Re: [PATCH 2/2] of: fdt: Allow the kernel to mark nomap regions received from fdt

[Rob Herring]

On Thu, Apr 6, 2023 at 10:14 AM Lucas Tanure <tanure@xxxxxxxxx> wrote:
>
> Reserved regions can be described in FDT and device trees, but FDT doesn't
> provide the related flags, like nomap.

It took me a minute to understand what you meant by FDT vs. device
trees. Use the exact things you are talking about: /memreserve/ and
/reserved-memory node.

> So allow the kernel to mark regions where the base and size received from
> the device tree are the same as the base and region on FDT.
> Here we trust that the device tree has a more updated description of the
> region than the one received from FDT.
>
> Signed-off-by: Lucas Tanure <tanure@xxxxxxxxx>
> ---
>  drivers/of/fdt.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index d1a68b6d03b3..754a7ea4f45c 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -482,11 +482,13 @@ static int __init early_init_dt_reserve_memory(phys_addr_t base,
>         if (nomap) {
>                 /*
>                  * If the memory is already reserved (by another region), we
> -                * should not allow it to be marked nomap, but don't worry
> -                * if the region isn't memory as it won't be mapped.
> +                * should not allow it to be marked nomap, unless is the exact same region
> +                * (same base and size), which the kernel knows better and should be allowed to mark
> +                *  it as nomap.
> +                * But don't worry if the region isn't memory as it won't be mapped.
>                  */
> -               if (memblock_overlaps_region(&memblock.memory, base, size) &&
> -                   memblock_is_region_reserved(base, size))
> +               if (memblock_overlaps_region(&memblock.memory, base, size) == MEMBLOCK_OVERLAPS &&
> +                   memblock_is_region_reserved(base, size) == MEMBLOCK_OVERLAPS)

Won't this fail to work as IIRC memblock will merge regions when they
are adjacent and have the same atrributes.

Perhaps instead, the DT code should ignore any /memreserve/ entries
that are also in /reserved-memory.

I would suggest just reverse the order they are processed, but I
suspect that might cause some regression. This code is all fragile
especially with platforms putting in 100 regions.

Finally, perhaps fix u-boot. The reason the reserved location goes in
both places was to support an OS not supporting /reserved-memory. I
think that support has been in place for a lot longer than anyone
would care about.

Rob