Re: [PATCH] Describe "fail" status for /cpus/cpu* nodes

[Rob Herring <robh+dt@kernel.org>]

On Fri, Oct 8, 2021 at 5:31 AM Matthias Schiffer
<matthias.schiffer@ew.tq-group.com> wrote:
>
> On Thu, 2021-09-16 at 16:10 +0200, Matthias Schiffer wrote:
> > There are situations where it is desirable to use the same base Device
> > Tree for devices with a different number of CPUs: There may be CPU
> > variants with different numbers of cores that can be used interchangably
> > on the same mainboard, or there are multiple CPU sockets. Not needing to
> > explicitly build a device tree for each such variant can make
> > maintenance significantly easier.
> >
> > For this to work, a system firmware / bootloader needs to adjust the
> > Device Tree by removing or disabling the excess CPU nodes. However, this
> > is currently not easily possible due to the special meaning of the
> > "disabled" status for CPU nodes:
> >
> > - A "disabled" CPU node is interpreted as inactive, but existent. The
> >   Linux kernel will attempt to enable such CPUs on boot, which will
> >   obviously fail for non-existent CPUs
> > - Removing the CPU node altogether from a Device Tree is much more
> >   complex than setting a single property, as it may leave dangling
> >   phandle references, often requiring specific knowledge of other nodes'
> >   structure to deal with them.
> >
> > In the discussion [1] it was suggested to introduce a new status value
> > for CPUs that should really not be used at all. Rob proposed to use the
> > value "fail", which already exists in the generic definitions of the
> > status property.
> >
> > [1] https://www.lkml.org/lkml/2020/8/26/1237
>
> Hi,
> I haven't received any feedback regarding this spec update yet. Should
> I also send a kernel patch that actually implements this behaviour?

Looks fine to me, just hadn't gotten around to applying.

> Do properties described in the spec also need to be documented in the
> kernel's Documentation/devicetree/bindings? It seems that there is no
> generic "cpu" binding documentation at the moment, only arch-specific
> variants.

https://github.com/devicetree-org/dt-schema/blob/main/schemas/cpus.yaml

Rob