How are Linux kernel compile config's determined?

Question

I am compiling my first Kernel (3.5 rc1) from source through menuconfig.

Certain configuration options are pre-set.

• Who / what determines if they are pre-set?
• Does the make menuconfig somehow detect my computer and its devices and characteristics and generates them?
• Or do the default configs come with the source, pre-determined by someone (who put the source out)?

Answer 1

make menuconfig doesn't dynamically determine your environment and tries to set the appropriate config but uses your .config file and the default entries in kconfig.

So yes the defaults come with the source and are specified in the kconfig files which also specify the help text, dependencies and other things. Have a look at a sample kconfig file like net/Kconfig.

make localmodconfig on the other hand tries to create a custom tailored kernel configuration for your system based on the loaded modules. It takes your current configuration (typically from your distribution) and will only enable the loaded modules.