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docs: Add device tree bindings for SBI PMU extension

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SBI PMU extension requires a firmware to be aware of the event to
counter/mhpmevent mappings supported by the hardware. One approach
is to encode that information in the device tree.

Define a device tree binding that allows a hardware to describe
all the PMU mappings required in concise format.

Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
This commit is contained in:
Atish Patra
2021-07-10 09:18:02 -07:00
committed by Anup Patel
parent 17e23b678d
commit 3e8b31aca9
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OpenSBI SBI PMU extension support
==================================
SBI PMU extension supports allow supervisor software to configure/start/stop
any performance counter at anytime. Thus, an user can leverage full
capability of performance analysis tools such as perf if 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, 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, 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 he MHPMEVENTx is
completely depends on platform. Generic platform writes the zero-extended event_idx
as the expected value for hardware cache/generic events as suggested by the SBI
specification.
SBI PMU Device Tree Bindings
----------------------------
Platforms may choose to describe PMU event selector and event to counter mapping
values via device tree. The following sections describes the PMU DT node
bindings in details.
* **compatible** (Mandatory) - The compatible string of SBI PMU device tree node.
This DT property must have the value **riscv,pmu**.
* **pmu,event-to-mhpmevent**(Optional) - It 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 a
table format where each row represents an event. The first column represent the
event idx where the 2nd & 3rd column 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.
* **pmu,event-to-mhpmcounters**(Optional) - It 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
a table format where each row represent a certain range of events and
corresponding counters. The first column represents starting of the pmu event id
and 2nd column represents the end of the pmu event id. The third column
represent a bitmap of all the MHPMCOUNTERx. This property is mandatory if
event-to-mhpmevent is present. Otherwise, it can be omitted. This property
shouldn't encode any raw event.
* **pmu,raw-event-to-mhpmcounters**(Optional) - It represents an ONE-to-MANY
mapping between a raw event and all the MHPMCOUNTERx in a bitmap format that can
be used to monitor that raw event. The information is encoded in a table format
where each raw represent a specific raw event. The first column stores the
expected event selector value that should be encoded in the expected value to be
written in MHPMEVENTx. The second column stores a bitmap of all the MHPMCOUNTERx
that can be used for monitoring the specified event.
*Note:* A platform may choose to provide the mapping between event & counters
via platform hooks rather than the device tree.
### Example
```
pmu {
compatible = "riscv,pmu";
interrupts = <0x100>;
interrupt-parent = <&plic>
pmu,event-to-mhpmevent = <0x0000B 0x0000 0x0001>,
pmu,event-to-mhpmcounters = <0x00001 0x00001 0x00000001>,
<0x00002 0x00002 0x00000004>,
<0x00003 0x0000A 0x00000ff8>,
<0x10000 0x10033 0x000ff000>,
pmu,raw-event-to-mhpmcounters = <0x0000 0x0002 0x00000f8>,
<0x0000 0x0003 0x00000ff0>,
};
```