Re: [PATCH 3/4] arch: define CONFIG_PAGE_SIZE_*KB on all architectures

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Arnd,

CC Greg

On Tue, Feb 27, 2024 at 11:59 AM Arnd Bergmann <arnd@arndb.de> wrote:
> On Tue, Feb 27, 2024, at 09:54, Geert Uytterhoeven wrote:
> >> diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
> >> index 9dcf245c9cbf..c777a129768a 100644
> >> --- a/arch/m68k/Kconfig.cpu
> >> +++ b/arch/m68k/Kconfig.cpu
> >> @@ -30,6 +30,7 @@ config COLDFIRE
> >>         select GENERIC_CSUM
> >>         select GPIOLIB
> >>         select HAVE_LEGACY_CLK
> >> +       select HAVE_PAGE_SIZE_8KB if !MMU
> >
> > .... if you would drop the !MMU-dependency here.
> >
> >>
> >>  endchoice
> >>
> >> @@ -45,6 +46,7 @@ config M68000
> >>         select GENERIC_CSUM
> >>         select CPU_NO_EFFICIENT_FFS
> >>         select HAVE_ARCH_HASH
> >> +       select HAVE_PAGE_SIZE_4KB
> >
> > Perhaps replace this by
> >
> >     config M68KCLASSIC
> >             bool "Classic M68K CPU family support"
> >             select HAVE_ARCH_PFN_VALID
> >   +         select HAVE_PAGE_SIZE_4KB if !MMU
> >
> > so it covers all 680x0 CPUs without MMU?
>
> I was a bit unsure about how to best do this since there
> is not really a need for a fixed page size on nommu kernels,
> whereas the three MMU configs clearly tie the page size to
> the MMU rather than the platform.
>
> There should be no reason for coldfire to have a different
> page size from dragonball if neither of them actually uses
> hardware pages, so one of them could be changed later.

Indeed, in theory, PAGE_SIZE doesn't matter for nommu, but the concept
of pages is used all over the place in Linux.

I'm mostly worried about some Coldfire code relying on the actual value
of PAGE_SIZE in some other context. e.g. for configuring non-cacheable
regions.

And does this impact running nommu binaries on a system with MMU?
I.e. if nommu binaries were built with a 4 KiB PAGE_SIZE, do they
still run on MMU systems with an 8 KiB PAGE_SIZE (coldfire and sun3),
or are there some subtleties to take into account?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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