Kernel Compile

[Cazzimma]

Question 1.
I am following the serveral tutorials for recompile source kernel.
(for example http://newbiedoc.sourceforge.net/system/kernel-pkg.html )
All goes ok BUT when i modify a single files
(for example just fs.h)
and i redo all the steps, it recompiles ALL the .o and all the files in the kernel without "touching" only the modified file.
why is that ? am i missing something? Is it normal ?

Question 2.
I got some kernel code how can i know for which kernel version has been written?
I ask this because this code is working for sure but it wont compile on my kernel version so I have reasons to believe that it is written for some particular kernel versions
(example i am using the 2.6.38.8, is it possible that the code suits for the 2.6.3x.y ? what is for sure that the code is written in 2.6.z.w)

p.s. i'm very sorry about my bad english :D

[dino99]

kernels ready to use:
http://kernel.ubuntu.com/~kernel-ppa/mainline/

download into an empty folder:
headers-all + header-i386 (or amd64) +image-i386 (or amd64)
then goto that folder and run: sudo dpkg -i *

kernel to compile:
https://help.ubuntu.com/community/Kernel/Compile

[Cazzimma]

@Dino that link is great and very useful. But how can it solve my problem? i need to recompile kernel because i changed some stuffs on kernel-side code. Why should i need pre-compiled images ?

Pheraps i did not explain well my problem.

Problem 1 "it recompiles all the .c again and again" :
t1 : compile kernel source.
t2 : compilation succesfull.
t3 : mount image ...
t4 : all happy.
t5 : modify kernel source. (a single file for example open.c )
t6 : compile ALL files again. (not only open.c but everithing) --> waste of time.

I am just doing the steps of
https://help.ubuntu.com/community/Kernel/Compile
this very guide.

why it recompiles all the files even if i modified only one??