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 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 -- 2.17.1
