Re: [PATCH v4 02/10] memblock: Add variables for usable memory limitation

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

Hi Mike,

On Tue, Jul 20, 2021 at 7:41 AM Mike Rapoport <rppt@kernel.org> wrote:
> On Mon, Jul 19, 2021 at 08:59:03AM +0200, Geert Uytterhoeven wrote:
> > 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.

Sorry, I misread "if memblock.memory is empty" as "if capmem 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.
>
> If I remember correctly, memblock_cap_memory_range() was added to support
> "linux,usable-memory-range" for crasdump kernels on arm64 and if it would
> be called before memory is registered we may silently corrupt the memory
> because the crash kernel will see all the memory as available.
>"
> So while WARN() maybe too much a pr_warn() seems to me quite appropriate.

Yes, makes perfect sense now.

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