1 | Clock Driver |
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2 | ############ |
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3 | |
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4 | Introduction |
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5 | ============ |
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6 | |
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7 | The purpose of the clock driver is to provide two services for the operating |
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8 | system. |
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9 | |
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10 | - A steady time basis to the kernel, so that the RTEMS primitives that need |
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11 | a clock tick work properly. See the *Clock Manager* chapter of the*RTEMS Application C User's Guide* for more details. |
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12 | |
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13 | - An optional time counter to generate timestamps of the uptime and wall |
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14 | clock time. |
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15 | |
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16 | The clock driver is usually located in the :file:`clock` directory of the BSP. |
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17 | Clock drivers should use the :dfn:`Clock Driver Shell` available via the:file:`clockdrv_shell.h` include file. |
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18 | |
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19 | Clock Driver Shell |
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20 | ================== |
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21 | |
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22 | The :dfn:`Clock Driver Shell` include file defines the clock driver functions |
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23 | declared in ``#include <rtems/clockdrv.h>`` which are used by RTEMS |
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24 | configuration file ``#include <rtems/confdefs.h>``. In case the application |
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25 | configuration defines ``#define CONFIGURE_APPLICATION_NEEDS_CLOCK_DRIVER``, |
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26 | then the clock driver is registered and should provide its services to the |
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27 | operating system. A hardware specific clock driver must provide some |
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28 | functions, defines and macros for the :dfn:`Clock Driver Shell` which are |
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29 | explained here step by step. A clock driver file looks in general like this. |
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30 | .. code:: c |
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31 | |
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32 | /* |
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33 | * A section with functions, defines and macros to provide hardware specific |
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34 | * functions for the Clock Driver Shell. |
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35 | \*/ |
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36 | #include "../../../shared/clockdrv_shell.h" |
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37 | |
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38 | Initialization |
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39 | -------------- |
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40 | |
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41 | Depending on the hardware capabilities one out of three clock driver variants |
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42 | must be selected. |
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43 | |
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44 | - The most basic clock driver provides only a periodic interrupt service |
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45 | routine which calls ``rtems_clock_tick()``. The interval is determined by |
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46 | the application configuration via ``#define |
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47 | CONFIGURE_MICROSECONDS_PER_TICK`` and can be obtained via``rtems_configuration_get_microseconds_per_tick()``. The timestamp |
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48 | resolution is limited to the clock tick interval. |
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49 | |
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50 | - In case the hardware lacks support for a free running counter, then the |
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51 | module used for the clock tick may provide support for timestamps with a |
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52 | resolution below the clock tick interval. For this so called simple |
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53 | timecounters can be used. |
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54 | |
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55 | - The desired variant uses a free running counter to provide accurate |
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56 | timestamps. This variant is mandatory on SMP configurations. |
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57 | |
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58 | Clock Tick Only Variant |
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59 | ~~~~~~~~~~~~~~~~~~~~~~~ |
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60 | |
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61 | .. code:: c |
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62 | |
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63 | static void some_support_initialize_hardware( void ) |
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64 | { |
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65 | /* Initialize hardware \*/ |
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66 | } |
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67 | #define Clock_driver_support_initialize_hardware() \\ |
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68 | some_support_initialize_hardware() |
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69 | /* Indicate that this clock driver lacks a proper timecounter in hardware \*/ |
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70 | #define CLOCK_DRIVER_USE_DUMMY_TIMECOUNTER |
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71 | #include "../../../shared/clockdrv_shell.h" |
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72 | |
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73 | Simple Timecounter Variant |
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74 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ |
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75 | |
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76 | .. code:: c |
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77 | |
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78 | #include <rtems/timecounter.h> |
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79 | static rtems_timecounter_simple some_tc; |
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80 | static uint32_t some_tc_get( rtems_timecounter_simple \*tc ) |
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81 | { |
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82 | return some.counter; |
