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* SUPPORT, LICENSE: New files. * Numerous files touched as part of merging the 4.5 branch onto the mainline development trunk and ensuring that the script that cuts snapshots and releases works on the documentation.
122 lines
3.6 KiB
Perl
122 lines
3.6 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 Timer_initialize
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Initialize the timer source.
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@example
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void 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{Read_timer} routine
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returns the number of benchmark time units (typically microseconds)
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that have elapsed since the last call to @code{Timer_initialize}.
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@example
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int Read_timer(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 An Empty Function
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This routine is invoked by the RTEMS Timing Test Suite to measure
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the cost of invoking a subroutine.
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@example
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rtems_status_code Empty_function (void)
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@{
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return RTEMS_SUCCESSFUL;
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@}
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@end example
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@subsection Set_find_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{Read_timer}
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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 Set_find_average_overhead(rtems_boolean 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{Timer_driver_Find_average_overhead} variable is usually
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used to 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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