Add basic documentation stub

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>

2 files changed, 181 insertions, 0 deletions

diff --git a/docs/bootup.md b/docs/bootup.md
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+# System Bootup Process
+
+## Initial Ram Disk to Rootfs transition
+
+After mounting the root filesystem, either the kernel or the initial ram disk
+startup process is expected to exec the init program from the root filesystem.
+
+At the current time, there is no support for re-scanning the service files
+*yet*, so when init is started, the final configuration in `/etc/init.d` has to
+be present. As a result, we currently cannot perform mounting of `/etc/` or
+packing init into the initial ram disk and doing the rootfs transition.
+
+Also, as a result of this, changing the service configuration requires a system
+reboot to be effective.
+
+This _will_ change in the future.
+
+
+## Processing Service Descriptions
+
+The init process reads service description files from `/etc/init.d` which are
+usually symlinks to actual files in `/usr/share/init`.
+
+The exact locations may be changed through configure flags when compiling init.
+
+Service files specify a *target* which is basically like a SystemV runlevel and
+can be one of the following:
+
+* boot
+* reboot
+* shutdown
+* ctrlaltdel
+
+After parsing the configuration files, the init process starts running the
+services for the `boot` target in a topological order as determined by their
+*before* and *after* dependencies.
+
+Services can be of one of the following *types*:
+
+* wait
+* once
+* respawn
+
+Services of type `wait` are started exactly once and the init process waits
+until they terminate before continuing with other services.
+
+The type `once` also only runs services once, but immediately continues
+starting other services in the mean time without waiting.
+
+Services of type `respawn` also don't stall the init process and are re-started
+whenever they terminate.
+
+## Service Process Setup
+
+If a service description contains only a single `exec` line, the init process
+forks and then execs the command directly in the child process.
+
+If the service description contains a `tty` field, the specified device file
+is opened in the child process and standard I/O is redirected to it before
+calling exec. Also, a new session is created.
+
+
+If a service description contains multiple `exec` lines, the init process forks
+off to a single child process that does the same setup as above, and then runs
+the command lines sequentially by forking a second time for each one, followed
+by an exec in the grand child and a wait in the original child.
+
+If a single command line returns something other than `EXIT_SUCCESS`,
+processing of multiple command lines is immediately stopped and the offending
+exit status is returned to init.
+
diff --git a/docs/services.md b/docs/services.md
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+# Service Files
+
+Services that can be started and managed by init are described by service
+description files stored in `/usr/share/init`.
+
+The init process actually reads from `/etc/init.d` which contains symlinks to
+the actual service files.
+
+Enabling a service means adding a symlink, disabling means removing a symlink.
+
+Service descriptions can be parameterized. The arguments are extracted from the
+name of the symlink. Currently only 1 parameter is supported. The argument
+value is separated from the service name by an '@' character in the symlink
+name.
+
+
+
+Below is an annotated example for a simple, service description for a
+generic, parameterized agetty service:
+
+    #
+    # The text that init should print out when the status of the
+    # service changes.
+    #
+    # The '%0' is replaced with the first argument extracted from the
+    # symlink name.
+    #
+    description "agetty on %0"
+
+    #
+    # How to run the service. 'respawn' means restart the service when it
+    # terminates, 'once' means run it only once and continue with other
+    # services in the mean while, 'wait' means run it once, but block until
+    # it exits.
+    #
+    type respawn
+
+    #
+    # When to start the service. 'boot' means when booting the system. Other
+    # options are 'reboot', 'shutdown' and 'ctrlaltdel'. The system always
+    # starts into the 'boot' target and then later transitions to one of the
+    # others.
+    #
+    target boot
+
+    #
+    # A list of service names that must be started before this service can
+    # be run, i.e. this services needs to be started after those.
+    #
+    # This can only refer to generic names, not specific instances. For
+    # instance, you can say "after getty" to make sure a service comes up after
+    # all gettys are started, but you cannot specify "after agetty@tty1".
+    #
+    # Similar to 'after', there is also a 'before' keyword for specifying
+    # dependencies.
+    #
+    after sysinit
+
+    #
+    # The 'tty' directive specifies a file to which all I/O of the process is
+    # redirected. The specified device file is used as a controlling tty for
+    # the process and a new session is created with the service process as
+    # session leader.
+    #
+    # In this example, we derive the controlling tty from the service
+    # description argument.
+    #
+    tty "/dev/%0"
+    
+    #
+    # The 'exec' directive specifies the command to execute in order to start
+    # the service. See in the example below on how to run multiple commands.
+    #
+    # Again we use the argument to specify what terminal our getty
+    # should run on.
+    #
+    exec agetty %0 linux
+
+As can be seen in this simple example, each line in a service description is
+made up of a keyword, followed by one or more arguments and terminated by a
+line break.
+
+Blank lines are ignored and shell-style comments can be used.
+
+Arguments are separated by space. Quotation marks can be used to treat
+something containing spaces or comment character as a single argument.
+
+In between quotation marks, C-style escape sequences can be used.
+
+Argument substitution (arguments derived from the symlink name) can be
+done using a '%' sign, followed by the argument index. A '%' sign can be
+escaped by writing '%%'.
+
+
+If a service should sequentially run multiple commands, they can be grouped
+inside braces as can be seen in the following, abbreviated example:
+
+    description "mount /var"
+    type wait
+    target boot
+    before vfs
+    exec {
+        mount -t tmpfs none /var
+        mkdir /var/log -m 0755
+        mkdir /var/spool -m 0755
+        mkdir /var/lib -m 0755
+        mkdir /var/tmp -m 0755
+        mount --bind /cfg/preserve/var_lib /var/lib
+    }
+