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

[Mike Rapoport <rppt@kernel.org>]

Hi,

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.

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.

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.
 
> Rob

-- 
Sincerely yours,
Mike.