Hi folks, Recently I was trying to build a kernel with the following options: CONFIG_KALLSYMS_ABSOLUTE_PERCPU=n CONFIG_KALLSYMS_BASE_RELATIVE=n They are defined in init/Kconfig like so: config KALLSYMS_ABSOLUTE_PERCPU bool depends on KALLSYMS default X86_64 && SMP config KALLSYMS_BASE_RELATIVE bool depends on KALLSYMS default y I was banging my head against the wall trying to understand why I couldn't edit these with any kconfig tool (e.g. make xconfig) and why my edits to the .config file were getting overwritten. A coworker finally explained to me that the reason was that the config options did not have a prompt, and so they are not user configurable. This is a really subtle behavior that was not obvious to me from the documentation. Though now I understand it, I can see why it's used widely. For example, KALLSYMS_ABSOLUTE_PERCPU has no prompt and is only set when X86_64 && SMP, because it is a trick that allows percpu variables to be encoded while still only using 32 bits to encode each address. This approach requires treating the 32-bit value as signed, which limits the range of addresses that could be encoded in other situations for other architectures, so it is implemented as a config option that is selected only where necessary: very clever. Since this is real behavior that's used for real purposes, let's document it here. I could only find oblique references in the document regarding config entries without prompts: - Describing the "visible" keyword: "It is similar to a conditional “prompt” attribute for individual menu entries." This implies preexisting knowledge about prompts that was never explained. - Describing the select keyword: "In general use select only for non-visible symbols (no prompts anywhere) and for symbols with no dependencies." This does spell out that symbols become invisible without prompts, but it's in an unrelated spot where a reader wouldn't expect to find it. It also isn't clear that "invisible" symbols can't even be altered by tweaking ".config". - Describing the default keyword: "If an input prompt is visible the default value is presented to the user and can be overridden by him." This doesn't state anything about a missing prompt, though maybe it invites the reader to assume the inverse, if a prompt is not set, the value cannot be overridden. Thanks, Stephen Stephen Brennan (1): Documentation: kbuild: explicitly document missing prompt Documentation/kbuild/kconfig-language.rst | 3 +++ 1 file changed, 3 insertions(+) -- 2.43.5
