In order for the OpenEmbedded build system to create an image or any target, it must be able to access source files. The general Yocto Project Development Environment figure represents source files using the "Upstream Project Releases", "Local Projects", and "SCMs (optional)" boxes. The figure represents mirrors, which also play a role in locating source files, with the "Source Mirror(s)" box.
The method by which source files are ultimately organized is a function of the project. For example, for released software, projects tend to use tarballs or other archived files that can capture the state of a release guaranteeing that it is statically represented. On the other hand, for a project that is more dynamic or experimental in nature, a project might keep source files in a repository controlled by a Source Control Manager (SCM) such as Git. Pulling source from a repository allows you to control the point in the repository (the revision) from which you want to build software. Finally, a combination of the two might exist, which would give the consumer a choice when deciding where to get source files.
BitBake uses the
SRC_URI
variable to point to source files regardless of their location.
Each recipe must have a SRC_URI
variable
that points to the source.
Another area that plays a significant role in where source files
come from is pointed to by the
DL_DIR
variable.
This area is a cache that can hold previously downloaded source.
You can also instruct the OpenEmbedded build system to create
tarballs from Git repositories, which is not the default behavior,
and store them in the DL_DIR
by using the
BB_GENERATE_MIRROR_TARBALLS
variable.
Judicious use of a DL_DIR
directory can
save the build system a trip across the Internet when looking
for files.
A good method for using a download directory is to have
DL_DIR
point to an area outside of your
Build Directory.
Doing so allows you to safely delete the Build Directory
if needed without fear of removing any downloaded source file.
The remainder of this section provides a deeper look into the source files and the mirrors. Here is a more detailed look at the source file area of the base figure: