These FPGA regions are children of FPGA bridges which are then children of the
+base FPGA region. The "Full Reconfiguration to add PRR's" example below shows
+this.
+
+If an FPGA Region does not specify a FPGA Manager, it will inherit the FPGA
+Manager specified by its ancestor FPGA Region. This supports both the case
+where the same FPGA Manager is used for all of a FPGA as well the case where
+a different FPGA Manager is used for each region.
+
+FPGA Regions do not inherit their ancestor FPGA regions' bridges. This prevents
+shutting down bridges that are upstream from the other active regions while one
+region is getting reconfigured (see Figure 1 above). During PR, the FPGA's
+hardware bridges remain enabled. The PR regions' bridges will be FPGA bridges
+within the static image of the FPGA.
+
+Required properties:
+- compatible : should contain "fpga-region"
+- fpga-mgr : should contain a phandle to an FPGA Manager. Child FPGA Regions
+ inherit this property from their ancestor regions. A fpga-mgr property
+ in a region will override any inherited FPGA manager.
+- #address-cells, #size-cells, ranges : must be present to handle address space
+ mapping for child nodes.
+
+Optional properties:
+- firmware-name : should contain the name of an FPGA image file located on the
+ firmware search path. If this property shows up in a live device tree
+ it indicates that the FPGA has already been programmed with this image.
+ If this property is in an overlay targeting a FPGA region, it is a
+ request to program the FPGA with that image.
+- fpga-bridges : should contain a list of phandles to FPGA Bridges that must be
+ controlled during FPGA programming along with the parent FPGA bridge.
+ This property is optional if the FPGA Manager handles the bridges.
+ If the fpga-region is the child of a fpga-bridge, the list should not
+ contain the parent bridge.
+- partial-fpga-config : boolean, set if partial reconfiguration is to be done,
+ otherwise full reconfiguration is done.
+- external-fpga-config : boolean, set if the FPGA has already been configured
+ prior to OS boot up.
+- region-unfreeze-timeout-us : The maximum time in microseconds to wait for
+ bridges to successfully become enabled after the region has been
+ programmed.
+- region-freeze-timeout-us : The maximum time in microseconds to wait for
+ bridges to successfully become disabled before the region has been
+ programmed.
+- child nodes : devices in the FPGA after programming.
+
+In the example below, when an overlay is applied targeting fpga-region0,
+fpga_mgr is used to program the FPGA. Two bridges are controlled during
+programming: the parent fpga_bridge0 and fpga_bridge1. Because the region is
+the child of fpga_bridge0, only fpga_bridge1 needs to be specified in the
+fpga-bridges property. During programming, these bridges are disabled, the
+firmware specified in the overlay is loaded to the FPGA using the FPGA manager
+specified in the region. If FPGA programming succeeds, the bridges are
+reenabled and the overlay makes it into the live device tree. The child devices
+are then populated. If FPGA programming fails, the bridges are left disabled
+and the overlay is rejected. The overlay's ranges property maps the lwhps
+bridge's region (0xff200000) and the hps bridge's region (0xc0000000) for use by
+the two child devices.
+
+Example:
+Base tree contains:
+
+ fpga_mgr: fpga-mgr@ff706000 {
+ compatible = "altr,socfpga-fpga-mgr";
+ reg = <0xff706000 0x1000
+ 0xffb90000 0x20>;
+ interrupts = <0 175 4>;
+ };
+
+ fpga_bridge0: fpga-bridge@ff400000 {
+ compatible = "altr,socfpga-lwhps2fpga-bridge";
+ reg = <0xff400000 0x100000>;
+ resets = <&rst LWHPS2FPGA_RESET>;
+ clocks = <&l4_main_clk>;
+
+ #address-cells = <1>;
+ #size-cells = <1>;
+ ranges;
+
+ fpga_region0: fpga-region0 {
+ compatible = "fpga-region";
+ fpga-mgr = <&fpga_mgr>;
+ };
+ };
+
+ fpga_bridge1: fpga-bridge@ff500000 {
+ compatible = "altr,socfpga-hps2fpga-bridge";
+ reg = <0xff500000 0x10000>;
+ resets = <&rst HPS2FPGA_RESET>;
+ clocks = <&l4_main_clk>;
+ };
+
+Overlay contains:
+
+/dts-v1/ /plugin/;
+/ {
+ fragment@0 {
+ target = <&fpga_region0>;
+ #address-cells = <1>;
+ #size-cells = <1>;
+ __overlay__ {
+ #address-cells = <1>;
+ #size-cells = <1>;
+
+ firmware-name = "soc_system.rbf";
+ fpga-bridges = <&fpga_bridge1>;
+ ranges = <0x20000 0xff200000 0x100000>,
+ <0x0 0xc0000000 0x20000000>;
+
+ gpio@10040 {
+ compatible = "altr,pio-1.0";
+ reg = <0x10040 0x20>;
+ altr,gpio-bank-width = <4>;
+ #gpio-cells = <2>;
+ clocks = <2>;
+ gpio-controller;
+ };
+
+ onchip-memory {
+ device_type = "memory";
+ compatible = "altr,onchipmem-15.1";
+ reg = <0x0 0x10000>;
+ };
+ };
+ };
+};
+
+
+Supported Use Models
+====================
+
+In all cases the live DT must have the FPGA Manager, FPGA Bridges (if any), and
+a FPGA Region. The target of the Device Tree Overlay is the FPGA Region. Some
+uses are specific to a FPGA device.
