[PATCH v3] dt-bindings: riscv: deprecate riscv,isa
Palmer Dabbelt
palmer at rivosinc.com
Mon Jun 26 12:53:37 PDT 2023
On Mon, 26 Jun 2023 10:38:43 PDT (-0700), apatel at ventanamicro.com wrote:
> On Mon, Jun 26, 2023 at 3:42 PM Conor Dooley <conor.dooley at microchip.com> wrote:
>>
>> intro
>> =====
>>
>> When the RISC-V dt-bindings were accepted upstream in Linux, the base
>> ISA etc had yet to be ratified. By the ratification of the base ISA,
>> incompatible changes had snuck into the specifications - for example the
>> Zicsr and Zifencei extensions were spun out of the base ISA.
>>
>> Fast forward to today, and the reason for this patch.
>> Currently the riscv,isa dt property permits only a specific subset of
>> the ISA string - in particular it excludes version numbering.
>> With the current constraints, it is not possible to discern whether
>> "rv64i" means that the hart supports the fence.i instruction, for
>> example.
>> Future systems may choose to implement their own instruction fencing,
>> perhaps using a vendor extension, or they may not implement the optional
>> counter extensions. Software needs a way to determine this.
>>
>> versioning schemes
>> ==================
>>
>> "Use the extension versions that are described in the ISA manual" you
>> may say, and it's not like this has not been considered.
>> Firstly, software that parses the riscv,isa property at runtime will
>> need to contain a lookup table of some sort that maps arbitrary versions
>> to versions it understands. There is not a consistent application of
>> version number applied to extensions, with a higgledy-piggledy
>> collection of tags, "bare" and versioned documents awaiting the reader
>> on the "recently ratified extensions" page:
>> https://wiki.riscv.org/display/HOME/Recently+Ratified+Extensions
>>
>> As an aside, and this is reflected in the patch too, since many
>> extensions have yet to appear in a release of the ISA specs,
>> they are defined by commits in their respective "working draft"
>> repositories.
>>
>> Secondly, there is an issue of backwards compatibility, whereby allowing
>> numbers in the ISA string, some parsers may be broken. This would
>> require an additional property to be created to even use the versions in
>> this manner.
>>
>> ~boolean properties~ string array property
>> ==========================================
>>
>> If a new property is needed, the whole approach may as well be looked at
>> from the bottom up. A string with limited character choices etc is
>> hardly the best approach for communicating extension information to
>> software.
>>
>> Switching to using properties that are defined on a per extension basis,
>> allows us to define explicit meanings for the DT representation of each
>> extension - rather than the current situation where different operating
>> systems or other bits of software may impart different meanings to
>> characters in the string.
>> Clearly the best source of meanings is the specifications themselves,
>> this just provides us the ability to choose at what point in time the
>> meaning is set. If an extension changes incompatibility in the future,
>> a new property will be required.
>>
>> Off-list, some of the RVI folks have committed to shoring up the wording
>> in either the ISA specifications, the riscv-isa-manual or
>> so that in the future, modifications to and additions or removals of
>> features will require a new extension. Codifying that assertion
>> somewhere would make it quite unlikely that compatibility would be
>> broken, but we have the tools required to deal with it, if & when it
>> crops up.
>> It is in our collective interest, as consumers of extension meanings, to
>> define a scheme that enforces compatibility.
>>
>> The use of individual properties, rather than elements in a single
>> string, will also permit validation that the properties have a meaning,
>> as well as potentially reject mutually exclusive combinations, or
>> enforce dependencies between extensions. That would not have be possible
>> with the current dt-schema infrastructure for arbitrary strings, as we
>> would need to add a riscv,isa parser to dt-validate!
>> That's not implemented in this patch, but rather left as future work (for
>> the brave, or the foolish).
>>
>> acpi
>> ====
>>
>> The current ACPI ECR is based on having a single ISA string unfortunately,
>> but ideally ACPI will move to another method, perhaps GUIDs, that give
>> explicit meaning to extensions.
>
> Drop this paragraph on ACPI.
>
> We clearly mentioned previously that ACPI will follow specs defined by RVI.
