[PATCH v2 5/5] Documentation: arm64: Document PMU counters access from userspace

[Raphael Gault <raphael.gault@arm.com>]

Add a documentation file to describe the access to the pmu hardware
counters from userspace

Signed-off-by: Raphael Gault <raphael.gault@arm.com>
---
 .../arm64/pmu_counter_user_access.txt         | 42 +++++++++++++++++++
 1 file changed, 42 insertions(+)
 create mode 100644 Documentation/arm64/pmu_counter_user_access.txt

diff --git a/Documentation/arm64/pmu_counter_user_access.txt b/Documentation/arm64/pmu_counter_user_access.txt
new file mode 100644
index 000000000000..6788b1107381
--- /dev/null
+++ b/Documentation/arm64/pmu_counter_user_access.txt
@@ -0,0 +1,42 @@
+Access to PMU hardware counter from userspace
+=============================================
+
+Overview
+--------
+The perf user-space tool relies on the PMU to monitor events. It offers an
+abstraction layer over the hardware counters since the underlying
+implementation is cpu-dependent.
+Arm64 allows userspace tools to have access to the registers storing the
+hardware counters' values directly.
+
+This targets specifically self-monitoring tasks in order to reduce the overhead
+by directly accessing the registers without having to go through the kernel.
+
+How-to
+------
+The focus is set on the armv8 pmuv3 which makes sure that the access to the pmu
+registers is enable and that the userspace have access to the relevent
+information in order to use them.
+
+In order to have access to the hardware counter it is necessary to open the event
+using the perf tool interface: the sys_perf_event_open syscall returns a fd which
+can subsequently be used with the mmap syscall in order to retrieve a page of memory
+containing information about the event.
+The PMU driver uses this page to expose to the user the hardware counter's
+index. Using this index enables the user to access the PMU registers using the
+`mrs` instruction.
+
+Have a look `at tools/perf/arch/arm64/tests/user-events.c` for an example. It can be
+run using the perf tool to check that the access to the registers works
+correctly from userspace:
+
+./perf test -v
+
+About chained events
+--------------------
+When the user requests for an event to be counted on 64 bits, two hardware
+counters are used and need to be combined to retrieve the correct value:
+
+val = read_counter(idx);
+if ((event.attr.config1 & 0x1))
+	val = (val << 32) | read_counter(idx - 1);
-- 
2.17.1