doc: userguide: add portability and usage info for odp time apis

Clarify and expand on portability and performance considerations
regarding the use of the ODP time APIs in fulfillment of JIRA
issue https://projects.linaro.org/browse/ODP-575

Signed-off-by: Bill Fischofer <bill.fischofer@linaro.org>
Signed-off-by: Maxim Uvarov <maxim.uvarov@linaro.org>

1 files changed, 21 insertions, 11 deletions

diff --git a/doc/users-guide/users-guide.adoc b/doc/users-guide/users-guide.adoc
index ead8da5e6..18d8cb8fb 100644
--- a/doc/users-guide/users-guide.adoc
+++ b/doc/users-guide/users-guide.adoc
@@ -362,31 +362,41 @@ PktIOs are represented by handles of abstract type `odp_pktio_t`.
 
 === Time
 The time API is used to measure time intervals and track time flow of an
-application and presents a convenient way to get access to a time source.
-The time API consists of two main parts: local time API and global time API.
+application and presents a convenient way to get access to an
+implementation-defined time source. The time API consists of two main parts:
+local time API and global time API.
 
 ==== Local time
-The local time API is designed to be used within one thread and can be faster
-than the global time API. The local time API cannot be used between threads as
-time consistency is not guaranteed, and in some cases that's enough.
-So, local time stamps are local to the calling thread and must not be shared
-with other threads. Current local time can be read with `odp_time_local()`.
+The local time API is designed to be used within one thread and obtaining
+local time may be more efficient in some implementations than global
+time. Local time stamps are local to the calling thread and should not be
+shared with other threads, as local time is not guaranteed to be consistent
+between threads. Current local time can be read with `odp_time_local()`.
 
 ==== Global time
 The global time API is designed to be used for tracking time between threads.
-So, global time stamps can be shared between threads. Current global time can
-be read with `odp_time_global()`.
+So, global time stamps may safely be shared between threads. Current global
+time can be read with `odp_time_global()`.
 
-Both, local and global time is not wrapped during the application life cycle.
+Both local and global time is not wrapped during the application life cycle.
 The time API includes functions to operate with time, such as `odp_time_diff()`,
 `odp_time_sum()`, `odp_time_cmp()`, conversion functions like
 `odp_time_to_ns()`, `odp_time_local_from_ns()`, `odp_time_global_from_ns()`.
 To get rate of time source `odp_time_local_res()`, `odp_time_global_res()`
 are used. To wait, `odp_time_wait_ns()` and `odp_time_wait_until()` are used,
-during witch a thread potentially busy loop the entire wait time.
+during which a thread potentially busy loops the entire wait time.
 
 The `odp_time_t` opaque type represents local or global timestamps.
 
+==== Portability Considerations
+The ODP Time APIs are designed to permit high-precision relative time
+measurement within an ODP application. No attempt is made to correlate an
+`odp_time_t` object with "wall time" or any other external time reference.
+As defined by the ODP specification, `odp_time_t` values are required to
+be unique over a span of at least 10 years. Most implementations will choose
+to implement time values using 64-bit values, whose wrap times exceed 500
+years, making wrapping concerns not relevant to ODP applications.
+
 === Timer
 Timers are how ODP applications measure and respond to the passage of time.
 Timers are drawn from specialized pools called timer pools that have their