Re: [PATCH] drivers: of: static DT reservations incorrectly added to dynamic list

Rob Herring [off-list ref] writes:

On Thu, Sep 14, 2017 at 5:24 AM, Stewart Smith
[off-list ref] wrote:

quoted

There are two types of memory reservations firmware can ask the kernel
to make in the device tree: static and dynamic.
See Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt

If you have greater than 16 entries in /reserved-memory (as we do on
POWER9 systems) you would get this scary looking error message:
[    0.000000] OF: reserved mem: not enough space all defined regions.

This is harmless if all your reservations are static (which with OPAL on
POWER9, they are).

It is not harmless if you have any dynamic reservations after the 16th.

In the first pass over the fdt to find reservations, the child nodes of
/reserved-memory are added to a static array in of_reserved_mem.c so that
memory can be reserved in a 2nd pass. The array has 16 entries. This is why,
on my dual socket POWER9 system, I get that error 4 times with 20 static
reservations.

We don't have a problem on ppc though, as in arch/powerpc/kernel/prom.c
we look at the new style /reserved-ranges property to do reservations,
and this logic was introduced in 0962e8004e974 (well before any powernv
system shipped).

Google shows up no occurances of that exact error message, so we're probably
safe in that no machine that people use has memory not being reserved when
it should be.

The fix is simple, as there's a different code path for static and dynamic
allocations, we just don't add the region to the list if it's static. Since
it's a static *OR* dynamic region, this is a perfectly valid thing to do
(although I have not checked every real world device tree on the planet
for this condition)

If the region is dynamic (i.e. no reg prop), then we bail from
__reserved_mem_reserve_reg. So wouldn't this change make the list be
empty always?

We get the dynamic node in __fdt_scan_reserved_mem() rather
than__reserved_mem_reserve_reg():


static int __init __reserved_mem_reserve_reg(unsigned long node,
                                             const char *uname)
{
...
        prop = of_get_flat_dt_prop(node, "reg", &len);
        if (!prop)
                return -ENOENT;
...
}

static int __init __fdt_scan_reserved_mem(unsigned long node, const char *uname,
                                          int depth, void *data)
{
....
....
        err = __reserved_mem_reserve_reg(node, uname);
        if (err == -ENOENT && of_get_flat_dt_prop(node, "size", NULL))
                fdt_reserved_mem_save_node(node, uname, 0, 0);

        /* scan next node */
        return 0;
}

So that should capture the dynamic reservations (as they're the ones
with the size property) to be handled by fdt_init_reserved_mem() later
in boot.

Won't we have problems with lookups for devices with a "memory-region"
property if static allocations are not in the list?

Ahh yep, I see the issue.

The array is being used for two things: reserving the memory and looking it
up during device init (seems like only used on ARM, which is why it
Worked For Me(TM) on POWER :)

It looks a bit more involved making that work, although not impossible.

I'm inclined to just make the safe and easy change of increasing the
array to 32 entries. That should be enough for everyone (TM).

that would certainly solve my immediate problem :)

(of course, given a CPU generation or two I'm sure we'd hit it again)

I'll send a patch that does that, and can asynchronously work on a patch
that addresses the device lookup of memory region problem (although
that'll be fairly down on my list of things to look at).

-- 
Stewart Smith
OPAL Architect, IBM.