+
+ * No FPGA Bridges
+ In this case, the FPGA Manager which programs the FPGA also handles the
+ bridges behind the scenes. No FPGA Bridge devices are needed for full
+ reconfiguration.
+
+ * Full reconfiguration with hardware bridges
+ In this case, there are hardware bridges between the processor and FPGA that
+ need to be controlled during full reconfiguration. Before the overlay is
+ applied, the live DT must include the FPGA Manager, FPGA Bridges, and a
+ FPGA Region. The FPGA Region is the child of the bridge that allows
+ register access to the FPGA. Additional bridges may be listed in a
+ fpga-bridges property in the FPGA region or in the device tree overlay.
+
+ * Partial reconfiguration with bridges in the FPGA
+ In this case, the FPGA will have one or more PRR's that may be programmed
+ separately while the rest of the FPGA can remain active. To manage this,
+ bridges need to exist in the FPGA that can gate the buses going to each FPGA
+ region while the buses are enabled for other sections. Before any partial
+ reconfiguration can be done, a base FPGA image must be loaded which includes
+ PRR's with FPGA bridges. The device tree should have a FPGA region for each
+ PRR.
+
+Device Tree Examples
+====================
+
+The intention of this section is to give some simple examples, focusing on
+the placement of the elements detailed above, especially:
+ * FPGA Manager
+ * FPGA Bridges
+ * FPGA Region
+ * ranges
+ * target-path or target
+
+For the purposes of this section, I'm dividing the Device Tree into two parts,
+each with its own requirements. The two parts are:
+ * The live DT prior to the overlay being added
+ * The DT overlay
+
+The live Device Tree must contain an FPGA Region, an FPGA Manager, and any FPGA
+Bridges. The FPGA Region's "fpga-mgr" property specifies the manager by phandle
+to handle programming the FPGA. If the FPGA Region is the child of another FPGA
+Region, the parent's FPGA Manager is used. If FPGA Bridges need to be involved,
+they are specified in the FPGA Region by the "fpga-bridges" property. During
+FPGA programming, the FPGA Region will disable the bridges that are in its
+"fpga-bridges" list and will re-enable them after FPGA programming has
+succeeded.
+
+The Device Tree Overlay will contain:
+ * "target-path" or "target"
+ The insertion point where the the contents of the overlay will go into the
+ live tree. target-path is a full path, while target is a phandle.
+ * "ranges"
+ The address space mapping from processor to FPGA bus(ses).
+ * "firmware-name"
+ Specifies the name of the FPGA image file on the firmware search
+ path. The search path is described in the firmware class documentation.
+ * "partial-fpga-config"
+ This binding is a boolean and should be present if partial reconfiguration
+ is to be done.
+ * child nodes corresponding to hardware that will be loaded in this region of
+ the FPGA.
