Linux – Compare two kernel config files from the menuconfig perspective

You have to use grep -r CONFIG_SND_SOC_MXS_SGTL5000.

Each of these config options just represents a #define macro. Many of them don't belong to a single file but instead are checked in multiple source files. CONFIG_64BIT for example appears in around 1k source code files.

I believe the picture is as in my last edit above; I just did not know enough about the process of reserving memory to realize when I asked the question: it all boiled down to the larger kernel having more of the drivers built in rather than modularized.

Incidentally, I did want to put this to some (remotely) practical use: I wanted to have as slim a kernel as I could, but still boot it without an initrd. I knew that

• the smaller kernel (moniker arch above) is very lightweight and fast to boot, but not without an initramfs;

• the larger kernel (slk) will boot without an initramfs, but takes longer.

So I made a compromise that I think I'll stick with for now. I took the two config files, call them large and small, and made sure that every unset config option in small is reflected in large.

First,

awk '/^# CO/ {print} small > staging'

grabs all lines in the small config file that are of the form

# CONFIG_BLAH is not set

and dumps them in a staging text file. Then,

for i in $(awk '{print $2}' staging) ; do sed -i "s/^$i=./# $i is not set/" large ; done

takes all lines from the config file large that set (via =y or =m) an option contained in staging and unsets them.

I then ran make oldconfig on the resulting file large in the kernel source directory and compiled. It all went through all right:

• the new kernel boots some 3 seconds faster

• without an initramfs

• is much smaller in the sense that its /lib/modules/<kernel>/modules.builtin got cut down in half, from 16K to 8K.

Those are not much to write home about, but as I said this was puzzling me.. I think I'm all set on the issue now.

Presumably the more straightforward thing to do would have been to figure out once and for all precisely which drivers I need on this machine in order to boot without an initramfs, but I'll leave that for some other day. Besides, playing off one config against the other was a fun exercise in its own right.