linux-kernel - Re: m68k Kconfig warning

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Date:   Wed, 4 Dec 2019 12:58:59 +0100
From:   Kars de Jong <karsdejong@...e.nl>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: m68k Kconfig warning

Hi Mike!

Op ma 2 dec. 2019 om 17:01 schreef Mike Rapoport <rppt@...ux.ibm.com>:
> The patches are here:
> https://www.spinics.net/lists/linux-m68k/msg13588.html
>
> Aside from some technicalities we had troubles deciding what should be the
> section size. With larger section size we might end up with wasted memory
> for memory maps and with smaller section size we'll have to limit the
> addressable physical memory...

I read through that thread. I believe our current page->flags needs 22
bits for the normal FLAGS and 2 bits for the ZONES, which leaves 8
bits for the SECTION. This is what Geert found out (it worked with
MAX_PHYSMEM_BITS = 30 and SECTION_SIZE_BITS = 22).
I think we can reduces ZONES to a single bit. We currently put all
memory in ZONE_DMA, so I think we might just as well put it in
ZONE_NORMAL and disable ZONE_DMA.

That would enable us to have 8 MB sections and use the full 32-bit
address space, correct? That may be a working compromise?

Kind regards,

Kars.