Re: [PATCH v1] dt-bindings: riscv: add SBI PMU event mappings

[Conor Dooley]

Hey Rob,

On Thu, Dec 22, 2022 at 12:06:27PM -0600, Rob Herring wrote:
> On Wed, Dec 21, 2022 at 02:15:49PM +0000, Conor Dooley wrote:
> > From: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > 
> > The SBI PMU extension requires a firmware to be aware of the event to
> > counter/mhpmevent mappings supported by the hardware. OpenSBI may use
> > DeviceTree to describe the PMU mappings. This binding is currently
> > described in markdown in OpenSBI (since v1.0 in Dec 2021) & used by QEMU
> > since v7.2.0.
> > 
> > Import the binding for use while validating dtb dumps from QEMU and
> > upcoming hardware (eg JH7110 SoC) that will make use of the event
> > mapping.
> > 
> > Link: https://github.com/riscv-software-src/opensbi/blob/master/docs/pmu_support.md
> > Co-developed-by: Atish Patra <atishp@xxxxxxxxxxxx>
> > Signed-off-by: Atish Patra <atishp@xxxxxxxxxxxx>
> > Signed-off-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > ---
> > I asked Rob on IRC about these bindings a few weeks ago & he said he
> > would be willing to take them. I have modified wording slightly in the
> > descriptions, but have mostly left things as close to the OpenSBI
> > documentation as possible.
> 
> Please CC the perf maintainers. Might be crickets, but so they at least 
> have a chance to see it.

Yah, I was kinda unsure who to CC. It does list them as being
specifically ARM so I probably made the wrong choice about inclusion.
I've added them now.

> > I'm not super sure about what I've done with the properties being
> > correct type wise, I went digging in bindings and am sorta using the
> > first thing that "fit".
> > 
> > Since you wrote the md doc Atish, I put your co-developed-by. OpenSBI
> > is BSD-2-Clause licensed so I am also unsure as to what license I can
> > use for this binding since that's where I took it from.
> > ---
> >  .../devicetree/bindings/perf/riscv,pmu.yaml   | 158 ++++++++++++++++++
> >  1 file changed, 158 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/perf/riscv,pmu.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/perf/riscv,pmu.yaml b/Documentation/devicetree/bindings/perf/riscv,pmu.yaml
> > new file mode 100644
> > index 000000000000..d65f937680af
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/perf/riscv,pmu.yaml
> > @@ -0,0 +1,158 @@
> > +# SPDX-License-Identifier: BSD-2-Clause
> > +%YAML 1.2
> > +---
> > +$id: http://devicetree.org/schemas/perf/riscv,pmu.yaml#
> > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > +
> > +title: RISC-V SBI PMU events
> > +
> > +maintainers:
> > +  - Atish Patra <atishp@xxxxxxxxxxxx>
> > +
> > +description: |
> > +  SBI PMU extension supports allow supervisor software to configure, start &
> > +  stop any performance counter at anytime. Thus, a user can leverage full
> > +  capability of performance analysis tools such as perf if the SBI PMU
> > +  extension is enabled. The OpenSBI implementation makes the following
> > +  assumptions about the hardware platform:
> > +    MCOUNTINHIBIT CSR must be implemented in the hardware. Otherwise, the SBI
> > +    PMU extension will not be enabled.
> > +
> > +    The platform must provide information about PMU event to counter mapping
> > +    via device tree or platform specific hooks. Otherwise, the SBI PMU
> > +    extension will not be enabled.
> > +
> > +    The platforms should provide information about the PMU event selector
> > +    values that should be encoded in the expected value of MHPMEVENTx while
> > +    configuring MHPMCOUNTERx for that specific event. This can be done via a
> > +    device tree or platform specific hooks. The exact value to be written to
> > +    the MHPMEVENTx is completely dependent on the platform. The generic
> > +    platform writes the zero-extended event_idx as the expected value for
> > +    hardware cache/generic events as suggested by the SBI specification.
> > +
> > +properties:
> > +  compatible:
> > +    const: riscv,pmu
> > +
> > +  riscv,event-to-mhpmevent:
> > +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
> > +    description:
> > +      Represents an ONE-to-ONE mapping between a PMU event and the event
> > +      selector value that platform expects to be written to the MHPMEVENTx CSR
> > +      for that event.
> > +      The mapping is encoded in an array format where each row represents an
> 
> s/array/matrix/
> 
> > +      event. The first element represents the event idx while the second &
> > +      third elements represent the event selector value that should be encoded
> > +      in the expected value to be written in MHPMEVENTx.
> > +      This property shouldn't encode any raw hardware event.
> > +    minItems: 1
> > +    maxItems: 255

