TipsAndTricks/QuickAndDirtyKernelConfig

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How to Quickly Set Some Options in the Kernel

The Real Way

The best in depth explanation/tutorial for working with the kernel in Yocto is located here: https://www.yoctoproject.org/training/kernel-lab. It has a pdf, and supporting layers/metadata to allow you to follow along. There is also a kernel development manual located at http://www.yoctoproject.org/docs/latest/kernel-dev/kernel-dev.html.
If you are going to do a lot with the kernel or are planning to ship it in a product/project, I'd strongly recommend going through the lab and reading the kernel manual.

The Quick and Dirty Way

But, sometimes, you really just want to be able to set an option. How can you do this?

Menuconfig

bitbake linux-yocto -c menuconfig 

This will put you into the standard ncurses menuconfig screen and you can select options as normal and then rebuild your kernel with:

bitbake linux-yocto 

This will modify the .config file that linux-yocto uses

tmp/work/<machine>-common-poky-linux/linux-yocto/<version>/linux-<machine>-common-standard-build/.config 

in the version I am working off of it looks like this for a machine=intel-corei7-64 and a linux-yocto near the time of Morty's (YP 2.2) release.

tmp/work/corei7-64-intel-common-poky-linux/linux-yocto/4.8+gitAUTOINC+552a83790b_67813e7efa-r0/linux-corei7-64-intel-common-standard-build/.config

The good news is that that was pretty easy. The bad news, is that

  • if I remove tmp, as I tend to do, then my .config will go away.
  • if I want to share my additions with someone else, I need to give them a whole .config file and tell them to read the above referenced lab for how to use it.

Just the Configs I want, nothing more

Suppose I know that I just want to set

CONFIG_DMA_API_DEBUG=y

and nothing else. What can I do then? With the standard settings, we get:

 
$grep CONFIG_DMA_API tmp/work/.../.config
# CONFIG_DMA_API_DEBUG is not set

So, the Quick & Dirty approach is to add the following to the end of my conf/local.conf file:

 
 FILESEXTRAPATHS_prepend_pn-linux-yocto := "/my/absolute/path/build/conf:"
 SRC_URI_append_pn-linux-yocto = " file://myKernelConfigs.cfg "

The file myKernelConfigs.cfg is in my conf directory. I could have put it anywhere I wanted, even into a separate git repo in ~/kernelconfigsrepo, for example. myKernelConfigs.cfg contains the kernel configs I want to set. Let's suppose I want to turn on CONFIG_DMA_API_DEBUG. So the file myKernelConfigs.cfg would have:

 
# yes, comments are ok
CONFIG_DMA_API_DEBUG=y

If I delete the tmp folder and rerun bitbake linux-yocto I can then see the following:


$grep CONFIG_DMA_API tmp/work/.../.config
CONFIG_DMA_API_DEBUG=y

The CONFIG_DMA_API_DEBUG got turned on as we hoped, yay!

Something Can Go Wrong...

This part is a silly example of how to tell what went wrong when you set config options. Let's suppose that I decide I also want to turn on the feature CONFIG_DMA_API_COW. I'm pretty certain that this is in my kernel so I'll just add it to the file myKernelConfigs.cfg:


# yes, comments are ok 
CONFIG_DMA_API_DEBUG=y
# I'm pretty sure this is in my kernel
CONFIG_DMA_API_COW=y

We can rm tmp and rerun bitbake linux-yocto . Sadly the grep of our config does not show a CONFIG_DMA_API_COW setting. Where did it go?


$grep CONFIG_DMA_API tmp/work/.../.config
CONFIG_DMA_API_DEBUG=y

It turns out that the CONFIG_DMA_API_COW option is not in the linux-yocto kernel. Big surprise! (This could also have been a typo, or an old config whose name got changed...). Config options that can't be applied for some reason appear in the file mismatch.cfg &/or invalid.cfg. These are in tmp/work-shared/<machine>/kernel-source/.kernel-meta/ . This is a useful place to look when you set something and it doesn't seem to be taking effect. For instance:


$grep -rH DMA_API_COW  tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/.config:CONFIG_DMA_API_COW=y
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/mismatch.cfg:CONFIG_DMA_API_COW
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/invalid.cfg:CONFIG_DMA_API_COW
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/mismatch-all.txt:Config: CONFIG_DMA_API_COW
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/mismatch-all.txt:Requested value:  CONFIG_DMA_API_COW=y
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/merge_config_build.log:Value requested for CONFIG_DMA_API_COW not in final .config
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/cfg/merge_config_build.log:Requested value:  CONFIG_DMA_API_COW=y
tmp/work-shared/intel-corei7-64/kernel-source/.kernel-meta/configs/standard/intel/myKernelConfigs.cfg:CONFIG_DMA_API_COW=y  

This tells us that the value CONFIG_DMA_API_COW is not in the final .config and is in the invalid.cfg file. Good to know!

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