[poky] Status of preempt-rt on raspberrypi

[Francis Giraldeau]

I'd like to create an image with linux preempt-rt for the raspberry-pi.

I merged the patches specific to the rpi on top of the linux 4.4 preempt-rt
branch [1]. Now, I'm trying to add the support in the meta-raspberrypi
layer [2].

However, I still have an issue setting the required config, such as
CONFIG_PREEMPT_FULL=y for the kernel. I saw that there is already a
preempt-rt branch and config by default in Yocto, but I haven't found the
way to select in the local.conf file when building for the rpi. My
local.conf file has this content:

MACHINE ??= "raspberrypi2"
PREFERRED_PROVIDER_virtual/kernel ?= "linux-raspberrypi-rt"

In fact, even with this local.conf:
MACHINE ??= "qemuarm"
KBRANCH ?= "standard/preempt-rt"

The resulting kernel config does not include PREEMPT_FULL:
$ grep PREEMPT
./tmp/work/qemuarm-poky-linux-gnueabi/linux-yocto/4.1.15+gitAUTOINC+46bb64d605_cf760f381c-r0/linux-qemuarm-standard-build/.config
CONFIG_PREEMPT_RCU=y
# CONFIG_PREEMPT_NONE is not set
# CONFIG_PREEMPT_VOLUNTARY is not set
CONFIG_PREEMPT=y
CONFIG_PREEMPT_COUNT=y
CONFIG_DEBUG_PREEMPT=y
# CONFIG_PREEMPT_TRACER is not set

And CONFIG_DEBUG_PREEMPT should be unset, because it is known to cause
slowdowns.

I followed the kernel dev manual and tried to add .cfg file, but it looks
like the kernel .config is generated from within do_configure_prepend() in
the meta-raspberrypi kernel recipe. That seems to overwrite other config
done by bitbake.

So, I'm a bit confused on how things should properly work. Any advice would
be greatly appreciated.

Thanks for the help,

Francis

[1] https://github.com/giraldeau/linux/tree/linux-4.4.y-rt-rebase-rpi
[2]
https://github.com/giraldeau/meta-raspberrypi/commit/56168d6b320c1b724d04b1e7f365255ffdb62e70
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