libfdt: Don't search for the terminator in fdt_get_string() Unless FDT_ASSUME_MASK includes ASSUME_VALID_INPUT, fdt_get_string() finds the end of the string with memchr(), bounded by the end of the string block, which also checks that the string is terminated. A string block normally ends with a NUL, and then every string in it is terminated. When the caller does not ask for the string's length, check that byte instead, so that fdt_string() doesn't scan the string at all. This helps callers that look up many property names. With a 612 KiB tree on x86-64, built with gcc -O2 and the default FDT_ASSUME_MASK, calling fdt_string() for the name of each of its 17,899 properties takes 108 us instead of 160 us, not counting the walk that finds them. libufdt, which Android bootloaders use to apply overlays, calls fdt_string() at least once for every property it reads: tens of thousands of times to apply one overlay to a 445 KiB tree. Reviewed-by: David Gibson <david@gibson.dropbear.id.au> Signed-off-by: Kelvin Zhang <zhangkelvin@google.com> Message-ID: <CAOSmRzixCf479QyhmGuwBBZrRLTYqoaSUw88HKZnkPyEh6=gcw@mail.gmail.com> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
The source tree contains the Device Tree Compiler (dtc) toolchain for working with device tree source and binary files and also libfdt, a utility library for reading and manipulating the binary format.
dtc and libfdt are maintained by:
dtc and libfdt are built with Meson:
$ meson setup build $ meson compile -C build $ meson test -C build $ meson install -C build
meson configure build lists the build options, e.g. -Dpython=disabled to skip the Python library. See the Meson quick guide for more, such as installing with --destdir. scripts/install-deps.sh installs the packages needed on common systems.
Building with make still works but is deprecated.
A Python library wrapping libfdt is also available. To build this you will need to install swig and Python development files. On Debian distributions:
$ sudo apt-get install swig python3-dev
The library provides an Fdt class which you can use like this, from build/tests after running the tests:
$ PYTHONPATH=../pylibfdt python3
>>> import libfdt
>>> fdt = libfdt.Fdt(open('test_tree1.dtb', mode='rb').read())
>>> node = fdt.path_offset('/subnode@1')
>>> print(node)
124
>>> prop_offset = fdt.first_property_offset(node)
>>> prop = fdt.get_property_by_offset(prop_offset)
>>> print('%s=%s' % (prop.name, prop.as_str()))
compatible=subnode1
>>> node2 = fdt.path_offset('/')
>>> print(fdt.getprop(node2, 'compatible').as_str())
test_tree1
You will find tests in tests/pylibfdt_tests.py showing how to use each method. Help is available using the Python help command, e.g.:
$ cd build/pylibfdt $ python3 -c "import libfdt; help(libfdt)"
If you add new features, please check code coverage:
$ sudo apt-get install python3-coverage $ cd build/tests # It's just 'coverage' on most other distributions $ python3-coverage run ../../tests/pylibfdt_tests.py $ python3-coverage html # Open 'htmlcov/index.html' in your browser
The library can be installed with pip from a local source tree:
$ pip install . [--user|--prefix=/path/to/install_dir]
Or directly from a remote git repo:
$ pip install git+git://git.kernel.org/pub/scm/utils/dtc/dtc.git@main
The install depends on libfdt shared library being installed on the host system first. Generally, using --user or --prefix is not necessary and pip will use the default location for the Python installation which varies if the user is root or not.
You can also install everything with meson if you like, but pip is recommended.
To install both libfdt and pylibfdt you can use:
$ meson setup [--prefix=/path/to/install_dir] build $ meson install -C build
To disable building the python library, even if swig and Python are available, use:
$ meson setup -Dpython=disabled build
More work remains to support all of libfdt, including access to numeric values.