+
+Device Tree Example: Full Reconfiguration without Bridges
+=========================================================
+
+Live Device Tree contains:
+ fpga_mgr0: fpga-mgr@f8007000 {
+ compatible = "xlnx,zynq-devcfg-1.0";
+ reg = <0xf8007000 0x100>;
+ interrupt-parent = <&intc>;
+ interrupts = <0 8 4>;
+ clocks = <&clkc 12>;
+ clock-names = "ref_clk";
+ syscon = <&slcr>;
+ };
+
+ fpga_region0: fpga-region0 {
+ compatible = "fpga-region";
+ fpga-mgr = <&fpga_mgr0>;
+ #address-cells = <0x1>;
+ #size-cells = <0x1>;
+ ranges;
+ };
+
+DT Overlay contains:
+/dts-v1/ /plugin/;
+/ {
+fragment@0 {
+ target = <&fpga_region0>;
+ #address-cells = <1>;
+ #size-cells = <1>;
+ __overlay__ {
+ #address-cells = <1>;
+ #size-cells = <1>;
+
+ firmware-name = "zynq-gpio.bin";
+
+ gpio1: gpio@40000000 {
+ compatible = "xlnx,xps-gpio-1.00.a";
+ reg = <0x40000000 0x10000>;
+ gpio-controller;
+ #gpio-cells = <0x2>;
+ xlnx,gpio-width= <0x6>;
+ };
+ };
+};
+
+Device Tree Example: Full Reconfiguration to add PRR's
+======================================================
+
+The base FPGA Region is specified similar to the first example above.
+
+This example programs the FPGA to have two regions that can later be partially
+configured. Each region has its own bridge in the FPGA fabric.
+
+DT Overlay contains:
+/dts-v1/ /plugin/;
+/ {
+ fragment@0 {
+ target = <&fpga_region0>;
+ #address-cells = <1>;
+ #size-cells = <1>;
+ __overlay__ {
+ #address-cells = <1>;
+ #size-cells = <1>;
+
+ firmware-name = "base.rbf";
+
+ fpga-bridge@4400 {
+ compatible = "altr,freeze-bridge";
+ reg = <0x4400 0x10>;
+
+ fpga_region1: fpga-region1 {
+ compatible = "fpga-region";
+ #address-cells = <0x1>;
+ #size-cells = <0x1>;
+ ranges;
+ };
+ };
+
+ fpga-bridge@4420 {
+ compatible = "altr,freeze-bridge";
+ reg = <0x4420 0x10>;
+
+ fpga_region2: fpga-region2 {
+ compatible = "fpga-region";
+ #address-cells = <0x1>;
+ #size-cells = <0x1>;
+ ranges;
+ };
+ };
+ };
+ };
+};
+
+Device Tree Example: Partial Reconfiguration
+============================================
+
+This example reprograms one of the PRR's set up in the previous example.
+
+The sequence that occurs when this overlay is similar to the above, the only
+differences are that the FPGA is partially reconfigured due to the
+"partial-fpga-config" boolean and the only bridge that is controlled during
+programming is the FPGA based bridge of fpga_region1.
+
+/dts-v1/ /plugin/;
+/ {
+ fragment@0 {
+ target = <&fpga_region1>;
+ #address-cells = <1>;
+ #size-cells = <1>;
+ __overlay__ {
+ #address-cells = <1>;
+ #size-cells = <1>;
+
+ firmware-name = "soc_image2.rbf";
+ partial-fpga-config;
+
+ gpio@10040 {
+ compatible = "altr,pio-1.0";
+ reg = <0x10040 0x20>;
+ clocks = <0x2>;
+ altr,gpio-bank-width = <0x4>;
+ resetvalue = <0x0>;
+ #gpio-cells = <0x2>;
+ gpio-controller;
+ };
+ };
+ };
+};
+
+Constraints
+===========
+
+It is beyond the scope of this document to fully describe all the FPGA design
+constraints required to make partial reconfiguration work[1] [2] [3], but a few
+deserve quick mention.
+
+A persona must have boundary connections that line up with those of the partion
+or region it is designed to go into.
+
+During programming, transactions through those connections must be stopped and
+the connections must be held at a fixed logic level. This can be achieved by
+FPGA Bridges that exist on the FPGA fabric prior to the partial reconfiguration.
+
+--
+[1] www.altera.com/content/dam/altera-www/global/en_US/pdfs/literature/ug/ug_partrecon.pdf
+[2] tspace.library.utoronto.ca/bitstream/1807/67932/1/Byma_Stuart_A_201411_MAS_thesis.pdf
+[3] http://www.xilinx.com/support/documentation/sw_manuals/xilinx14_1/ug702.pdf