Re: Preempt-rt kernel build for beagleboard


Bruce Ashfield <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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