> There are scalability issues in using GUIDs for each ISA extension.
Which spec are we following for the ACPI ISA string?
> Regards,
> Anup
>
>>
>> parser simplicity
>> =================
>>
>> Many systems that parse DT at runtime already implement an function that
>> can check for the presence of a string in an array of string, as it is
>> similar to the process for parsing a list of compatible strings, so a
>> bunch of new, custom, DT parsing should not be needed.
>> Getting rid of "riscv,isa" parsing would be a nice simplification, but
>> unfortunately for backwards compatibility with old dtbs, existing
>> parsers may not be removable - which may greatly simplify
>> dt parsing code. In Linux, for example, checking for whether a hart
>> supports an extension becomes as simple as:
>> of_property_match_string(node, "riscv,isa-extensions", "zicbom")
>>
>> vendor extensions
>> =================
>>
>> Compared to riscv,isa, this proposed scheme promotes vendor extensions,
>> oft touted as the strength of RISC-V, to first-class citizens.
>> At present, extensions are defined as meaning what the RISC-V ISA
>> specifications say they do. There is no realistic way of using that
>> interface to provide cross-platform definitions for what vendor
>> extensions mean. Vendor extensions may also have even less consistency
>> than RVI do in terms of versioning, or no care about backwards
>> compatibility.
>> The new property allows us to assign explicit meanings on a per vendor
>> extension basis, backed up by a description of their meanings.
>>
>> fin
>> ===
>>
>> Create a new file to store the extension meanings and a new
>> riscv,isa-base property to replace the aspect of riscv,isa that is
>> not represented by the new property - the base ISA implemented by a hart.
>>
>> As a starting point, add properties for extensions currently used in
>> Linux.
>>
>> Finally, mark riscv,isa as deprecated, as removing support for it in
>> existing programs would be an ABI break.
>>
>> CC: Palmer Dabbelt <palmer at dabbelt.com>
>> CC: Paul Walmsley <paul.walmsley at sifive.com>
>> CC: Rob Herring <robh+dt at kernel.org>
>> CC: Krzysztof Kozlowski <krzysztof.kozlowski+dt at linaro.org>
>> CC: Alistair Francis <alistair.francis at wdc.com>
>> CC: Andrew Jones <ajones at ventanamicro.com>
>> CC: Anup Patel <apatel at ventanamicro.com>
>> CC: Atish Patra <atishp at atishpatra.org>
>> CC: Jessica Clarke <jrtc27 at jrtc27.com>
>> CC: Rick Chen <rick at andestech.com>
>> CC: Leo <ycliang at andestech.com>
>> CC: Oleksii <oleksii.kurochko at gmail.com>
>> CC: linux-riscv at lists.infradead.org
>> CC: qemu-riscv at nongnu.org
>> CC: u-boot at lists.denx.de
>> CC: devicetree at vger.kernel.org
>> CC: linux-kernel at vger.kernel.org
>> Reviewed-by: Palmer Dabbelt <palmer at rivosinc.com>
>> Acked-by: Palmer Dabbelt <palmer at rivosinc.com>
>> Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
>> ---
>> Changes in v3:
>> - Per Rob's suggestion, switch to an array of strings. Cuts down on the
>> size, compared to booleans. It has a standard mechanism for parsing
>> (you need to parse arrays of strings for compatibles). It still allows
>> for having a limited set of explicitly defined properties - so the
>> advantages over a free-form string still apply.
>> - Pick up Palmer's Ack and Review (although I expect that he will be the
>> one to apply this).
>> ---
>> .../devicetree/bindings/riscv/cpus.yaml | 43 ++-
>> .../devicetree/bindings/riscv/extensions.yaml | 245 ++++++++++++++++++
>> 2 files changed, 265 insertions(+), 23 deletions(-)
>> create mode 100644 Documentation/devicetree/bindings/riscv/extensions.yaml
>>
>> diff --git a/Documentation/devicetree/bindings/riscv/cpus.yaml b/Documentation/devicetree/bindings/riscv/cpus.yaml
>> index 67bd239ead0b..74bc92591086 100644
>> --- a/Documentation/devicetree/bindings/riscv/cpus.yaml
>> +++ b/Documentation/devicetree/bindings/riscv/cpus.yaml
>> @@ -25,6 +25,7 @@ description: |
>>
>> allOf:
>> - $ref: /schemas/cpu.yaml#
>> + - $ref: extensions.yaml
>>
>> properties:
>> compatible:
>> @@ -82,25 +83,6 @@ properties:
>> description:
>> The blocksize in bytes for the Zicboz cache operations.
