[PATCH] Documentation, dt, numa: Add note to empty NUMA node

[Ard Biesheuvel]

On Tue, 21 Sept 2021 at 21:45, Rob Herring <robh at kernel.org> wrote:
>
> On Sun, Sep 5, 2021 at 11:16 PM Gavin Shan <gshan at redhat.com> wrote:
> >
> > The empty memory nodes, where no memory resides in, are allowed.
> > For these empty memory nodes, the 'len' of 'reg' property is zero.
> > The NUMA node IDs are still valid and parsed, but memory may be
> > added to them through hotplug afterwards. Currently, QEMU fails
> > to boot when multiple empty memory nodes are specified. It's
> > caused by device-tree population failure and duplicated memory
> > node names.

Those memory regions are known in advance, right? So wouldn't it be
better to use something like 'status = "disabled"' here?

>
> I still don't like the fake addresses. I can't really give suggestions
> on alternative ways to fix this with you just presenting a solution.
>

Agreed. Please try to explain what the problem is, and why this is the
best way to solve it. Please include other solutions that were
considered and rejected if any exist.

> What is the failure you see? Can we relax the kernel's expectations?
> What about UEFI boot as the memory nodes aren't used (or maybe they
> are for NUMA?) How does this work with ACPI?
>

The EFI memory map only needs to describe the memory that was present
at boot. More memory can be represented as ACPI objects, including
coldplugged memory that is already present at boot. None of this
involves the memory nodes in DT.

> > As device-tree specification indicates, the 'unit-address' of
> > these empty memory nodes, part of their names, are the equivalents
> > to 'base-address'. Unfortunately, I finds difficulty to get where
> > the assignment of 'base-address' is properly documented for these
> > empty memory nodes. So lets add a section for empty memory nodes
> > to cover this in NUMA binding document. The 'unit-address',
> > equivalent to 'base-address' in the 'reg' property of these empty
> > memory nodes is specified to be the summation of highest memory
> > address plus the NUMA node ID.
> >
> > Signed-off-by: Gavin Shan <gshan at redhat.com>
> > Acked-by: Randy Dunlap <rdunlap at infradead.org>
> > ---
> >  Documentation/devicetree/bindings/numa.txt | 60 +++++++++++++++++++++-
> >  1 file changed, 59 insertions(+), 1 deletion(-)
> >
> > diff --git a/Documentation/devicetree/bindings/numa.txt b/Documentation/devicetree/bindings/numa.txt
> > index 21b35053ca5a..82f047bc8dd6 100644
> > --- a/Documentation/devicetree/bindings/numa.txt
> > +++ b/Documentation/devicetree/bindings/numa.txt
> > @@ -103,7 +103,65 @@ Example:
> >                 };
> >
> >  ==============================================================================
> > -4 - Example dts
> > +4 - Empty memory nodes
> > +==============================================================================
> > +
> > +Empty memory nodes, which no memory resides in, are allowed. The 'length'
> > +field of the 'reg' property is zero. However, the 'base-address' is a
> > +dummy and invalid address, which is the summation of highest memory address
> > +plus the NUMA node ID. The NUMA node IDs and distance maps are still valid
> > +and memory may be added into them through hotplug afterwards.
> > +
> > +Example:
> > +
> > +       memory at 0 {
> > +               device_type = "memory";
> > +               reg = <0x0 0x0 0x0 0x80000000>;
> > +               numa-node-id = <0>;
> > +       };
> > +
> > +       memory at 80000000 {
> > +               device_type = "memory";
> > +               reg = <0x0 0x80000000 0x0 0x80000000>;
> > +               numa-node-id = <1>;
> > +       };
> > +
> > +       /* Empty memory node */
> > +       memory at 100000002 {
> > +               device_type = "memory";
> > +               reg = <0x1 0x2 0x0 0x0>;
> > +               numa-node-id = <2>;
> > +       };
> > +
> > +       /* Empty memory node */
> > +       memory at 100000003 {
> > +               device_type = "memory";
> > +               reg = <0x1 0x3 0x0 0x0>;
> > +               numa-node-id = <3>;
> > +       };
>
> Do you really need the memory nodes here or just some way to define
> numa node id's 2 and 3 as valid?
>
>
> > +
> > +       distance-map {
> > +               compatible = "numa-distance-map-v1";
> > +               distance-matrix = <0 0  10>,
> > +                                 <0 1  20>,
> > +                                 <0 2  40>,
> > +                                 <0 3  20>,
> > +                                 <1 0  20>,
> > +                                 <1 1  10>,
> > +                                 <1 2  20>,
> > +                                 <1 3  40>,
> > +                                 <2 0  40>,
> > +                                 <2 1  20>,
> > +                                 <2 2  10>,
> > +                                 <2 3  20>,
> > +                                 <3 0  20>,
> > +                                 <3 1  40>,
> > +                                 <3 2  20>,
> > +                                 <3 3  10>;
> > +       };
> > +
> > +==============================================================================
> > +5 - Example dts
> >  ==============================================================================
> >
> >  Dual socket system consists of 2 boards connected through ccn bus and
> > --
> > 2.23.0
> >