1.14. Multi Image Support

Traditionally a single configuration only works for a single board. Sometimes even variants of a single board like different amount of memory require a new config. This has the effect that the number of defconfig files increases dramatically. All the configs have to be kept in sync manually. Multi Image Support solves this problem.

With Multi Image Support binaries for multiple boards can be generated from a single config. A single board can only support compilation with or without Multi Image Support. Multi Image Support exposes several user visible changes:

• In make menuconfig it becomes possible to select multiple boards instead of only one.
• The barebox-flash-image link is no longer generated since there is no single binary to use anymore.
• images are generated under images/. The build process shows all images generated at the end of the build.

1.14.1. Technical background

With Multi Image Support enabled, the main barebox binary (barebox.bin) will be shared across different boards. For board specific code this means that it has to test whether it actually runs on the board it is designed for. Typically board specific code has this:

static int efikamx_init(void)
{
        if (!of_machine_is_compatible("genesi,imx51-sb"))
                return 0;

        ... board specific code ...
}
device_initcall(efikamx_init);

Multi Image Support always uses PBL to generate compressed images. A board specific PBL image is prepended to the common barebox binary. The PBL image contains the devicetree which is passed to the common barebox binary to let the common binary determine the board type.

The board specific PBL images are generated from a single set of object files using the linker. The basic trick here is that the PBL objects have multiple entry points, specified with the ENTRY_POINT macro. For each PBL binary generated a different entry point is selected using the -e option to ld. The linker will throw away all unused entry points and only keep the functions used by a particular entry point.

The Multi Image PBL files can be disassembled with make <entry-function-name.pbl.S