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

[Neil Armstrong]

On 06/04/2023 17:48, Rob Herring wrote:
> 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.

Fixing U-Boot won't fix already tagged and in-the-field mainline u-boot
releases that adds /memreserve/ entries, so yes u-boot should be definitely
fixed but Linux should ignore the /memreserve/ entries when they matches
an /reserved-memory node like when the U-Boot /memreserve/ code was added.

Neil

> Rob
>
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