PAE40 without HIGHMEM

[Vineet Gupta]

+CC arcml

Hi Alexey,

On 05/05/2017 01:44 AM, Alexey Brodkin wrote:
> Hi Vineet,
>
> Playing with PAE40 on axs103 v1.2-rc2 (where PAE really seems to work quite well)
> Eugeniy bumped into the fact that PAE could be enabled without HIGHMEM.
>
> Personally I din't face that trouble because I used to enable HIGHMEM manually before
> even thinking of PAE.
>
> So we were surprised why there's no dependency which I was pretty sure was there
> back in the day. And to no surprise I found you removed such a dependency in
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7cab91b87dd8eeee5911ec34be8bb0288ebba18b
> "ARCv2: Allow enabling PAE40 w/o HIGHMEM".
>
> Now what we have another [and this time real HW] platform with PAE support
> and DDR mapped above 4Gb do you think we may revert this change and
> put dependency on HIGHMEM for PAE?
>
> If you like I may send a patch to that effect.

I understand your point, and PAE40 when really exercising > 32 bits certainly
needs HIGHMEM.

However in a setup where we only had 32 bit paddr, we can technically still build
a PAE40 enabled kernel - which uses PAE40 enabled MMU cmds etc (with upper words
always happening to be 0) and should in theory work as well. Agreed it is not a
production config, still it is a useful "test" config or a "developer" config.

The next question is why was Euginey seeing the errors - because he had DT setup
memory > 1_0000_0000 and CONFIG_HIGHMEM was not enabled ? That certainly is broken
and needs to be prevented at config or early run time.

-Vineet