>>
>> - riscv,isa:
>> - description:
>> - Identifies the specific RISC-V instruction set architecture
>> - supported by the hart. These are documented in the RISC-V
>> - User-Level ISA document, available from
>> - https://riscv.org/specifications/
>> -
>> - Due to revisions of the ISA specification, some deviations
>> - have arisen over time.
>> - Notably, riscv,isa was defined prior to the creation of the
>> - Zicntr, Zicsr, Zifencei and Zihpm extensions and thus "i"
>> - implies "zicntr_zicsr_zifencei_zihpm".
>> -
>> - While the isa strings in ISA specification are case
>> - insensitive, letters in the riscv,isa string must be all
>> - lowercase.
>> - $ref: /schemas/types.yaml#/definitions/string
>> - pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
>> -
>> # RISC-V has multiple properties for cache op block sizes as the sizes
>> # differ between individual CBO extensions
>> cache-op-block-size: false
>> @@ -139,8 +121,17 @@ properties:
>> DMIPS/MHz, relative to highest capacity-dmips-mhz
>> in the system.
>>
>> +oneOf:
>> + - required:
>> + - riscv,isa
>> + - required:
>> + - riscv,isa-base
>> +
>> +dependencies:
>> + riscv,isa-base: [ "riscv,isa-extensions" ]
>> + riscv,isa-extensions: [ "riscv,isa-base" ]
>> +
>> required:
>> - - riscv,isa
>> - interrupt-controller
>>
>> unevaluatedProperties: false
>> @@ -160,7 +151,9 @@ examples:
>> i-cache-sets = <128>;
>> i-cache-size = <16384>;
>> reg = <0>;
>> - riscv,isa = "rv64imac";
>> + riscv,isa-base = "rv64i";
>> + riscv,isa-extensions = "i", "m", "a", "c";
>> +
>> cpu_intc0: interrupt-controller {
>> #interrupt-cells = <1>;
>> compatible = "riscv,cpu-intc";
>> @@ -183,8 +176,10 @@ examples:
>> i-tlb-size = <32>;
>> mmu-type = "riscv,sv39";
>> reg = <1>;
>> - riscv,isa = "rv64imafdc";
>> tlb-split;
>> + riscv,isa-base = "rv64i";
>> + riscv,isa-extensions = "i", "m", "a", "f", "d", "c";
>> +
>> cpu_intc1: interrupt-controller {
>> #interrupt-cells = <1>;
>> compatible = "riscv,cpu-intc";
>> @@ -202,8 +197,10 @@ examples:
>> device_type = "cpu";
>> reg = <0>;
>> compatible = "riscv";
>> - riscv,isa = "rv64imafdc";
>> mmu-type = "riscv,sv48";
>> + riscv,isa-base = "rv64i";
>> + riscv,isa-extensions = "i", "m", "a", "f", "d", "c";
>> +
>> interrupt-controller {
>> #interrupt-cells = <1>;
>> interrupt-controller;
>> diff --git a/Documentation/devicetree/bindings/riscv/extensions.yaml b/Documentation/devicetree/bindings/riscv/extensions.yaml
>> new file mode 100644
>> index 000000000000..af98307f2c2c
>> --- /dev/null
>> +++ b/Documentation/devicetree/bindings/riscv/extensions.yaml
>> @@ -0,0 +1,245 @@
>> +# SPDX-License-Identifier: (GPL-2.0 OR MIT)
>> +%YAML 1.2
>> +---
>> +$id: http://devicetree.org/schemas/riscv/extensions.yaml#
>> +$schema: http://devicetree.org/meta-schemas/core.yaml#
>> +
>> +title: RISC-V ISA extensions
>> +
>> +maintainers:
>> + - Paul Walmsley <paul.walmsley at sifive.com>
>> + - Palmer Dabbelt <palmer at sifive.com>
>> + - Conor Dooley <conor at kernel.org>
>> +
>> +description: |
>> + RISC-V has a large number of extensions, some of which are "standard"
>> + extensions, meaning they are ratified by RISC-V International, and others
>> + are "vendor" extensions.
