Hello,
I'm on Slackware 14.1

What I'm trying to do: 'make O=/my/build/folder' from /usr/src/linux-3.10.17, as a normal user.
I'm using a slightly modified config-generic-smp as .config

First error I get is about the source directory no being clean and that I need to 'make mrproper', but of course I can only do that as root. I figure there must be a good reason the kernel-source package ships in an unclean state.

If I run 'make mrproper' as root, then running 'make O=/my/build/folder' afterwards(as non-root) works for a while but then fails because of a missing header 'hz.h'
Turns out that 'make mrproper' deletes a bunch of headers in ./include/config so I had to reinstall the kernel-source package.

I also tried from a copied source directory (cp -s -r ...) so I could run 'make mrproper' as non-root and then recopied those headers in ./include/config but it still complains that mrproper should be run.

At this point, I'm stumped.

Why not make life easier and copy /usr/src/linux-3.10.17 content in your /my/build/folder, then change permissions and compile from here

I did, the problem is those missing headers in include/config, the ones that get deleted by 'make mrproper'. Are they supposed to be regenerated? Because they are not.

Here is make's output tail I get:
...
CC kernel/fork.o
CC kernel/exec_domain.o
CC kernel/panic.o
CC kernel/printk.o
CC kernel/cpu.o
CC kernel/exit.o
CC kernel/itimer.o
make[2]: *** No rule to make target `/tmp/build/kernel/include/config/hz.h', needed by `kernel/hz.bc'. Stop.
make[1]: *** [kernel] Error 2
make: *** [sub-make] Error 2

Ok so I did a real copy of the source (cp -r instead of cp -s -r) and it's compiling fine now.
Thanks.
Still irks me that I have to do a whole copy when it's already installed once.

You don't have to. Why not just customize the LOCALVERSION in "General setup"?

I always do that. I added '-custom-'. If I didn't, 'make modules_install' would overwrite the modules installed by the 'kernel-modules-smp' package, which is a bad. I learned that the hard way years ago.
Not sure what you're getting at. That setting doesn't affect where Make looks for headers.

Actually I don't understand why you need to use "make O=". Why not build the kernel and modules in place?

The whole point of the 'O=' functionality is to allow making kernels builds with different .config, which allows minimal rebuild times when you tweak each.
For example I could do
'make O=/my/build/folder1' where LOCALVERSION is set to 'custom1'
'make O=/my/build/folder2' where LOCALVERSION is set to 'custom2'

Because of the errors I get with 'make O=', instead I have to make a whole source copy for each config and build in place.
Follow me? I don't know how to explain it better.

I never used O option, but personally I prefer don't touch /usr/src/linux* dir, as I consider it like system dir (distribution kernel source). I used to always compile custom kernels as user in separate directory (I think I read somewhere a suggestion by Linus Torvalds to not compile kernels in /usr/src)

Now if I read the Makefile about O option:
At this point, I don't know how mrproper would deal with this, no experience on that

If you've done an in-tree build (one without 'O=/somewhere') then you can't use that source tree to build another kernel. You have to do a 'make mrproper' on it first, but once you've done that 'make mrproper' you can use the tree for any number of builds if they all use 'O=/somewhere-unique'.

The reason Pat ships /usr/src/linux in an unclean state is that its contents are required if you ever need to build out-of-tree kernel modules for use with the stock kernel. If he shipped pristine sources, then you wouldn't be able to do that. Unless that is Pat himself started building the stock kernels with O=/somewhere in order to preserve /usr/src/linux.

O= is very useful if you're building from kernel sources under the control of git. Without it a 'git checkout -f' would delete the results of your last build. If you've used O= you can switch back and forth at will, without having to give each build its own copy of the kernel sources.

I use /usr/obj/$VERSION$LOCALVERSION for my O= build directories. Once built, I do a 'make O=... clean && make O=... prepare' on the builddir to reclaim a little disk space.

I use a build script that does the build and nicely packages all this up for me. I end up
with a single package that looks something like this:
May seem over-engineered to some, but I've been using this for around 4 years now and it works well for me, but we each have our own ways of doing things.

Good to know, thanks.

You can never automate too much, time is so precious