Re: PowerPC PPC32 CONFIG_HIGHMEM dependency question

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

* Robert Johnson | 2014-03-17 17:41:25 [+0000]:

>I am using an embedded single board computer (SBC) that has a Freescale P2020 processor.  The vendor board support package (BSP) for this product currently uses a 3.0.4 Linux kernel.  In looking through the RT patch for various Linux kernel versions even up through 3.12.13-rt21, I see that CONFIG_HIGHMEM is not allowed if CONFIG_PREEMPT_RT_FULL is selected (see arch/powerpc/Kconfig).
>
>The result for my SBC is that if I enable CONFIG_PREEMPT_RT_FULL in the kernel, I am then only able to access 1GB of RAM instead of the 4GB that is resident.

For 2 GiB you could move the memory mapping to something like 1.5GiB
userland, 2.GiB kernel and the remaining 500MiB for virtual.

>Is there any way to get CONFIG_PREEMPT_RT_FULL to work with CONFIG_HIGHMEM?

Yes. The trick is to look either at the ARM code or at the x86 code and
do something similar for powerpc.

>If I want to use CONFIG_HIGHMEM, should I consider the use of CONFIG_PREEMPT_RTB which does not appear to have a dependency?

You could CONFIG_PREEMPT_RT_BASE with CONFIG_HIGHMEM, yes. But then
don't complain about your latency which will be higher compared to
_FULL. Again: CONFIG_PREEMPT_RT_BASE is only debugging option.

Sebastian