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

From: Arnd Bergmann <arnd@arndb.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
	<devicetree@vger.kernel.org>,
	"Grant Likely" <grant.likely@arm.com>,
	"Nicolas Pitre" <nico@fluxnic.net>,
	"Masahiro Yamada" <masahiroy@kernel.org>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"Lukasz Stelmach" <l.stelmach@samsung.com>,
	"Russell King - ARM Linux admin" <linux@armlinux.org.uk>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	Linux-Renesas <linux-renesas-soc@vger.kernel.org>,
	"Chris Brandt" <chris.brandt@renesas.com>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>,
	"Eric Miao" <eric.miao@nvidia.com>,
	"Dmitry Osipenko" <digetx@gmail.com>,
	"Ard Biesheuvel" <ardb@kernel.org>,
	"Linux ARM" <linux-arm-kernel@lists.infradead.org>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>
Subject: Re: [PATCH v6] ARM: boot: Obtain start of physical memory from DTB
Date: Tue, 19 May 2020 13:28:24 +0200	[thread overview]
Message-ID: <CAK8P3a3H=7qx+Rz9sScTVCSMKWGwQ_ROnyoyK73A5yUd+_jbTw@mail.gmail.com> (raw)
In-Reply-To: <CAMuHMdU5DG06G4H=+PH+OONMT_9oE==KS=wP+bLgY9xVCez6Ww@mail.gmail.com>

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.

Would extending this to arm work?

      Arnd

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