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

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

Hi Arnd,

On Tue, May 19, 2020 at 1:28 PM Arnd Bergmann <arnd@arndb.de> wrote:
> On Tue, May 19, 2020 at 1:21 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Tue, May 19, 2020 at 11:46 AM Russell King - ARM Linux admin
> > <linux@armlinux.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@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