Hi, RFC: Post-build control of serial console
It is annoying to have to create and maintain two completely different builds of coreboot just to enable or disable the console. It would be much more convenient to have a 'silent' flag in the image, which can be updated as needed, without needing to rebuild coreboot. For example, if something goes wrong and coreboot hangs, it would be nice to be able to enable serial on the same image, then boot it again to see how far it gets. I propose a 'Coreboot Control Block' (CCB) which can hold a small number of such settings. It is designed to be available very early in bootblock, before CBFS is ready. It is able to control the output of the very first bootblock banner. early silicon-init, etc. It is built as part of the bootblock image, so can be accessed simply as a static C struct within the bootblock code. That means that the code overhead is very low and we could perhaps enable it by default. The bootblock can have a CBFS file attribute that indicates that it contains a CCB and the offset where it is stored. Other coreboot stages can read this as well, or it could be duplicated in a separate file. We can provide options in cbfstool to get and set settings in the CCB. This makes it easy to use this feature, e.g. to enable silent: cbfstool coreboot.rom ccb-set -n serial -V silent Why not use a separate CBFS file? - Boards typically read the entire bootblock and start execution from it. The console is started early so the settings are needed before CBFS is available. By putting the CCB inside the bootblock, it can control things from an early stage. Why not CMOS RAM / VVRAM? - If we allocate some space in CMOS for console / logging settings, then it would allow a similar feature. But it involves changing settings on the device. Each board would need to provide some CMOS options for this feature as part of the layout file. It would not be possible to enable console output without running some code on the device to update the CMOS RAM. Why not use Firmware Handoff to pass the CCB to following stages? - We could do that, particularly if CCB attracts some additional features, such as logging. Regards, Simon _______________________________________________ coreboot mailing list -- coreboot@coreboot.org To unsubscribe send an email to coreboot-le...@coreboot.org