>> + This document defines properties that indicate whether a hart supports a
>> + given extension.
>> +
>> + Once a standard extension has been ratified, no changes in behaviour can be
>> + made without the creation of a new extension.
>> + The properties for standard extensions therefore map to their originally
>> + ratified states, with the exception of the I, Zicntr & Zihpm extensions.
>> + See the "i" property for more information.
>> +
>> +select:
>> + properties:
>> + compatible:
>> + contains:
>> + const: riscv
>> +
>> +properties:
>> + riscv,isa:
>> + description:
>> + Identifies the specific RISC-V instruction set architecture
>> + supported by the hart. These are documented in the RISC-V
>> + User-Level ISA document, available from
>> + https://riscv.org/specifications/
>> +
>> + Due to revisions of the ISA specification, some deviations
>> + have arisen over time.
>> + Notably, riscv,isa was defined prior to the creation of the
>> + Zicntr, Zicsr, Zifencei and Zihpm extensions and thus "i"
>> + implies "zicntr_zicsr_zifencei_zihpm".
>> +
>> + While the isa strings in ISA specification are case
>> + insensitive, letters in the riscv,isa string must be all
>> + lowercase.
>> + $ref: /schemas/types.yaml#/definitions/string
>> + pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
>> + deprecated: true
>> +
>> + riscv,isa-base:
>> + description:
>> + The base ISA implemented by this hart, as described by the 20191213
>> + version of the unprivileged ISA specification.
>> + enum:
>> + - rv32i
>> + - rv64i
>> +
>> + riscv,isa-extensions:
>> + $ref: /schemas/types.yaml#/definitions/string-array
>> + minItems: 1
>> + description: Extensions supported by the hart.
>> + items:
>> + anyOf:
>> + # single letter extensions, in canonical order
>> + - const: i
>> + description: |
>> + The base integer instruction set, as ratified in the 20191213
>> + version of the unprivileged ISA specification, with the exception of
>> + counter access.
>> + Counter access was removed after the ratification of the 20191213
>> + version of the unprivileged specification and shunted into the
>> + Zicntr and Zihpm extensions.
>> +
>> + - const: m
>> + description:
>> + The standard M extension for integer multiplication and division, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: a
>> + description:
>> + The standard A extension for atomic instructions, as ratified in the
>> + 20191213 version of the unprivileged ISA specification.
>> +
>> + - const: f
>> + description:
>> + The standard F extension for single-precision floating point, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: d
>> + description:
>> + The standard D extension for double-precision floating-point, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: q
>> + description:
>> + The standard Q extension for quad-precision floating-point, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: c
>> + description:
>> + The standard C extension for compressed instructions, as ratified in
>> + the 20191213 version of the unprivileged ISA specification.
>> +
>> + - const: v
>> + description:
>> + The standard V extension for vector operations, as ratified
>> + in-and-around commit 7a6c8ae ("Fix text that describes vfmv.v.f
>> + encoding") of the riscv-v-spec.
>> +
>> + - const: h
>> + description:
>> + The standard H extension for hypervisors as ratified in the 20191213
>> + version of the privileged ISA specification.
>> +
>> + # multi-letter extensions, sorted alphanumerically
>> + - const: smaia
>> + description: |
>> + The standard Smaia supervisor-level extension for the advanced
>> + interrupt architecture for machine-mode-visible csr and behavioural
>> + changes to interrupts as frozen at commit ccbddab ("Merge pull
>> + request #42 from riscv/jhauser-2023-RC4") of riscv-aia.
