dtc.1

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.\" Copyright (c) 2013 David Chisnall
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.\" Cambridge Computer Laboratory under DARPA/AFRL contract (FA8750-10-C-0237)
.\" ("CTSRD"), as part of the DARPA CRASH research programme.
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.Dd February 26, 2022
.Dt DTC 1
.Os
.Sh NAME
.Nm dtc
.Nd device tree compiler
.Sh SYNOPSIS
.Nm
.Op Fl @fhsv
.Op Fl b Ar boot_cpu_id
.Op Fl d Ar dependency_file
.Op Fl i Ar include_path
.Op Fl E Ar [no-]checker_name
.Op Fl H Ar phandle_format
.Op Fl I Ar input_format
.Op Fl O Ar output_format
.Op Fl o Ar output_file
.Op Fl R Ar entries
.Op Fl S Ar bytes
.Op Fl p Ar bytes
.Op Fl V Ar blob_version
.Op Fl W Ar [no-]checker_name
.Op Fl P Ar predefined_properties
.Ar input_file
.Sh DESCRIPTION
The
.Nm
utility converts between flattened device tree (FDT) representations.
It is most commonly used to generate device tree blobs (DTB), the binary
representation of an FDT, from device tree sources (DTS), the ASCII text source
representation.
.Pp
The binary can be written in two formats, binary and assembly.
The binary is identical to the in-memory representation and can be used
directly by firmware, loaders, and so on.
The assembly format, documented in
.Sx "ASM FORMAT" ,
will produce the same binary format when assembled, but also includes some
global variables that refer to parts of the table.
This format is most commonly used to produce a kernel specific to a device,
with the device tree blob compiled in.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl d Ar dependency_file
Writes a dependency file understandable by make to the specified file.
This file can be included in a Makefile and will ensure that the output file
depends on the input file and any files that it includes.
This argument is only useful when the input is DTS, as only the source format
has a notion of inclusions.
.It Fl i Ar include_path
Adds a path to search for include files.
.It Fl E Ar [no-]checker_name
Enable or disable a specified checker.
The argument is the name of the checker.
The full list of checkers is given in
.Sx CHECKERS .
.It Fl @
Emit a __symbols__ node to allow plugins to be loaded.
.It Fl f
Force the tool to attempt to generate the output, even if the input had errors.
.It Fl h
Display the help text and exit.
.It Fl H Ar phandle_format
Specifies the type of phandle nodes to generate in the output.
Valid values are:
.Pp
.Bl -tag -width indent -compact
.It Ar linux
Generate the legacy linux,phandle nodes expected by older systems.
.It Ar epapr
Generate the phandle nodes, as described in the ePAPR specification.
This is the most sensible option for device trees being used with
.Fx .
.It Ar both
Generate both, for maximum compatibility.
.El
.It Fl I Ar input_format
Specifies the input format.
Valid values are:
.Pp
.Bl -tag -width indent -compact
.It Ar dtb
Device tree blob.
The binary representation of the FDT.
.It Ar dts
Device tree source.
The ASCII representation of the FDT.
This is the default if the input format is not explicitly stated.
.El
.It Fl O Ar output_format
Specifies the output format.
Valid values are:
.Pp
.Bl -tag -width indent -compact
.It Ar asm
Assembler source for generating a device tree blob, as described in
.Sx "ASM FORMAT" .
.It Ar dtb
Device tree blob.
The binary representation of the FDT.
This is the default if the output format is not explicitly stated.
.It Ar dts
Device tree source.
The ASCII representation of the FDT.
.El
.It Fl o Ar output_file
The file to which to write the output.
.It Fl P Ar predefined_macro
Defines a macro, in the form
.Ar name=value
or
.Ar name
to be used for device tree source files that contain conditional components.
This tool supports two extensions to the standard to support conditional
compilation of device trees.
The first is an
.Ar /include/if [property]/ "file.dts"
directive that is allowed at the start of a file and which will only include
the specified file if it the specified property is passed with this flag.
The second is the
.Ar $NAME
format for property values.
