Re: [PATCH 2/2] ARM: Allow either FLATMEM or SPARSEMEM on the multiplatform build

[Arnd Bergmann <arnd@arndb.de>]

On Mon, May 18, 2020 at 9:45 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
> On Mon, May 18, 2020 at 08:58:36AM -0700, Florian Fainelli wrote:
> > On 5/7/2020 1:11 PM, Florian Fainelli wrote:
> > > On 5/7/2020 12:27 AM, Mike Rapoport wrote:
> > >> On Wed, May 06, 2020 at 04:50:09PM -0700, Florian Fainelli wrote:
> > >>> From: Gregory Fong <gregory.0xf0@gmail.com>
> > >>>
> > >>> ARMv7 chips with LPAE can often benefit from SPARSEMEM, as portions of
> > >>> system memory can be located deep in the 36-bit address space.  Allow
> > >>> FLATMEM or SPARSEMEM to be selectable at compile time; FLATMEM remains
> > >>> the default.
> > >>>
> > >>> This is based on Kevin's "[PATCH 3/3] ARM: Allow either FLATMEM or
> > >>> SPARSEMEM on the multi-v7 build" from [1] and shamelessly rips off his
> > >>> commit message text above.  As Arnd pointed out at [2] there doesn't
> > >>> seem to be any reason to tie this specifically to ARMv7, so this has
> > >>> been changed to apply to all multiplatform kernels.
> > >>>
> > >>> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-September/286837.html
> > >>> [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-October/298950.html
> > >>>
> > >>> Cc: Kevin Cernekee <cernekee@gmail.com>
> > >>> Tested-by: Stephen Boyd <sboyd@codeaurora.org>
> > >>> Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
> > >>> Signed-off-by: Doug Berger <opendmb@gmail.com>
> > >>> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> > >>> ---
> > >>>  arch/arm/Kconfig | 5 +++++
> > >>>  1 file changed, 5 insertions(+)
> > >>>
> > >>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > >>> index 5986277296c3..7bb5264a9c3a 100644
> > >>> --- a/arch/arm/Kconfig
> > >>> +++ b/arch/arm/Kconfig
> > >>> @@ -312,6 +312,8 @@ choice
> > >>>  config ARCH_MULTIPLATFORM
> > >>>   bool "Allow multiple platforms to be selected"
> > >>>   depends on MMU
> > >>> + select ARCH_FLATMEM_ENABLE
> > >>> + select ARCH_SPARSEMEM_ENABLE
> > >>
> > >> The logic in mm/Kconfig is quite convoluted, so selecting
> > >> ARCH_SPARSEMEM_ENABLE will automatically make SPARSEMEM the only option.
> > >>
> > >> On top of this you would need to enable ARCH_SELECT_MEMORY_MODEL, e.g.
> > >> something like:
> > >
> > > Yes indeed, thanks that does allow me to select between flatmem and
> > > sparsemem from menuconfig correctly now.
> >
> > Mike, do you want to make a formal submission to Russell's patch
> > tracker? If so, feel free to add:
>
> I actually hoped to hear from people what do they think about switching
> over to SPARSEMEM for the multiplatform builds. I think at least v7 CPUs
> it would make sense.

It seems that at least s5p/exynos and clps711x (armv4) used to default to
sparsemem, and that got lost in the multiplatform conversion.

I also see discontiguous memory ranges in multiple broadcom chips,
on TI dm8168, ecx-2000 and imx.

> Russel, Arnd, can you comment please?

I see no problem with giving users the choice for all multiplatform
builds. No idea on what the default should be really, i.e. whether
only v7 configurations should make it the default, or rather none of
them or all of them.

Maybe lets leave the default unchanged with flatmem but enable it
in multi_v7_defconfig and the configurations for chips that are known
to have discontiguous memory (clps, bcm, imx, exynos, ...).

      Arnd

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