Re: [PATCH v2 2/2] amr64: map FDT as RW for early_init_dt_scan()

[Stephen Boyd <swboyd@chromium.org>]

Quoting Hsin-Yi Wang (2019-05-13 04:14:32)
> On Mon, May 13, 2019 at 4:59 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
> 
> >
> > This makes the fdt mapped without the call to meblock_reserve(fdt) which
> > makes the fdt memory available for memblock allocations.
> >
> > Chances that is will be actually allocated are small, but you know, things
> > happen.
> >
> > IMHO, instead of calling directly __fixmap_remap_fdt() it would be better
> > to add pgprot parameter to fixmap_remap_fdt(). Then here and in kaslr.c it
> > can be called with PAGE_KERNEL and below with PAGE_KERNEL_RO.
> >
> > There is no problem to call memblock_reserve() for the same area twice,
> > it's essentially a NOP.
> >
> Thanks for the suggestion. Will update fixmap_remap_fdt() in next patch.
> 
> However, I tested on some arm64 platform, if we also call
> memblock_reserve() in kaslr.c, would cause warning[1] when
> memblock_reserve() is called again in setup_machine_fdt(). The warning
> comes from https://elixir.bootlin.com/linux/latest/source/mm/memblock.c#L601
> ```
> if (type->regions[0].size == 0) {
>   WARN_ON(type->cnt != 1 || type->total_size);
>   ...
> ```
> 
> Call memblock_reserve() multiple times after setup_machine_fdt()
> doesn't have such warning though.
> 
> I didn't trace the real reason causing this. But in this case, maybe
> don't call memblock_reserve() in kaslr?
> 

Why not just have fixmap_remap_fdt() that maps it as RW and reserves
memblock once, and then call __fixmap_remap_fdt() with RO after
early_init_dt_scan() or unflatten_device_tree() is called? Why the
desire to call memblock_reserve() twice or even three times?