These allow property value to be specified on the command line.
.It Fl R Ar entries
The number of empty reservation table entries to pad the table with.
This is useful if you are generating a device tree blob for bootloader or
similar that needs to reserve some memory before passing control to the
operating system.
.It Fl S Ar bytes
The minimum size in bytes of the blob.
The blob will be padded after the strings table to ensure that it is the
correct size.
This is useful for environments where the device tree blob must be modified in
place.
.It Fl p Ar bytes
The number of bytes of padding to add to the blob.
The blob will be padded after the strings table to ensure that it is the
correct size.
This is useful for environments where the device tree blob must be modified in
place.
.It Fl W Ar [no-]checker_name
Enable or disable a specified checker.
This is an alias for
.Fl E .
.It Fl s
Sorts the properties and nodes in the tree.
This is mainly useful when using tools like
.Xr diff 1
to compare two device tree sources.
.It Fl V Ar output_version
The version of the format to output.
This is only relevant for binary outputs, and only a value of 17 is currently
supported.
.It Fl v
Display the tool version and exit.
.It Ar input_file
The source file.
.El
.Sh "ASM FORMAT"
The assembly format defines several globals that can be referred to from other
compilation units, in addition to any labels specified in the source.
These are:
.Pp
.Bl -tag -width "dt_strings_start" -compact -offset indent
.It dt_blob_start
start of the device tree blob.
.It dt_header
start of the header, usually identical to the start of the blob.
.It dt_reserve_map
start of the reservation map.
.It dt_struct_start
start of the structure table.
.It dt_struct_end
end of the structure table.
.It dt_strings_start
start of the strings table.
.It dt_strings_end
end of the strings table.
.It dt_blob_end
end of the device tree blob.
.El
.Sh CHECKERS
The utility provides a number of semantic checks on the correctness of the
tree.
These can be disabled with the
.Fl W
flag.
For example,
.Fl W Ar no-type-phandle
will disable the phandle type check.
The supported checks are:
.Pp
.Bl -tag -width "no-type-phandle" -compact -offset indent
.It type-compatible
Checks the type of the
.Va compatible
property.
.It type-model
Checks the type of the
.Va model
property.
.It type-compatible
Checks the type of the
.Va compatible
property.
.It cells-attributes
Checks that all nodes with children have both
.Va #address-cells
and
.Va #size-cells
properties.
.It deleted-nodes
Checks that all
.Va /delete-node/
statements refer to nodes that are merged.
.El
.Sh OVERLAYS
The utility provides support for generating overlays, also known as plugins.
Overlays are a method of patching a base device tree that has been compiled with
the
.Fl @
flag, with some limited support for patching device trees that were not compiled
with the
.Fl @
flag.
.Pp
To denote that a DTS is intended to be used as an overlay,
.Va /plugin/\&;
should be included in the header, following any applicable
.Va /dts-v1/\&;
tag.
.Pp
Conventional overlays are crafted by creating
.Va fragment
nodes in a root.
Each fragment node must have either a
.Va target
property set to a label reference, or a
.Va target-path
string property set to a path.
It must then have an
.Va __overlay__
child node, whose properties and child nodes are merged into the base device
tree when the overlay is applied.
.Pp
Much simpler syntactic sugar was later invented to simplify generating overlays.
Instead of creating targeted fragments manually, one can instead create a root
node that targets a label in the base FDT using the
.Va &label
syntax supported in conventional DTS.
This will indicate that a fragment should be generated for the node, with the
given
.Va label
being the target, and the properties and child nodes will be used as the
__overlay__.
.Pp
Additionally, a path-based version of this syntactic sugar is supported.
A root node may target a path in the base FDT using a name of the form
.Va &{/path} .
A fragment will be generated for the node as it is in the
.Va &label
case, except the
.Va target-path
property will be set to
.Va /path
and no
.Va target
will be set.
.Pp
Both conventional overlays and the later-added syntactic sugar are supported.
.Pp
Overlay blobs can be applied at boot time by setting
.Va fdt_overlays
in
.Xr loader.conf 5 .
