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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Mon, 19 Jul 2021 08:59:03 +0200
Message-ID: <CAMuHMdXfFhzm48U2Hvjz8yrjPsQbagW4aC_L-QE_Q6yx1Lo=tA@mail.gmail.com>
Subject: Re: [PATCH v4 02/10] memblock: Add variables for usable memory limitation
To:     Mike Rapoport <rppt@kernel.org>
Cc:     Rob Herring <robh@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Nicolas Pitre <nico@fluxnic.net>,
        Ard Biesheuvel <ardb@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Nick Kossifidis <mick@ics.forth.gr>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
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Hi Mike,

On Sun, Jul 18, 2021 at 11:31 AM Mike Rapoport <rppt@kernel.org> wrote:
> On Wed, Jul 14, 2021 at 07:51:01AM -0600, Rob Herring wrote:
> > On Wed, Jul 14, 2021 at 02:50:12PM +0200, Geert Uytterhoeven wrote:
> > > Add two global variables (cap_mem_addr and cap_mem_size) for storing a
> > > base address and size, describing a limited region in which memory may
> > > be considered available for use by the kernel.  If enabled, memory
> > > outside of this range is not available for use.
> > >
> > > These variables can by filled by firmware-specific code, and used in
> > > calls to memblock_cap_memory_range() by architecture-specific code.
> > > An example user is the parser of the "linux,usable-memory-range"
> > > property in the DT "/chosen" node.
> > >
> > > Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
> > > ---
> > > This is similar to how the initial ramdisk (phys_initrd_{start,size})
> > > and ELF core headers (elfcorehdr_{addr,size})) are handled.
> > >
> > > Does there exist a suitable place in the common memblock code to call
> > > "memblock_cap_memory_range(cap_mem_addr, cap_mem_size)", or does this
> > > have to be done in architecture-specific code?
> >
> > Can't you just call it from early_init_dt_scan_usablemem? If the
> > property is present, you want to call it. If the property is not
> > present, nothing happens.

I will have a look...

> For memblock_cap_memory_range() to work properly it should be called after
> memory is detected and added to memblock with memblock_add[_node]()
>
> I'm not huge fan of adding more globals to memblock so if such ordering can
> be implemented on the DT side it would be great.

Me neither ;-)

> I don't see a way to actually enforce this ordering, so maybe we'd want to
> add warning in memblock_cap_memory_range() if memblock.memory is empty.

"linux,usable-memory-range" is optional, and typically used only in
crashdump kernels, so it would be a bad idea to add such a warning.

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