[Letux-kernel] Nokia N900 not hitting OFF mode since 5.9 is caused by proactive memory compaction

[H. Nikolaus Schaller]

Hi,


> Am 11.12.2021 um 12:14 schrieb Andreas Kemnade <andreas at kemnade.info>:
> 
> Hi,
> 
> On Fri, 10 Dec 2021 13:39:28 +0100
> "H. Nikolaus Schaller" <hns at goldelico.com> wrote:
> 
>> Hi Andreas,
>> I have checked:
>> 
>> 	http://download.goldelico.com/letux-kernel/letux-5.10.83/src/FULLCONFIG
>> 
>> has CONFIG_COMPACTION=y
>> 
>> But
>> 
>> 	http://download.goldelico.com/letux-kernel/letux-5.10.83/src/DEFCONFIG
>> 
>> has not. So it needs to be disabled explicitly. Interestingly letux-5.4.163 has it enabled as well.
>> So we should also look if the patch identified by Merlijn was already backported...
>> 
>> Since I can't test it (have no proper power-management-test-setup any more),
>> I will proactively disable it for letux-5.10.y, letux-5.15.y and letux-5.16-rc...
>> 
> For a distribution kernel I am a bit unsure if that is a good idea.
> Lets look at Kconfig
> 
> 
>          Compaction is the only memory management component to form
>          high order (larger physically contiguous) memory blocks
>          reliably. The page allocator relies on compaction heavily and
>          the lack of the feature can lead to unexpected OOM killer
>          invocations for high order memory requests. You shouldn't
>          disable this option unless there really is a strong reason for
>          it and then we would be really interested to hear about that at
>          linux-mm at kvack.org.

Indeed I got some doubts, especially since CONFIG_COMPACTION=y is there in older kernels
as well. So it would not be the reason for problems but just a workaround.

So let's wait what the disussion on the phone-devel or linux-omap lists will reveal.

> 
> ok, the reason is power management. I will recheck things with
> a recent kernel but with the gta04 I have not seen these extreme things
> and it is also not about suspend current. I guess there is something
> special with the N900. People have seen off-mode on omap3 after thet
> compaction thing. Anyways, if we do not have a well-crafted module
> blacklist, we have some peripherals which are not idling properly, so
> runtime pm is not that aggressive anyways on the gta04.

Yes, there are subtle differences. The main difference having the real omap3430 inside
of the N900. GTA04 uses the dm3730 i.e. second generation silicon. Only the 600MHz
Pandora with omap3530 is quite similar to N900.

Despite having found my N900 unit (and unsuccessfully tried to charge&boot) I did
not yet find the box where I have stored spare time :) And it would be good to have/build
a console adapter (which needs very difficile mechanics: https://n900.elektranox.org/serial-adapter.html).
Maybe after Xmas... And next year is long...

BR,
Nikolaus