Multiple overlays may be specified, and they will be applied in the order given.
.Sh NODE OMISSION
This utility supports the
.Va /omit-if-no-ref/
statement to mark nodes for omission if they are ultimately not referenced
elsewhere in the device tree.
This may be used in more space-constrained environments to remove nodes that may
not be applicable to the specific device the tree is being compiled for.
.Pp
When the
.Fl @
flag is used to write symbols, nodes with labels will be considered referenced
and will not be removed from the tree.
.Sh EXAMPLES
The command:
.Pp
.Dl "dtc -o blob.S -O asm device.dts"
.Pp
will generate a
.Pa blob.S
file from the device tree source
.Pa device.dts
and print errors if any occur during parsing or property checking.
The resulting file can be assembled and linked into a binary.
.Pp
The command:
.Pp
.Dl "dtc -o - -O dts -I dtb device.dtb"
.Pp
will write the device tree source for the device tree blob
.Pa device.dtb
to the standard output.
This is useful when debugging device trees.
.Pp
The command:
.Pp
.Dl "dtc -@ -O dtb -I dts -o device.dtb device.dts"
.Pp
will generate a
.Pa device.dtb
file from the device tree source
.Pa device.dts
with a __symbols__ node included so that overlays may be applied to it.
.Pp
The command:
.Pp
.Dl "dtc -@ -O dtb -I dts -o device_overlay.dtbo device_overlay.dts"
.Pp
will generate a
.Pa device_overlay.dtbo
file, using the standard extension for a device tree overlay, from the device
tree source
.Pa device_overlay.dts .
A __symbols__ node will be included so that overlays may be applied to it.
The presence of a
.Va /plugin/\&;
directive in
.Pa device_overlay.dts
will indicate to the utility that it should also generate the underlying
metadata required in overlays.
.Sh COMPATIBILITY
This utility is intended to be compatible with the device tree compiler
provided by elinux.org.
Currently, it implements the subset of features
required to build
.Fx
and others that have been requested by
.Fx
developers.
.Pp
The
.Ar fs
input format is not supported.
This builds a tree from a Linux
.Pa  /proc/device-tree ,
a file system hierarchy not found in
.Fx ,
which instead exposes the DTB directly via a sysctl.
.Pp
The warnings and errors supported by the elinux.org tool are not documented.
This tool supports the warnings described in the
.Sx CHECKERS
section.
.Sh SEE ALSO
.Xr fdt 4
.Sh STANDARDS
The device tree formats understood by this tool conform to the Power.org
Standard for Embedded Power Architecture Platform Requirements
.Pq Vt ePAPR ,
except as noted in the
.Sx BUGS
section and with the following exceptions for compatibility with the elinux.org
tool:
.Pp
.Bl -bullet -compact
.It
The target of cross references is defined to be a node name in the
specification, but is in fact a label.
.El
.Pp
The /include/ directive is not part of the standard, however it is implemented
with the semantics compatible with the elinux.org tool.
It must appear in the top level of a file, and imports a new root definition.
If a file, plus all of its inclusions, contains multiple roots then they are
merged.
All nodes that are present in the second but not the first are imported.
Any that appear in both are recursively merged, with properties from the second
replacing those from the first and properties child nodes being recursively
merged.
.Sh HISTORY
A dtc tool first appeared in
.Fx 9.0 .
This version of the tool first appeared in
.Fx 10.0 .
.Sh AUTHORS
.Nm
was written by
.An David T. Chisnall .
Some features were added later by
.An Kyle Evans .
.Pp
Note: The fact that the tool and the author share the same initials is entirely
coincidental.
.Sh BUGS
The device tree compiler does not yet support the following features:
.Pp
.Bl -bullet -compact
.It
Labels in the middle of property values.
This is only useful in the assembly output, and only vaguely useful there, so
is unlikely to be added soon.
.It
Full paths, rather than labels, as the targets for phandles.
This is not very hard to add, but will probably not be added until something
actually needs it.
.El
.Pp
The current version performs a very limited set of semantic checks on the tree.
This will be improved in future versions.