skiboot/doc: Add doc/imc.rst documentation

Add imc.rst documentation to detail the In-Memory Collection
Counters infrastructure and interface

Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>

2 files changed, 55 insertions, 0 deletions

diff --git a/doc/imc.rst b/doc/imc.rst
new file mode 100644
index 00000000..3324aa36
--- /dev/null
+++ b/doc/imc.rst
@@ -0,0 +1,54 @@
+.. _imc:
+
+OPAL/Skiboot In-Memory Collection (IMC) interface Documentation
+===============================================================
+
+Overview:
+---------
+
+In-Memory-Collection (IMC) is performance monitoring infrastrcuture
+for counters that (once started) can be read from memory at any time by
+an operating system. Such counters include those for the Nest and Core
+units, enabling continuous monitoring of resource utilisation on the chip.
+
+The API is agnostic as to how these counters are implemented. For the
+Nest units, they're implemented by having microcode in an on-chip
+microcontroller and for core units, they are implemented as part of core logic
+to gather data and periodically write it to the memory locations.
+
+Nest (On-Chip, Off-Core) unit:
+------------------------------
+
+Nest units have dedicated hardware counters which can be programmed
+to monitor various chip resources such as memory bandwidth,
+xlink bandwidth, alink bandwidth, PCI, NVlink and so on. These Nest
+unit PMU counters can be programmed in-band via scom. But alternatively,
+programming of these counters and periodically moving the counter data
+to memory are offloaded to a hardware engine part of OCC (On-Chip Controller).
+
+Microcode, starts to run at system boot in OCC complex, initialize these
+Nest unit PMUs and periodically accumulate the nest pmu counter values
+to memory. List of supported events by the microcode is packages as a DTS
+and stored in IMA_CATALOG partition.
+
+Core unit:
+----------
+
+Core IMC PMU counters are handled in the core-imc unit. Each core has
+4 Core Performance Monitoring Counters (CPMCs) which are used by Core-IMC logic.
+Two of these are dedicated to count core cycles and instructions.
+The 2 remaining CPMCs have to multiplex 128 events each.
+
+Core IMC hardware does not support interrupts and it peridocially (based on
+sampling duration) fetches the counter data and accumulate to main memory.
+Memory to accumulate counter data are refered from "PDBAR" (per-core scom)
+and "LDBAR" per-thread spr.
+
+OPAL APIs:
+----------
+
+The OPAL API is simple: a call to init a counter type, and calls to
+start and stop collection. The memory locations are described in the
+device tree.
+
+See :ref:`opal-imc-counters` and :ref:`device-tree/imc`
diff --git a/doc/index.rst b/doc/index.rst
index 5464ae25..d350acc3 100644
--- a/doc/index.rst
+++ b/doc/index.rst
@@ -34,6 +34,7 @@ Developer Guide and Internals
    pci-slot
    xscom-node-bindings
    xive
+   imc
 
 
 OPAL ABI