Level 3 cache not detected

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Hypex
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Re: Level 3 cache not detected

Post by Hypex »

sailorMH wrote: Tue Aug 29, 2023 7:24 am
And is possible to enable L2 with some kernel parameter? Like l2cr or cachesize= ? Or is it done by parameters during kernel compilation?
Yes, it was a kernel parameter for a long time now, from early kernels.
I checked commandline kernel parameters, and there are l2cr=0x80000000
My 7455 has 256 kB L2, i.e. 0x40000 and 0x80000000 is 2 GB...
I see what you mean here. I never looked at it that way before. As you found out it describes cache register which is a little technical and not memory size but that's a good guess!
But, kernel parameter meaning should differ from CPU l2cr register and maybe other L2 parameters should be set. But this I don't understand.
I will try l2cr=3 on weekend and we will see.
I think you must have done the most research of anyone! It is a good idea to look it up. So, though it is PPC specific, there should be info in the kernel command line documentation but it only says [ PPC ] and nothing else. The most I can find is in an article below. What I do recall is it being set to 0x80000000 for on or 0x00000000 for off but I've seen nothing else mentioned.

https://home.samfundet.no/~olegil/amiga/

Also, there is l3cr if you want to experiment. Unfortunately you will need to consult the technical guide again. :)

https://docs.kernel.org/admin-guide/ker ... eters.html
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sailorMH
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Re: Level 3 cache not detected

Post by sailorMH »

Thank you for link to olegil/amiga.

it is at least some explanation. I found also some codes with l2cr settings for some powermac cards, and it is more complex. I copy them here tomorrow.

It means, that kernel parameter l2cr ( and l3cr ) is not exact copy of CPU l2cr/l3cr register, but write into them some important values. Like on / off ;-). It would be great to know other bits meaning.

Back to XE testing:
my kernel cmdline is:

Code: Select all

cat /proc/cmdline 
root=UUID=a7128a20-0df5-4cc8-8400-2b3dc958a5c7 console=tty0 l2cr=0x80000000 video=radeonfb:off libata.dma=1 radeon.modeset=1
so the l2cr parameter is correctly set. But:

In /proc/sys/kernel/ after boot is no l2cr file.

Code: Select all

echo 0x80000000 > /proc/sys/kernel/l2cr
ends with no such file or directory

I am not kernel expert, but it look like the kernel is compilled without L2 support, and cmdline parameter l2cr and /proc/sys/kernel/l2cr has no effect.

I checked Debian 8 on Pegasos 2:
there is also no file /proc/sys/kernel/l2cr
but lshw - C memory shows both L1 and L2 cache
Micro A1-C (G3/1.2 GHz), AmigaOne XE (G4/1.4 GHz), Pegasos II (G4/1.33 GHz), Sam440ep, Sam440ep-flex, AmigaOne X1000
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