ibm,powerpc-cpu-features Binding¶
This device tree binding describes CPU features available to software, with enablement, privilege, and compatibility metadata.
More general description of design and implementation of this binding is found in design.txt, which also points to documentation of specific features.
/cpus/ibm,powerpc-cpu-features node binding¶
Node: ibm,powerpc-cpu-features
Description: Container of CPU feature nodes.
The node name must be “ibm,powerpc-cpu-features”.
It is implemented as a child of the node “/cpus”, but this must not be assumed by parsers.
The node is optional but should be provided by new OPAL firmware.
Properties:
device_type
- Usage:
required
- Value type:
string
- Definition:
“cpu-features”
compatible
- Usage:
required
- Value type:
string
- Definition:
“ibm,powerpc-cpu-features”
This compatibility refers to backwards compatibility of the overall design with parsers that behave according to these guidelines. This can be extended in a backward compatible manner which would not warrant a revision of the compatible property.
isa
- Usage:
required
- Value type:
<u32>
Definition:
isa that the CPU is currently running in. This provides instruction set compatibility, less the individual feature nodes. For example, an ISA v3.0 implementation that lacks the “transactional-memory” cpufeature node should not use transactional memory facilities.
Value corresponds to the “Power ISA Version” multiplied by 1000. For example, <3000> corresponds to Version 3.0, <2070> to Version 2.07. The minor digit is available for revisions.
/cpus/ibm,powerpc-cpu-features/example-feature node bindings¶
Each child node of cpu-features represents a CPU feature / capability.
Node: A string describing an architected CPU feature, e.g., “floating-point”.
Description: A feature or capability supported by the CPUs.
The name of the node is a human readable string that forms the interface used to describe features to software. Features are currently documented in the code where they are implemented in skiboot/core/cpufeatures.c
Presence of the node indicates the feature is available.
Properties:
isa
- Usage:
required
- Value type:
<u32>
Definition:
First level of the Power ISA that the feature appears in. Software should filter out features when constraining the environment to a particular ISA version.
Value is defined similarly to /cpus/features/isa
usable-privilege
- Usage:
required
- Value type:
<u32> bit mask
- Definition:
Bit numbers are LSB0
- bit 0:
PR (problem state / user mode)
- bit 1:
OS (privileged state)
- bit 2:
HV (hypervisor state)
All other bits reserved and should be zero.
This property describes the privilege levels and/or software components that can use the feature.
If bit 0 is set, then the hwcap-bit-nr property will exist.
hv-support
- Usage:
optional
- Value type:
<u32> bit mask
- Definition:
Bit numbers are LSB0
- bit 0:
HFSCR
All other bits reserved and should be zero.
This property describes the HV privilege support required to enable the feature to lesser privilege levels. If the property does not exist then no support is required.
If no bits are set, the hypervisor must have explicit/custom support for this feature.
If the HFSCR bit is set, then the hfscr-bit-nr property will exist and the feature may be enabled by setting this bit in the HFSCR register.
os-support
- Usage:
optional
- Value type:
<u32> bit mask
- Definition:
Bit numbers are LSB0
- bit 0:
FSCR
All other bits reserved and should be zero.
This property describes the OS privilege support required to enable the feature to lesser privilege levels. If the property does not exist then no support is required.
If no bits are set, the operating system must have explicit/custom support for this feature.
If the FSCR bit is set, then the fscr-bit-nr property will exist and the feature may be enabled by setting this bit in the FSCR register.
hfscr-bit-nr
- Usage:
optional
- Value type:
<u32>
- Definition:
HFSCR bit position (LSB0)
This property exists when the hv-support property HFSCR bit is set. This property describes the bit number in the HFSCR register that the hypervisor must set in order to enable this feature.
This property also exists if an HFSCR bit corresponds with this feature. This makes CPU feature parsing slightly simpler.
fscr-bit-nr
- Usage:
optional
- Value type:
<u32>
- Definition:
FSCR bit position (LSB0)
This property exists when the os-support property FSCR bit is set. This property describes the bit number in the FSCR register that the operating system must set in order to enable this feature.
This property also exists if an FSCR bit corresponds with this feature. This makes CPU feature parsing slightly simpler.
hwcap-bit-nr
- Usage:
optional
- Value type:
<u32>
- Definition:
Linux ELF AUX vector bit position (LSB0)
This property may exist when the usable-privilege property value has PR bit set. This property describes the bit number that should be set in the ELF AUX hardware capability vectors in order to advertise this feature to userspace. Bits 0-31 correspond to bits 0-31 in AT_HWCAP vector. Bits 32-63 correspond to 0-31 in AT_HWCAP2 vector, and so on. Missing AT_HWCAPx vectors implies that the feature is not enabled or can not be advertised. Operating systems may provide a number of unassigned hardware capability bits to allow for new features to be advertised.
Some properties representing features created before this binding are advertised to userspace without a one-to-one hwcap bit number may not specify this bit. Operating system will handle those bits specifically. All new features usable by userspace will have a hwcap-bit-nr property.
dependencies
- Usage:
optional
- Value type:
<prop-encoded-array>
Definition:
If this property exists then it is a list of phandles to cpu feature nodes that must be enabled for this feature to be enabled.
Custom properties of the feature
- Usage:
optional
Definition:
Particular features may define their own properties.
Example¶
/cpus/ibm,powerpc-cpu-features {
device_type = "ibm,powerpc-cpu-features";
isa = <3020>;
darn {
isa = <3000>;
usable-privilege = <1 | 2 | 4>;
hwcap-bit-nr = <xx>;
};
scv {
isa = <3000>;
usable-privilege = <1 | 2>;
os-support = <0>;
hwcap-bit-nr = <xx>;
};
stop {
isa = <3000>;
usable-privilege = <2 | 4>;
hv-support = <0>;
os-support = <0>;
};
vsx2 (hypothetical) {
isa = <3010>;
usable-privilege = <1 | 2 | 4>;
hv-support = <0>;
os-support = <0>;
hwcap-bit-nr = <xx>;
};
vsx2-newinsns {
isa = <3020>;
usable-privilege = <1 | 2 | 4>;
os-support = <1>;
fscr-bit-nr = <xx>;
hwcap-bit-nr = <xx>;
dependencies = <&vsx2>;
};
};