Re: Application of standard kernel patch and then RT patch with Petalinux

["Ahmed S. Darwish" <a.darwish@linutronix.de>]

Hi Jack,

On Mar 25, 2021, Jack wrote:
>
> I am having problems applying kernel patches in the context of a Xilinx
> Petalinux project (Yocto-based).
>
> Xilinx provides a 5.4.0 kernel, according to make kernelconfig, though
> how this corresponds to the upstream 5.4.0 kernel, I'm not sure.
>

What Xilinx provides in its linux-xlnx tree is quite different from the
mainline v5.4 kernel. You can see all the differences by diffing against
219d54332a09 ("Linux 5.4").

> My understanding is that, in order to apply the latest 5.4 rt patch
> (106-rt54), I have to apply the upstream 106 patch first. Is this
> correct?
>

No, it doesn't work out that way, especially that the Xilinx kernel is
quite different at core kernel components.

The RT kernel releases are meant to be taken as a whole, by picking one
of the stable versions shown at:

  https://wiki.linuxfoundation.org/realtime/preempt_rt_versions

> When applying the patches using petalinux-config -c kernel, the
> upstream patch appears to be accepted, but the rt patch is not. I get
> the sequence of errors below, picked out in red.
>

FYI, in general, it's advised to separate kernel development from
"integration" development (e.g. yocto, peta linux, etc.)

> I am not familiar enough with the kernel, or the Petalinux tools, to
> get a handle on the problem. The log file mentioned at the bottom does
> not exist following the completion of the command.
>
> Is anyone able to help with this?

Unfortunately, if you're going to do RT things on top of an SoC kernel,
you're on your own — and some kernel development expertise will be
needed.

There are only two sane ways out:

1) Clone one of the stable RT kenrels trees (e.g., v5.4.106-rt45), then
*carefully* pull in what you need from the Xilinx kernel tree. Then do
proper stress and latency tests to make sure that everything is still in
line.

2) Ask a supplier with sufficient embedded/kernel/RT expertise to do it
for you.

Good luck,

--
Ahmed S. Darwish
Linutronix GmbH