linux-kernel - Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

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Message-ID: <CAMuHMdV0_GTop+YTPeu+aSFB=1YHsyzLXn-+0fa8upkNMq10bQ@mail.gmail.com>
Date:   Tue, 19 May 2020 14:11:14 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Lukasz Stelmach <l.stelmach@...sung.com>,
        Dmitry Osipenko <digetx@...il.com>,
        Nicolas Pitre <nico@...xnic.net>,
        Eric Miao <eric.miao@...dia.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Chris Brandt <chris.brandt@...esas.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        Grant Likely <grant.likely@....com>
Subject: Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB

Hi Arnd,

On Tue, May 19, 2020 at 1:28 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Tue, May 19, 2020 at 1:21 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin
> > <linux@...linux.org.uk> wrote:
>
> > >
> > > > However, something under /chosen should work.
> > >
> > > Yet another sticky plaster...
> >
> > IMHO the old masking technique is the hacky solution covered by
> > plasters.
> >
> > DT describes the hardware.  In general, where to put the kernel is a
> > software policy, and thus doesn't belong in DT, except perhaps under
> > /chosen.  But that would open another can of worms, as people usually
> > have no business in specifying where the kernel should be located.
> > In the crashkernel case, there is a clear separation between memory to
> > be used by the crashkernel, and memory to be solely inspected by the
> > crashkernel.
> >
> > Devicetree Specification, Release v0.3, Section 3.4 "/memory node" says:
> >
> >     "The client program may access memory not covered by any memory
> >      reservations (see section 5.3)"
> >
> > (Section 5.3 "Memory Reservation Block" only talks about structures in
> > the FDT, not about DTS)
> >
> > Hence according to the above, the crashkernel is rightfully allowed to
> > do whatever it wants with all memory under the /memory node.
> > However, there is also
> > Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt.
> > This suggests the crashkernel should be passed a DTB that contains a
> > /reserved-memory node, describing which memory cannot be used freely.
> > Then the decompressor needs to take this into account when deciding
> > where the put the kernel.
> >
> > Yes, the above requires changing code. But at least it provides a
> > path forward, getting rid of the fragile old masking technique.
>
> There is an existing "linux,usable-memory-range" property documented
> in Documentation/devicetree/bindings/chosen.txt, which as I understand
> is exactly what you are looking for, except that it is currently only
> documented for arm64.

Thank you, that looks appropriate!

It seems this is not really used by the early startup code.
Is that because the early startup code always runs in-place, and the
kernel image is not even copied?

> Would extending this to arm work?

Let's see.... Th arm early boot code seems to be more complex than the
arm64 code ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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