On 05/05/2017 12:42 PM, Alexey Brodkin wrote: > Hi Vineet, > > On Fri, 2017-05-05 at 12:31 -0700, Vineet Gupta wrote: >> +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. > > Actually it was not an error that Eugeniy saw but absence of any differences after: > 1) Adding the second memory region "0x1 0xa000_0000" in axc003_idu.dtsi > 2) Enabling PAE40 in menuconfig > > We had roughly the same situation as with PAE disabled and pristine DTS. > Well the only difference was mention of PAE40 being in use... but still only > 256Mb (read the same lowmem) was available. So you mean PAE40 + !HIGHMEM behaved as !PAE40 and it didn't crash or tell the user that it was not being in user ! I'd say this is good and cool ;-) > I'd say for "tests" we may manually overcome HIGHMEM dependency (with local arch/arc/Kconfig mod) > but keep the dependency in place so users who really want to access memory above 4Gb are not > misleaded. Fair enough, please send the revert of my orig patch to enforce the dependency: as you rightly say now we have a PAE40 enabled platform to actually test the real deal and not made up stuff ! -Vineet