>> +
>> + - const: ssaia
>> + description: |
>> + The standard Ssaia supervisor-level extension for the advanced
>> + interrupt architecture for supervisor-mode-visible csr and
>> + behavioural changes to interrupts as frozen at commit ccbddab
>> + ("Merge pull request #42 from riscv/jhauser-2023-RC4") of riscv-aia.
>> +
>> + - const: sscofpmf
>> + description: |
>> + The standard Sscofpmf supervisor-level extension for count overflow
>> + and mode-based filtering as ratified at commit 01d1df0 ("Add ability
>> + to manually trigger workflow. (#2)") of riscv-count-overflow.
>> +
>> + - const: sstc
>> + description: |
>> + The standard Sstc supervisor-level extension for time compare as
>> + ratified at commit 3f9ed34 ("Add ability to manually trigger
>> + workflow. (#2)") of riscv-time-compare.
>> +
>> + - const: svinval
>> + description:
>> + The standard Svinval supervisor-level extension for fine-grained
>> + address-translation cache invalidation as ratified in the 20191213
>> + version of the privileged ISA specification.
>> +
>> + - const: svnapot
>> + description:
>> + The standard Svnapot supervisor-level extensions for napot
>> + translation contiguity as ratified in the 20191213 version of the
>> + privileged ISA specification.
>> +
>> + - const: svpbmt
>> + description:
>> + The standard Svpbmt supervisor-level extensions for page-based
>> + memory types as ratified in the 20191213 version of the privileged
>> + ISA specification.
>> +
>> + - const: zba
>> + description: |
>> + The standard Zba bit-manipulation extension for address generation
>> + acceleration instructions as ratified at commit 6d33919 ("Merge pull
>> + request #158 from hirooih/clmul-fix-loop-end-condition") of
>> + riscv-bitmanip.
>> +
>> + - const: zbb
>> + description: |
>> + The standard Zbb bit-manipulation extension for basic bit-manipulation
>> + as ratified at commit 6d33919 ("Merge pull request #158 from
>> + hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
>> +
>> + - const: zbc
>> + description: |
>> + The standard Zbc bit-manipulation extension for carry-less
>> + multiplication as ratified at commit 6d33919 ("Merge pull request
>> + #158 from hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
>> +
>> + - const: zbs
>> + description: |
>> + The standard Zbs bit-manipulation extension for single-bit
>> + instructions as ratified at commit 6d33919 ("Merge pull request #158
>> + from hirooih/clmul-fix-loop-end-condition") of riscv-bitmanip.
>> +
>> + - const: zicbom
>> + description:
>> + The standard Zicbom extension for base cache management operations as
>> + ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
>> +
>> + - const: zicbop
>> + description:
>> + The standard Zicbop extension for cache-block prefetch instructions
>> + as ratified in commit 3dd606f ("Create cmobase-v1.0.pdf") of
>> + riscv-CMOs.
>> +
>> + - const: zicboz
>> + description:
>> + The standard Zicboz extension for cache-block zeroing as ratified
>> + in commit 3dd606f ("Create cmobase-v1.0.pdf") of riscv-CMOs.
>> +
>> + - const: zicntr
>> + description:
>> + The standard Zicntr extension for base counters and timers, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: zicsr
>> + description:
>> + The standard Zicsr extension for control and status register
>> + instructions, as ratified in the 20191213 version of the
>> + unprivileged ISA specification.
>> +
>> + - const: zifencei
>> + description:
>> + The standard Zifencei extension for instruction-fetch fence, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: zihintpause
>> + description:
>> + The standard Zihintpause extension for pause hints, as ratified in
>> + commit d8ab5c7 ("Zihintpause is ratified") of the riscv-isa-manual.
>> +
>> + - const: zihpm
>> + description:
>> + The standard Zihpm extension for hardware performance counters, as
>> + ratified in the 20191213 version of the unprivileged ISA
>> + specification.
>> +
>> + - const: ztso
>> + description:
>> + The standard Ztso extension for total store ordering, as ratified
>> + in commit 2e5236 ("Ztso is now ratified.") of the
>> + riscv-isa-manual.
>> +
>> +additionalProperties: true
>> +...
>> --
>> 2.40.1
>>
More information about the linux-riscv
mailing list