Linux – Safely changing Linux kernel config programmatically

configurationkernellinux-kernel

I'm writing a script that has to automatically change a number of Linux kernel configuration options based on an input file.
The easiest way to do this would of course be to edit the .config file directly, but this is discouraged and may cause issues.
I can't seem to find a way to do this "the right way" though.

"make help" doesn't show a way to change single options, nor does ./scripts/kconfig/conf –help. This file on kernel.org doesn't seem to have a way either.
Even Gentoo's Genkernel manually changes the .config file.

So, basically:
Is there a safe way to change kernel configuration options programmatically, complete with dependency tracking, etc.?
Can Kconfig be run manually to achieve this?

Best Answer

What I do is first:

make defconfig

Then append settings changes to .config followed by:

make olddefconfig

This should "fix up" any inconsistencies introduced in the configuration. YMMV since it may not fix them in the way you expect.

Related Solutions

Linux – Search through kernel make menuconfig configuration menu entries

When you press /, it says

Enter CONFIG_ (sub)string to search for (with or without "CONFIG_")

which means it's looking for the names of the options, not the labels of the options.

With Linux 3.3, I found your option using grep...

$ find . -name Kconfig -exec grep 'config.*PCI' {} + | grep DMA
./drivers/ide/Kconfig:config BLK_DEV_IDEDMA_PCI

And then opened the file ./drivers/ide/Kconfig to see more information

config BLK_DEV_IDEDMA_PCI
    bool
    select BLK_DEV_IDEPCI
    select BLK_DEV_IDEDMA_SFF

Since it doesn't have a tristate or bool line like the others, that suggests it doesn't appear in the menu.

Searching in the same file for BLK_DEV_IDEDMA_PCI, you can see lots of entries that refer to it, e.g.

config BLK_DEV_AMD74XX
    tristate "AMD and nVidia IDE support"
    depends on !ARM
    select IDE_TIMINGS
    select BLK_DEV_IDEDMA_PCI

So it looks like you're not supposed to enable DMA explicitly: the drivers that need DMA will enable it automatically.

Linux – Manual Kernel Build Results in Large Binary

Despite what file says, it turns out to be debugging symbols after all. A thread about this on the LKML led me to try:

make INSTALL_MOD_STRIP=1 modules_install

And low and behold, a comparison from within the /lib/modules/x.x.x directory; before:

> ls -hs kernel/crypto/anubis.ko 
112K kernel/crypto/anubis.ko

And after:

> ls -hs kernel/crypto/anubis.ko 
16K kernel/crypto/anubis.ko

More over, the total size of the directory (using the same .config) as reported by du -h went from 185 MB to 13 MB.

Keep in mind that beyond the use of disk space, this is not as significant as it may appear. Debugging symbols are not loaded during normal runtime, so the actual size of each module in memory is probably identical regardless of the size of the .ko file. I think the only significant difference it will make is in the size of the initramfs file, and the only difference it will make there is in the time needed to uncompress the fs. I.e., if you use an uncompressed initramfs, it won't matter.

strip --strip-all also works, and file reports them correctly as stripped either way. Why it says not stripped for the distro ones remains a mystery.

Best Answer

Related Solutions

Linux – Search through kernel make menuconfig configuration menu entries

Linux – Manual Kernel Build Results in Large Binary

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