Re: [PATCH] of/fdt: Don't worry about non-memory region overlap for no-map

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, May 19, 2021 at 6:27 PM Stephen Boyd <swboyd@chromium.org> wrote:
>
> In commit 8a5a75e5e9e5 ("of/fdt: Make sure no-map does not remove
> already reserved regions") we returned -EBUSY when trying to mark
> regions as no-map when they're in the reserved memory node. This if
> condition will still trigger though if the DT has a /memreserve/ that
> completely subsumes the no-map memory carveouts in the reserved memory
> node. Let's only consider this to be a problem if we're trying to mark a
> region as no-map and it is actually memory. If it isn't memory,
> presumably it was removed from the memory map via /memreserve/ and thus
> can't be mapped anyway.
>
> This silences a warning seen at boot for me on sc7180-trogdor.dtsi
> boards that have /memreserve/ coming from the bootloader and those
> reserved regions overlap with the carveouts that we want to use in DT
> for other purposes like communicating with remote processors.
>
> Cc: Douglas Anderson <dianders@chromium.org>
> Cc: Nicolas Boichat <drinkcat@chromium.org>
> Cc: Quentin Perret <qperret@google.com>
> Fixes: 8a5a75e5e9e5 ("of/fdt: Make sure no-map does not remove already reserved regions")
> Signed-off-by: Stephen Boyd <swboyd@chromium.org>
> ---
>  drivers/of/fdt.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index ba17a80b8c79..be13b4b6c2d8 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -1161,7 +1161,8 @@ int __init __weak early_init_dt_reserve_memory_arch(phys_addr_t base,
>                  * If the memory is already reserved (by another region), we
>                  * should not allow it to be marked nomap.
>                  */
> -               if (memblock_is_region_reserved(base, size))
> +               if (memblock_is_region_memory(base, size) &&
> +                   memblock_is_region_reserved(base, size))
>                         return -EBUSY;

I'm not an expert on this code, so take review comments w/ a grain of salt.

That being said, while the change looks right on the surface, I'm not
sure it's 100% right when I dig. The names of
memblock_is_region_memory() and memblock_is_region_reserved() make
them sound more similar than they actually are. One of the two tests
for intersection and the other for "subset of". I think if
memblock_is_region_memory() used "intersects" instead of "subset of"
then your patch would be correct.

Specifically if you've got memory regions:

0x1000 - 0x2000 - memory
0x3000 - 0x4000 - memory

Then you check memblock_is_region_memory(0x2800, 0x1000) or
memblock_is_region_memory(0x1800, 0x1000) then I think it will return
false, right? Because those aren't _subsets_ of memory even though
they intersect memory.

I don't know if cases like that show up in practice, but it seems
better to be safe?

-Doug