build/docs/build.md

# The Pygos Build System

The Pygos system can be built by running the `mk.sh` shell script with
the desired product configuration as argument.


The shell script can be run from anywhere on the file system. All
configuration files and scripts are accessed relative to the source location
of the script and all generated files are accessed relative to the current
working directory.

Actually it is even strongly encouraged to run the build system from outside
the git tree to have the generated files cleanly separated from the build
system.


A second script named `check_update.sh` is provided to automatically query
all upstream package sources to check if newer versions are available.


The `mk.sh` creates a `download` and a `src` directory. In the former it stores
downloaded package tar balls, in the later it extracts the tar balls and
applies patches.

For all other files and directories, a sub directory named after the product
configuration is created. Throughout the build system, this directory is
referred to as *build root*.

Inside the build root a `deploy` directory is created. Build output for each
package is deployed to a sub directory named after the package.

The cross toolchain is stored in `<build root>/toolchain`.

Outputs and diagnostic messages of the build processes are stored in
`<build root>/toolchain/log/<package>-<stage>.log`.


## Package Build Scripts

The directory `pkg` contains a sub directory for each package. Each package
directory is expected to contain a shell script named `build`.

The build script is expected to set the following variables:

* `VERSION` containing a package version number.
* `URL` containig a URL from which to download a source tar ball.
* `TARBALL` containing the name of the source tar ball. This is appended to
  the URL to download the package.
* `SHA256SUM` containing the SHA-256 check sum of the source tar ball.
* `SRCDIR` containing the name of the source directory unpacked from the
  tar ball.
* `DEPENDS` containing a space separated list of packages that the package
  in question depends on. Those packages are built first. Their headers and
  libraries are copied into the cross toolchain before building the current
  package and removed after building it.


Using the specified variables, the build system automatically downloads,
verifies and unpacks the source tar balls (unless that has already been done)
and determines the order in which to build the packages.


The `build` script is also expected to implement the following functions:

* `prepare` is run after unpacking the source tar ball. The current working
  directory is set to the source directory. The path to the package directory
  is passed as first argument, so the function can easily access patch files
  stored in the package directory. All output and error messages from the
  script are stored in `<packagename>-prepare.log`.
* `build` is run to compile the package. The current working directory is a
  temporary directory inside the build root directory. The source directory
  is passed as first argument. The second argument is a path to the *deploy*
  directory where generated files are installed. All standard output and error
  messages from the script are piped to `<packagename>-build.log`.
* `deploy` is run after compilation to install the build output to the deploy
  directory. Arguments and working directory are the same as for `build`. All
  output and error messages from the script are piped to
  `<packagename>-deploy.log`. The `deploy` function is also expected to
  generate a file named `rootfs_files.txt` that contains a listing of all files
  in the deploy directory that should be included in the root filesystem and
  what permissions should be set on them. Once the function returns,
  the `mk.sh` script strips everything installed to `bin` and `lib`, so the
  implementation doesn't have to do that. In fact `install-strip` Makefile
  targets should not be used since many implementations are broken for cross
  compilation. Further common steps are executed for packages that
  produce `libtool` archives and `pkg-config` files.
* `check_update` is only used by the `check_update.sh` script. It is supposed
  to find out if the package has a newer version available, and if so, echo it
  to stdout.


### Environment Variables

The `mk.sh` sets a number of shell variables that package scripts can use.

The following variables describe the target system and the build environment:

* `BOARD` contains the target board specified on the command line
* `PRODUCT` contains the product name specified on the command line
* `TARGET` specifies the host triplet of the target board
* `OS_NAME` is statically set to `Pygos`
* `OS_RELEASE` holds a version string generated using `git-describe`
* `NUMJOBS` contains the number of processors available for parallel builds
* `HOSTTUPLE` contains the host triplet of the machine that the build system
  is running on for compiling toolchain packages.
* `CMAKETCFILE` contains the absolute path to a CMake toolchain file that can
  be used for compiling CMake based packages with the cross toolchain.


And a number of variables containing special directories:

* `BUILDROOT` contains the absolute path to the build root directory, i.e. the
  output directory within the working directory of the `mk.sh` script.
* `SCRIPTDIR` contains the absolute path to the script directory, i.e. the git
  tree with the build system in it.
* `TCDIR` contains the absolute path to the cross toolchain directory.
* `PKGBUILDDIR` contains the absolute path of the temporary directory in which
  the package is being built.
* `PKGSRCDIR` contains the root directory of all unpacked package tar balls
* `PKGDEPLOYDIR` contains the root directory of all package deploy directories
* `PKGLOGDIR` holds the absolute path of the directory containing all log files
* `PKGDOWNLOADDIR` holds the absolute path of the directory containing all
  package tar balls

The cross toolchain directory containing the executable prefixed with `$TARGET-`
is also prepended to `PATH`.

