[RFC 01/14] docs: Add device tree bindings for SBI PMU extension

[Atish Patra]

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.

Signed-off-by: Atish Patra <atish.patra at wdc.com>
---
 docs/pmu_support.md | 83 +++++++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 docs/pmu_support.md

diff --git a/docs/pmu_support.md b/docs/pmu_support.md
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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 platform specific. Generic platform writes a default value <xyz> to
+the MHPMEVENTx CSR where <xyz> is formatted as described below.
+```
+  	xyz[0:19]    : event_idx
+        xyz[20:XLEN] : event_data[0:(XLEN-20)]
+
+```
+
+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**.
+
+* **opensbi,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.
+
+* **opensbi,event-to-counters**(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.
+
+* **opensbi,raw-event-to-counters**(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>
+      opensbi,event-to-mhpmevent 	= <0x0000B  0x0000 0x0001>,
+      opensbi,event-to-counters 	= <0x00001 0x00001 0x00000001>,
+      					  <0x00002 0x00002 0x00000004>,
+      					  <0x00003 0x0000A 0x00000ff8>,
+      				 	  <0x10000 0x10033 0x000ff000>,
+      opensbi,raw-event-to-counters 	= <0x0000 0x0002 0x00000f8>,
+					  <0x0000 0x0003 0x00000ff0>,
+};
+
+```
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