On 12/02/2014 8:27 pm, Sebastian Huber wrote:
Add a CPU counter interface to allow access to a free-running counter.
It is useful to measure short time intervals.
What is meant by a short time interval and also what would a long time
mean ?
Is an overflow or wrap around status available and/or important ?
This can be used for
example to enable profiling of critical low-level functions.
Why use this rather than the existing rtems_clock_get_uptime_nanoseconds ?
Are you looking at sub-nano second times ?
I would be concerned about trace data based on this unless the over was
handled or interrupts are masked.
Add two busy wait functions rtems_counter_delay_ticks() and
rtems_counter_delay_nanoseconds() implemented via the CPU counter.
How is the user protected in a portable way across different cpus ?
Is there range checking or is overflows used ?
Are the calls re-entrant ?
diff --git a/doc/cpu_supplement/general.t b/doc/cpu_supplement/general.t
index cf28eef..3608705 100644
--- a/doc/cpu_supplement/general.t
+++ b/doc/cpu_supplement/general.t
@@ -341,6 +341,26 @@ _TLS_Size = _TLS_BSS_end - _TLS_Data_begin;
_TLS_Alignment = ALIGNOF (.tdata);
@end example
+@section CPU counter
+
+The CPU support must implement the CPU counter interface. A CPU counter is
+some free-running counter. It ticks usually with a frequency close to the CPU
+or system bus clock. On some architectures the actual implementation is board
+support package dependent. The CPU counter is used for profiling of low-level
+functions. It is also used to implement two busy wait functions
+@code{rtems_counter_delay_ticks()} and @code{rtems_counter_delay_nanoseconds()}
+which may be used in device drivers. It may be also used as an entropy source
+for random number generators.
+
+The CPU counter interface uses a CPU port specific unsigned integer type
+@code{CPU_Counter_Ticks} to represent CPU counter values. The CPU port must
+provide the following two functions
+
+@itemize
+@item @code{_CPU_counter_Read()} to read the current CPU counter value, and
+@item @code{_CPU_counter_Subtract()} to subtract two the CPU counter values.
+@end itemize
Is overflows handled in the maths ? I could not see it if important.
Should the user be able to ask the range of the counter ?
Chris
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