### Utility Functions

Some utility functions are provided for common package build tasks:

* `apply_patches` can be used inside the `prepare` function to automatically
  apply patches stored in the package directory to the source tree.
* `strip_files` takes a list of files as argument and runs the cross toolchain
  strip program on those that are valid ELF binaries. If a directory is
  encountered, the function recursively processes the sub directory. Usually
  you don't need to use this. The `mk.sh` script uses this function to after
  the deploy step to process the `bin` and `lib` directories.
* `verson_find_greatest` can be used in `check_update` to find the largest
  version number from a list. The list of version numbers is read from stdin.
  Version numbers can have up to four dot separated numbers or characters.
* `run_configure` can be used to run `autoconf` generated `configure` scripts
  with all the required options set for cross compilation. Extra options can
  be to the options passed to `configure`.

## Configuration Files

The configuration for the build system is organized in layers, stored in
the `layer` directory in the git tree.

The configuration on how to build an image for a specific target is a file
in the `product` sub directory that specifies, what configuration layers
to use and how to stack them on top of each other. Layers that are further
down in the file override the ones before them.

From the layer configuartion, the build system itself merges (in layer
precedence order) and processes the following configuration files:

* `ROOTFS` contains a list of packages that should be built and installed to
  the root filesystem.
* `TOOLCHAIN` contains shell variables for the cross compiler toolchain.
  See below for more detail.
* `LDPATH` contains a list of directories where the loader should look
  for dynamic libraries.
* `INIT` contains shell variables configuring the init system. See below
  for more detail.

### Utility Functions

For working with configuration files, the following utility functions
can be used:

* `file_path_override` takes a file name and looks for the last layer that
  contains it. The absolute path of the first found file is `echo`ed.
* `cat_file_override` looks for the last layer that contains a file and
  prints it to standard output.
* `cat_file_merge` prints the content of a file to standard output, for every
  layer that contains the file, in layer precedence order.
* `include_override` includes a file using the `source` builtin from the last
  layer that contains the file.
* `include_merge` includes a file using the `source` builtin from every layer
  that contains the file, in layer precedence order.


### Toolchain Configuration

The toolchain configuration file contains a list of shell variables for
configuring the cross toolchain packages, as well as some other packages
that need to know information about the target system.

Currently, the following variables are used:

* `RELEASEPKG` contains the name of the release package to build to trigger a
  build of the entire system. Typically this package depends on the `rootfs`
  package, which in turn pulls all configured packages as dependencies. It gets
  built last and packages the root filesystem image and boot loader files in
  some device specific way, so they can be installed easily on the target
  hardware.
* `LINUXPKG` contains the name of the kernel package. There is a default
  package called 'linux' that builds a standard, main line LTS kernel. Other
  packages can be specified for building vendor kernels.
* `TARGET` specifies the target triplet for the cross toolchain, which is also
  the host triplet for packages cross compiled with autotools.
* `GCC_CPU` specifies the target processor for GCC.
* `GCC_EXTRACFG` extra configure arguments passed to GCC. For instance, this
  may contain FPU configuration for ARM targets.
* `MUSL_CPU` contains the target CPU architecture for the Musl C library.
* `LINUX_CPU` contains the value of the `ARCH` variable passed to the kernel
  build system. Used by the generic main line kernel package.
* `LINUX_TGT` contains the space seperated make targets for the generic,
  main line, LTS kernel package.
* `OPENSSL_TARGET` contains the target architecture for the OpenSSL package.


### Init System Configuration

The INIT configuration file contains a list of shell variables for configuring
the init system.

Currently, the following variables are used:

* `GETTY_TTY` contains a space separated list of ttys on which to start agetty
  on system boot.
* `HWCLOCK` is set to yes if the system has a hardware clock that the time
  should be synchronized with during system boot and shutdown. If set to
  anything else, the init system is configured to keep track of time using
  `ntpdate` and a file on persistent storage.
* `DHCP_PORTS` contains a space separated list of network interfaces on which
  to operate a DHCP client for network auto configuration.
* `SERVICES` contains a space separated list of raw service names to enable.
* `MODULES` contains a space seperated list of kernel modules that should be
  loaded during system boot.


For configuring network interfaces, a file `ifrename` exists that assigns
persistent, predictable names to network interfaces.

The default naming scheme of the Pygos system is to rename the Ethernet
interfaces installed on the board to *port<X>* where X is an index starting
with 0.

For each network interface, addresses, mtu, offloading, etc can be configured
in a file `interfaces/<name>`, where *name* is the interface name *after*
renaming.

If the files `nftables.rules` or `sysctl.conf` are found, they are copied to
the target system image and the coresponding services are enabled.

For more details, please refer to the not yet existing network documentation.

### Package Specific Configuration Files

Additional configuration files may be present that are used by various packages.

The following files are currently used (with default override behavior):

* `linux.config` contains the kernel build configuration. The same name is
  currently used by both the main line and the board specific vendor kernels.
* `dnsmasq.conf` is installed to `/etc` by the dnsmasq package.
* `unbound.conf` is installed to `/etc` by the unbound package.
* `dhcpcd.conf` is installed to `/etc` by the dhcpcd package.