Please make it clear for me, what is the difference of the ff:

make config
make defconfig

make oldconfig
make olddefconfig

Did you bother to search - this is documented all over the web. Or run "make help".

I do that. But still can't pin point. Please enlighten me.

Look at the Makefile (generated by ./configure.)

Then, actually read man make, where it talks about "make-targets."

See if you can find the words that you're looking for in the Makefile, and if you can then puzzle-out what they must mean.

I did make help. Correct me if I'm wrong.

make config : Ask me all options from the new kernel source.
make defconfig : It will auto answer the defaults for each options in new kernel source.
make oldconfig : Ask me the new options that is not present in current kernel.
make olddefconfig : It will auto answer the defaults for new options.
make menuconfig : Text baseed GUI to make some tweaks in .config file.

If I run olddefconfig and run make menuconfig and make some changes then save, it will take effect on .config the olddefconfig produced or it will create new config file?

It will take effect on .config the olddefconfig produced. That is if you copied your old .config into the new /use/src/linux directory and then ran the above commands.

Thanks. So it is safe to run olddefconfig/oldconfig rather than config/defconfig?

It is indeed safe to run olddefconfig. I typically run make silentoldconfig. My typical flow would be to:
1. backup my current .config
2. Change the symbolic link from my old kernel to my new kernel directory
3. cd into the new kernel directory
4. Copy the old .config into the new kernel directory
5. Make silentoldconfig or make olddefconfig
And then if you need to add options. Run make menuconfig->adjust/add options->save and then you are good to compile. Hopefully I didn’t sidestep your question too much.

Wow nice, I am doing same thing as you. I'm glad to found out lol. Thanks!

You’re welcome. Hope whatever you are doing works out.

There are even more (there were somewhere a readme I read a few years ago in the kernel directory)

I usually emerge new gentoo-sources
set the symlink to /usr/src/linux new
cd /usr/src/linux
zcat /proc/config.gz > .config (reduces a copy error)
make oldconfig
nano .config (to change the version string, as it is faster in my eyes, could be also done with menuconfig)
make --jobs 8 && make --jobs 8 modules_install (8 threads, so 8 jobs for make)
copy files, adapt bootloader by hand.
emerge nvidia-drivers (rebuild gpu driver)
reboot box and test new kernel

I think nconfig is the same as menuconfig. It just means "use ncurses for the menu". xconfig is graphical as is gconfig. I think xconfig is for KDE and gconfig for gtk-based desktops like gnome and xfce.

You should never just use config. It's only there for historical reasons, to show people what building a kernel used to be like in the bad old days. It asks hundreds of questions, and if (in a moment of inattention or tiredness) you give the wrong answer to any one of them, you have to go back again to the beginning. You can't just go back and change the error like you can in the menu-based methods.

Most-importantly, the kernel configuration parameters are stored in a hidden file, named (IIRC ...) .config.

You should make it your business to make an un-hidden, locked, backup copy of this file, say in /root, each and every time you make a configuration change ... or, "just because."

I-f ... ... you have this information, and know that it is always current, you can do nice things like ... restore a known-good configuration instantly, or use the diff command to conclusively identify the changes between any one and any other.

Thanks for the tip. I will surely put it in my mind.

I usually use these days from a working kernel

zcat /proc/config.gz > .config (reduces a copy error)

as there can not be any copy errors. And why else should I keep that option than enabled in the kernel
even when you build test kernels, it is kinda easy to know which was the last known good config.
I also tried keeping .config in unencrypted boot, but that is also a hassle. And most of the time I forgot to copy that .config file over to boot and name it properly