[yocto] [meta-raspberrypi][PATCH] linux-raspberrypi-rt: add

[Trevor Woerner]

On Wed 2017-01-04 @ 01:05:27 AM, Andreas Müller wrote:
> On Tue, Jan 3, 2017 at 8:38 PM, Trevor Woerner <twoerner at gmail.com> wrote:
> > Add an -rt kernel option for Raspberry Pi that includes the FIQ patch which
> > adds stability to the -rt kernel (with full preemption enabled) and is an
> > improvement to simply disabling FIQ on the kernel cmdline.
> >
> > Signed-off-by: Trevor Woerner <twoerner at gmail.com>
> I played around with rt-kernel too but had no solution for the crashes
> so far. Will test soon and hope you are my hero..

Without any tweak I couldn't get a kernel with CONFIG_PREEMPT_RT_FULL=y
enabled to run for more than 10 minutes before failing.

Simply adding the two kernel cmdline options to disable the FIQ completely
also works, but (compared to using the FIQ patch) simply disabling the FIQ
gives results from cyclictest that aren't as good as the results from using
the FIQ patch.

I ran one test on a raspi3 with the -rt kernel, with CONFIG_PREEMPT_RT_FULL=y,
with the FIQ disabled from the kernel cmdline, running both:

	# stress --cpu 5 --io 4 --vm 2 --vm-bytes 128M&
	# cyclictest -p 80 -t5 -n

It ran for 3 days successfully before I shut it down.

I then ran another test with everything the same, but instead of a disabled
FIQ, I updated the FIQ patch for raspi-4.4.35 and applied it. The above
stress/cyclictest ran for 6 days successfully before I stopped it.

So I have pretty good confidence in this recipe ;-)