fdt — Flattened
Device Tree support
options FDT
makeoptions FDT_DTS_FILE=<board name>.dts
options FDT_DTB_STATIC
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.
- 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.
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).
There is a couple of options for managing
fdt support at the FreeBSD
kernel level.
- 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. The indicated DTS file will be converted (compiled) into a binary
form along with building the kernel itself. The DTS file name is relative
to the default location of DTS sources i.e.
sys/dts. 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).
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).
The fdt support first appeared in
FreeBSD 9.0.
The fdt support was developed by Semihalf
under sponsorship from the FreeBSD Foundation. This manual page was written
by Rafal Jaworowski.