I copied these 255s from dt-schema somewhere as a sane max.
Atish, is there a cap here or should we allow as many as someone likes?
The md binding doesn't mention any limits - is it in the SBI spec?
The strongest wording I saw there was "limited" & event idx is 20 bits
wide as per the spec [0]. Does that make 2^20 the hard limit here?

0 - https://github.com/riscv-non-isa/riscv-sbi-doc/blob/master/riscv-sbi.adoc#11-performance-monitoring-unit-extension-eid-0x504d55-pmu

> > +    items:
> > +      $ref: /schemas/types.yaml#/definitions/uint32-array
> 
> Huh? A property can only have 1 type. I wonder what the tools do with 
> this...

I suppose I left this over (by accident) from when I had it arranged
slightly differently. I guess I never actually noticed as the tools
don't appear to complain. Now dropped :)

> > +      maxItems: 3
> 
> Better to do:
> 
>          items:
>            - description: what's in the 1st word
>            - description: what's in the 2nd word
>            - description: what's in the 3rd word
> 
> And rework the overall description to not say the same thing.

Yah, good idea. I'll do that for the next version.

> > +  riscv,event-to-mhpmcounters:
> > +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
> > +    description:
> > +      Represents a MANY-to-MANY mapping between a range of events and all the
> > +      MHPMCOUNTERx in a bitmap format that can be used to monitor these range
> > +      of events. The information is encoded in an array format where each
> > +      row represents a certain range of events and corresponding counters.
> > +      The first element represents starting of the pmu event id and 2nd column
> > +      represents the end of the pmu event id. The third element represent a
> > +      bitmap of all the MHPMCOUNTERx.
> > +      This property is mandatory if event-to-mhpmevent is present. Otherwise,
> > +      it can be omitted.
> 
> No need to state this in freeform text, use 'dependencies'.

Oh! I didn't know that that existed. I was going to go and do some sort
of required properties dance but perhaps that's not needed now (at least
to the same extent).

> > +      This property shouldn't encode any raw event.
> > +    minItems: 1
> > +    maxItems: 255
> > +    items:
> > +      $ref: /schemas/types.yaml#/definitions/uint32-array
> > +      maxItems: 3
> > +
> > +  riscv,raw-event-to-mhpmcounters:
> > +    $ref: /schemas/types.yaml#/definitions/uint32-matrix
> > +    description:
> > +      Represents an ONE-to-MANY or MANY-to-MANY mapping between the rawevent(s)
> > +      and all the MHPMCOUNTERx in a bitmap format that can be used to monitor
> > +      that raw event.
> > +      The encoding of the raw events are platform specific. The information is
> > +      encoded in an array format where each row represents the specific raw
> > +      event(s). The first element is a 64-bit match value where the invariant
> > +      bits of range of events are set. The second element is a 64-bit mask that
> > +      will have all the variant bits of the range of events cleared. All other
> > +      bits should be set in the mask. The third element is a 32-bit value to
> > +      represent bitmap of all MHPMCOUNTERx that can monitor these set of
> > +      event(s). If a platform directly encodes each raw PMU event as a unique
> > +      ID, the value of select_mask must be 0xffffffff_ffffffff.
> > +    minItems: 1
> > +    maxItems: 255
> > +    items:
> > +      $ref: /schemas/types.yaml#/definitions/uint32-array
> > +      maxItems: 5
> > +
> > +required:
> > +  - compatible
> 
> I assume at least one of the other properties must be present?

Atish: (and +CC the OpenSBI list)
Which properties are actually needed here? Or are any? The md binding
in the OpenSBI sources doesn't seem to require anything other than the
compatible?

Thanks Rob,
Conor.

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