Provided by: freebsd-manpages_12.2-2_all 
NAME
fdt — Flattened Device Tree support
SYNOPSIS
options FDT
makeoptions FDT_DTS_FILE=<board name>.dts
options FDT_DTB_STATIC
DESCRIPTION
Flattened Device Tree is a mechanism for describing computer hardware resources, which
cannot be probed or self enumerated, in a uniform and portable way. The primary consumers
of this technology are embedded systems, where a lot of designs are based on similar chips,
but have different assignment of pins, memory layout, addresses bindings, interrupts routing
and other resources.
Configuration data, which cannot be self discovered in run-time, has to be supplied from
external source. The concept of a flattened device tree is a platform and architecture
independent approach for resolving such problems. The idea is inherited from Open Firmware
IEEE 1275 device-tree notion, and has been successfully adopted by the embedded industry.
The scheme works in the following way:
• Hardware platform resources are manually described in a human readable text source
format, where all non self-enumerating information is gathered.
• This source description is converted (compiled) into a binary object i.e. a flattened
device tree blob which is passed to the kernel at boot time.
• The kernel (driver) learns about hardware resources details and dependencies from this
[externally supplied] blob, which eliminates the need for embedding any information
about the underlying platform hardware resources in the kernel.
• The flattened device tree mechanism in principle does not depend on any particular
first-stage bootloader or firmware features. The only overall requirement for the
environment is to provide a complete device tree description to the kernel.
The fdt layer allows any platform code in the kernel to retrieve information about hardware
resources from a unified origin, which brings advantages to the embedded applications
(eliminates hard-coded configuration approach, enforces code to be data driven and
extensible) leading to easier porting and maintenance.
DEFINITIONS
Device tree source (DTS)
The device tree source is a text file which describes hardware resources of a
computer system in a human-readable form, with certain hierarchical structure
(a tree). The default location for DTS files in the FreeBSD source repository
is sys/dts directory.
Device tree blob (DTB)
The textual device tree description (DTS file) is first converted (compiled)
into a binary object (the device tree blob) i.e. the DTB, which is handed over
to the final consumer (typically kernel) for parsing and processing of its
contents.
Device tree compiler (DTC)
A utility program executed on the host, which transforms (compiles) a textual
description of a device tree (DTS) into a binary object (DTB).
Device tree bindings
While the device tree textual description and the binary object are media to
convey the hardware configuration information, an actual meaning and
interpretation of the contents are defined by the device tree bindings. They
are certain conventions describing definitions (encoding) of particular nodes
in a device tree and their properties, allowed values, ranges and so on. Such
reference conventions were provided by the legacy Open Firmware bindings,
further supplemented by the ePAPR specification.
BUILDING THE WORLD
In order for the system to support fdt it is required that FreeBSD world be built with the
WITH_FDT build knob supplied either via src.conf(5) or command line defined with -D.
This creates the user space dtc compiler and enables fdt support in loader(8).
BUILDING KERNEL
There is a couple of options for managing fdt support at the FreeBSD kernel level.
makeoptions DTS+=<board name>.dts
Specifies device tree source (DTS) files for a given kernel. The indicated
DTS files will be converted (compiled) into a binary form along with building
the kernel itself. Any DTS file names not written as an absolute path must be
specified relative to the default location of DTS sources i.e., sys/dts.
makeoptions DTSO+=<overlay name>.dtso
Specifies device tree source overlay (DTSO) files for a given kernel. Overlay
files will be built with the kernel as with the makeoption DTS described
above. Overlay files specified as relative paths will be relative to the
default location of DTS overlays for the platform being built i.e.,
sys/dts/arm/overlays.
options FDT The primary option for enabling fdt support in the kernel. It covers all low-
level and infrastructure parts of fdt kernel support, which primarily are the
fdtbus(4) and simplebus(4) drivers, as well as helper routines and libraries.
makeoptions FDT_DTS_FILE=<board name>.dts
Specifies a preferred (default) device tree source (DTS) file for a given
kernel. It will be built along with the kernel as if it were supplied via the
makeoption DTS described above. This makeoption is not mandatory unless
FDT_DTB_STATIC is also defined (see below).
options FDT_DTB_STATIC
Typically, the device tree blob (DTB) is a stand-alone file, physically
separate from the kernel, but this option lets statically embed a DTB file
into a kernel image. Note that when this is specified the FDT_DTS_FILE
makeoption becomes mandatory (as there needs to be a DTS file specified in
order to embed it into the kernel image).
SEE ALSO
fdtbus(4), openfirm(4), simplebus(4)
STANDARDS
IEEE Std 1275: IEEE Standard for Boot (Initialization Configuration) Firmware: Core
Requirements and Practices (Open Firmware).
Power.org Standard for Embedded Power Architecture Platform Requirements (ePAPR).
HISTORY
The fdt support first appeared in FreeBSD 9.0.
AUTHORS
The fdt support was developed by Semihalf under sponsorship from the FreeBSD Foundation.
This manual page was written by Rafal Jaworowski.