forked from Imagelibrary/rtems
107 lines
3.4 KiB
Perl
107 lines
3.4 KiB
Perl
@c
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@c COPYRIGHT (c) 1988-2002.
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@c On-Line Applications Research Corporation (OAR).
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@c All rights reserved.
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@c
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@c $Id$
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@c
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@chapter Timer Driver
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The timer driver is primarily used by the RTEMS Timing Tests.
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This driver provides as accurate a benchmark timer as possible.
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It typically reports its time in microseconds, CPU cycles, or
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bus cycles. This information can be very useful for determining
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precisely what pieces of code require optimization and to measure the
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impact of specific minor changes.
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The gen68340 BSP also uses the Timer Driver to support a high performance
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mode of the on-CPU UART.
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@section Benchmark Timer
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The RTEMS Timing Test Suite requires a benchmark timer. The
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RTEMS Timing Test Suite is very helpful for determining
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the performance of target hardware and comparing its performance
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to that of other RTEMS targets.
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This section describes the routines which are assumed to exist by
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the RTEMS Timing Test Suite. The names used are @b{EXACTLY} what
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is used in the RTEMS Timing Test Suite so follow the naming convention.
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@subsection benchmark_timer_initialize
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Initialize the timer source.
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@example
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void benchmark_timer_initialize(void)
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@{
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initialize the benchmark timer
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@}
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@end example
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@subsection Read_timer
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The @code{benchmark_timer_read} routine returns the number of benchmark
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time units (typically microseconds) that have elapsed since the last
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call to @code{benchmark_timer_initialize}.
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@example
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int benchmark_timer_read(void)
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@{
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stop time = read the hardware timer
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if the subtract overhead feature is enabled
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subtract overhead from stop time
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return the stop time
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@}
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@end example
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Many implementations of this routine subtract the overhead required
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to initialize and read the benchmark timer. This makes the times reported
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more accurate.
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Some implementations report 0 if the harware timer value change is
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sufficiently small. This is intended to indicate that the execution time
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is below the resolution of the timer.
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@subsection benchmark_timer_disable_subtracting_average_overhead
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This routine is invoked by the "Check Timer" (@code{tmck}) test in the
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RTEMS Timing Test Suite. It makes the @code{benchmark_timer_read}
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routine NOT subtract the overhead required
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to initialize and read the benchmark timer. This is used
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by the @code{tmoverhd} test to determine the overhead
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required to initialize and read the timer.
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@example
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void benchmark_timer_disable_subtracting_average_overhead(bool find_flag)
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@{
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disable the subtract overhead feature
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@}
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@end example
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The @code{benchmark_timer_find_average_overhead} variable is used to
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indicate the state of the "subtract overhead feature".
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@section gen68340 UART FIFO Full Mode
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The gen68340 BSP is an example of the use of the timer to support the UART
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input FIFO full mode (FIFO means First In First Out and roughly means
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buffer). This mode consists in the UART raising an interrupt when n
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characters have been received (@i{n} is the UART's FIFO length). It results
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in a lower interrupt processing time, but the problem is that a scanf
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primitive will block on a receipt of less than @i{n} characters. The solution
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is to set a timer that will check whether there are some characters
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waiting in the UART's input FIFO. The delay time has to be set carefully
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otherwise high rates will be broken:
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@itemize @bullet
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@item if no character was received last time the interrupt subroutine was
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entered, set a long delay,
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@item otherwise set the delay to the delay needed for @i{n} characters
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receipt.
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@end itemize
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