[yocto] [meta-raspberrypi] how to apply custom kernel configuration?

[Andrea Galbusera]

Hi!

I was wondering how to apply some extra kernel configuration from a custom
layer via a bbappend. I know that with meta-raspberrypi there exist some
variables you can define in local.conf that will drive how the kernel is
configured: but what about some very specific device driver nobody than me
is interest in? To me it seems it doesn't deserve a dedicated global oe
variable.

Some investigation showed this topic already came up on the list from time
to time in the past (most recently in [1]). To me it looks like there is
some common desire to get rid of the current "magic" carried out by
linux-rpi.inc by moving the linux-raspberrypi recipes into the
configuration fragments style.

In the short term, my question are: how do you guys manage to add your
custom kernel configurations? Do you rely on full blown static defconfigs?
Have you found any reliable way to do "differential" configurations with
the current state of metadata?

In a longer term, I'd really like to help moving towards supporting
configuration fragments for linux-raspberrypi, but I suspect it'd be a too
much hard task for my current understanding of the whole kernel
configuration workflow. Do you know of anyone already working on this or
does anyone have a clear idea of what is needed to get the work done? Some
times ago I tried to resurrect [2], but with no luck, probably due to some
changes that happened in the linux-yocto files in the meanwhile.

Comments from gurus are welcome! ;-)

[1] https://lists.yoctoproject.org/pipermail/yocto/2016-December/033559.html
[2] https://lists.yoctoproject.org/pipermail/yocto/2015-October/027034.html
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