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? 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. > > Suggested-by: Rob Herring <robh+dt@xxxxxxxxxx> > Signed-off-by: Matthias Schiffer <matthias.schiffer@xxxxxxxxxxxxxxx> > --- > source/chapter3-devicenodes.rst | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/source/chapter3-devicenodes.rst b/source/chapter3-devicenodes.rst > index 1cea148..9d8ef6e 100644 > --- a/source/chapter3-devicenodes.rst > +++ b/source/chapter3-devicenodes.rst > @@ -614,8 +614,8 @@ standard properties with specific applicable detail. > CPU. This property shall be present for nodes > representing CPUs in a symmetric > multiprocessing (SMP) configuration. For a CPU > - node the meaning of the ``"okay"`` and > - ``"disabled"`` values are as follows: > + node the meaning of the ``"okay"``, ``"disabled"`` > + and ``"fail"`` values are as follows: > > ``"okay"`` : > The CPU is running. > @@ -623,6 +623,9 @@ standard properties with specific applicable detail. > ``"disabled"`` : > The CPU is in a quiescent state. > > + ``"fail"`` : > + The CPU is not operational or does not exist. > + > A quiescent CPU is in a state where it cannot > interfere with the normal operation of other > CPUs, nor can its state be affected by the > @@ -639,6 +642,11 @@ standard properties with specific applicable detail. > loop, held in reset, and electrically isolated > from the system bus or in another > implementation dependent state. > + > + A CPU with ``"fail"`` status does not affect the > + system in any way. > + The status is assigned to nodes for which no > + corresponding CPU exists. > ``enable-method`` | SD | ``<stringlist>`` Describes the method by which a CPU in a > disabled state is enabled. This property is > required for CPUs with a status property with
