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103 lines
3.6 KiB
ReStructuredText
103 lines
3.6 KiB
ReStructuredText
.. comment SPDX-License-Identifier: CC-BY-SA-4.0
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.. COMMENT: COPYRIGHT (c) 1988-2002.
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.. COMMENT: On-Line Applications Research Corporation (OAR).
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.. COMMENT: All rights reserved.
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Timer Driver
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############
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The timer driver is primarily used by the RTEMS Timing Tests. This driver
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provides as accurate a benchmark timer as possible. It typically reports its
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time in microseconds, CPU cycles, or bus cycles. This information can be very
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useful for determining precisely what pieces of code require optimization and
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to measure the impact of specific minor changes.
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The gen68340 BSP also uses the Timer Driver to support a high performance mode
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of the on-CPU UART.
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Benchmark Timer
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===============
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The RTEMS Timing Test Suite requires a benchmark timer. The RTEMS Timing Test
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Suite is very helpful for determining the performance of target hardware and
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comparing its performance to that of other RTEMS targets.
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This section describes the routines which are assumed to exist by the RTEMS
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Timing Test Suite. The names used are *EXACTLY* what is used in the RTEMS
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Timing Test Suite so follow the naming convention.
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benchmark_timer_initialize
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--------------------------
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Initialize the timer source.
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.. code-block:: c
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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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Read_timer
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----------
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The ``benchmark_timer_read`` routine returns the number of benchmark time units
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(typically microseconds) that have elapsed since the last call to
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``benchmark_timer_initialize``.
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.. code-block:: c
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benchmark_timer_t 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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Many implementations of this routine subtract the overhead required to
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initialize and read the benchmark timer. This makes the times reported more
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accurate.
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Some implementations report 0 if the harware timer value change is sufficiently
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small. This is intended to indicate that the execution time is below the
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resolution of the timer.
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benchmark_timer_disable_subtracting_average_overhead
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----------------------------------------------------
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This routine is invoked by the "Check Timer" (``tmck``) test in the RTEMS
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Timing Test Suite. It makes the ``benchmark_timer_read`` routine NOT subtract
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the overhead required to initialize and read the benchmark timer. This is used
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by the ``tmoverhd`` test to determine the overhead required to initialize and
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read the timer.
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.. code-block:: c
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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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The ``benchmark_timer_find_average_overhead`` variable is used to indicate the
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state of the "subtract overhead feature".
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gen68340 UART FIFO Full Mode
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============================
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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 characters
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have been received (*n* is the UART's FIFO length). It results in a lower
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interrupt processing time, but the problem is that a scanf primitive will block
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on a receipt of less than *n* characters. The solution is to set a timer that
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will check whether there are some characters waiting in the UART's input
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FIFO. The delay time has to be set carefully otherwise high rates will be
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broken:
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- if no character was received last time the interrupt subroutine was entered,
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set a long delay,
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- otherwise set the delay to the delay needed for ``n`` characters receipt.
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