[yocto] Preempt-rt kernel build for beagleboard

[Bruce Ashfield]

On 12-11-27 05:23 AM, Yu Pan wrote:
> I started looking at Yocto 2 weeks back. Despite the complexity in
> kernel building, I found it quite hard to locate information on
> preempt-rt build. I was trying to build linux-yocto-rt for beagleboard
> from "danny" and the linux-yocto-3.4 kernel, and it seems to be working.
> I'd like to share how one can do that in this thread.
>
> Since the kernel does not include an official preempt-rt branch for
> beagleboard yet, I choose to directly use the standard/preempt-rt/base
> branch. I guess this should work for the preempt-rt kernel, from the
> fact that the standard kernel (non preempt-rt) used for beagleboard from
> the standard/beagleboard branch is basically identical to that from the
> standard/base branch.

Correct. If you don't have any board specific patches, working with
an existing branch in the tree is fine.

>
> Here is what I did.
> 1. Under poky/meta-yocto-bsp/recipes-kernel/linux, I added a
> linux-yocto-rt_3.4.bbappend recipe containing the following lines. The
> SRCREV value used is the same as the one in the base
> linux-yocto-rt_3.4.bb <http://linux-yocto-rt_3.4.bb> under
> poky/meta/recipes/kernel.
>
>    KBRANCH_beagleboard = "standard/preempt-rt/base"
>    SRCREV_machine_beagleboard ?= "5705c8037d2c47938034ead87c70ae3ebef552f7"
>    COMPATIBLE_MACHINE_beagleboard = "beagleboard"

Also note that the yocto-bsp tool allows the extension of an existing
BSP or kernel to a new board. It should provide some of what you
did via editing the files, I added TomZ in case I've misspoken about
what the tools do in this area.

But what you have above is correct.

>
> 2. Modified the PREFERRED_PROVIDER_virtual/kernel variable in
> poky/meta-yocto-bsp/conf/machine/beagleboard.conf to "linux-yocto-rt".
>
> 3. "bitbake linux-yocto-rt -c menuconfig" to set the Preemption Model to
> "Fully Preemptible Kernel (RT)".

You shouldn't have needed to do this part, since the board already has
a preempt-rt BSP description:

http://git.yoctoproject.org/cgit/cgit.cgi/linux-yocto-3.4/tree/meta/cfg/kernel-cache/bsp/beagleboard/beagleboard-preempt-rt.scc?h=meta

And when you build the linux-yocto-rt kernel with that board, that
description should be found, and it enables all the base options already.

If you look in your build, what does the file 'linux/meta/top_tgt'
contain ?

Cheers,

Bruce

>
> 4. "bitbake linux-yocto-rt"
>
> 5. "bitbake core-image-rt" which includes some realtime test programs on
> top of core-image-minimal.
>
> I did some cyclictest on beagleboard using the generated kernel and
> image with "/usr/bin/cyclictest -p95 -m". The results did show huge
> improvement on max latency especially under load.
>
> Hope it helps.
>
> Cheers
> Pan Yu
>
>
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