EFISTUB: If I have both CMDLINE configured in kernel, AND via efibootmgr, which one gets executed / takes precedence?

In arch/x86/Kconfig of the kernel tree it says for CMDLINE:

	  Enter arguments here that should be compiled into the kernel
	  image and used at boot time.  If the boot loader provides a
	  command line at boot time, it is appended to this string to
	  form the full kernel command line, when the system boots.

	  However, you can use the CONFIG_CMDLINE_OVERRIDE option to
	  change this behavior.

	  In most cases, the command line (whether built-in or provided
	  by the boot loader) should specify the device for the root
	  file system.

and for CMDLINE_OVERRIDE:

	  Set this option to 'Y' to have the kernel ignore the boot loader
	  command line, and use ONLY the built-in command line.

	  This is used to work around broken boot loaders.  This should
	  be set to 'N' under normal conditions.

So both commandlines will probably be used. I don't think an initramfs will normally interfere with the kernel commandline. In any case you can make sure you got what you wanted with cat /proc/cmdline.

Looking in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/init/main.c?h=v6.5-rc5 there's a function setup_command_line that seems to set up the built-in command line which is called after setup_boot_config

ok idk what that all was. Here's something more interesting:

In arch/x86/kernel/setup.c it says /* append boot loader cmdline to builtin */. I think that suggests that the builtin comes first. And I assume that the code that queries the command line scans left to right and selects the first instance of an option because there doesn't seem to be anywhere that "loads" args into some kind of structure.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/setup.c?h=v6.5-rc5#n972

#ifdef CONFIG_CMDLINE_BOOL
#ifdef CONFIG_CMDLINE_OVERRIDE
	strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
#else
	if (builtin_cmdline[0]) {
		/* append boot loader cmdline to builtin */
		strlcat(builtin_cmdline, " ", COMMAND_LINE_SIZE);
		strlcat(builtin_cmdline, boot_command_line, COMMAND_LINE_SIZE);
		strscpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
	}
#endif
#endif

I guess the best thing to do would be to run linux in QEMU with the EFI system that's provided by a third party thing and test it out.