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83 | } |
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84 | static bool some_tc_is_pending( rtems_timecounter_simple \*tc ) |
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85 | { |
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86 | return some.is_pending; |
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87 | } |
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88 | static uint32_t some_tc_get_timecount( struct timecounter \*tc ) |
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89 | { |
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90 | return rtems_timecounter_simple_downcounter_get( |
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91 | tc, |
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92 | some_tc_get, |
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93 | some_tc_is_pending |
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94 | ); |
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95 | } |
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96 | static void some_tc_tick( void ) |
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97 | { |
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98 | rtems_timecounter_simple_downcounter_tick( &some_tc, some_tc_get ); |
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99 | } |
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100 | static void some_support_initialize_hardware( void ) |
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101 | { |
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102 | uint32_t frequency = 123456; |
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103 | uint64_t us_per_tick = rtems_configuration_get_microseconds_per_tick(); |
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104 | uint32_t timecounter_ticks_per_clock_tick = |
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105 | ( frequency * us_per_tick ) / 1000000; |
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106 | /* Initialize hardware \*/ |
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107 | rtems_timecounter_simple_install( |
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108 | &some_tc, |
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109 | frequency, |
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110 | timecounter_ticks_per_clock_tick, |
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111 | some_tc_get_timecount |
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112 | ); |
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113 | } |
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114 | #define Clock_driver_support_initialize_hardware() \\ |
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115 | some_support_initialize_hardware() |
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116 | #define Clock_driver_timecounter_tick() \\ |
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117 | some_tc_tick() |
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118 | #include "../../../shared/clockdrv_shell.h" |
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119 | |
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120 | Timecounter Variant |
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121 | ~~~~~~~~~~~~~~~~~~~ |
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122 | |
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123 | This variant is preferred since it is the most efficient and yields the most |
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124 | accurate timestamps. It is also mandatory on SMP configurations to obtain |
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125 | valid timestamps. The hardware must provide a periodic interrupt to service |
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126 | the clock tick and a free running counter for the timecounter. The free |
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127 | running counter must have a power of two period. The ``tc_counter_mask`` |
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128 | must be initialized to the free running counter period minus one, e.g. for a |
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129 | 32-bit counter this is 0xffffffff. The ``tc_get_timecount`` function must |
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130 | return the current counter value (the counter values must increase, so if the |
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131 | counter counts down, a conversion is necessary). Use``RTEMS_TIMECOUNTER_QUALITY_CLOCK_DRIVER`` for the ``tc_quality``. Set``tc_frequency`` to the frequency of the free running counter in Hz. All |
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132 | other fields of the ``struct timecounter`` must be zero initialized. |
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133 | Install the initialized timecounter via ``rtems_timecounter_install()``. |
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134 | .. code:: c |
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135 | |
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136 | #include <rtems/timecounter.h> |
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137 | static struct timecounter some_tc; |
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138 | static uint32_t some_tc_get_timecount( struct timecounter \*tc ) |
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139 | { |
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140 | some.free_running_counter; |
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141 | } |
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142 | static void some_support_initialize_hardware( void ) |
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143 | { |
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144 | uint64_t us_per_tick = rtems_configuration_get_microseconds_per_tick(); |
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145 | uint32_t frequency = 123456; |
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146 | /* |
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147 | * The multiplication must be done in 64-bit arithmetic to avoid an integer |
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148 | * overflow on targets with a high enough counter frequency. |
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149 | \*/ |
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150 | uint32_t interval = (uint32_t) ( ( frequency * us_per_tick ) / 1000000 ); |
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151 | /* |
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152 | * Initialize hardware and set up a periodic interrupt for the configuration |
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153 | * based interval. |
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154 | \*/ |
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155 | some_tc.tc_get_timecount = some_tc_get_timecount; |
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156 | some_tc.tc_counter_mask = 0xffffffff; |
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157 | some_tc.tc_frequency = frequency; |
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158 | some_tc.tc_quality = RTEMS_TIMECOUNTER_QUALITY_CLOCK_DRIVER; |
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159 | rtems_timecounter_install( &some_tc ); |
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160 | } |
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161 | #define Clock_driver_support_initialize_hardware() \\ |
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162 | some_support_initialize_hardware() |
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163 | #include "../../../shared/clockdrv_shell.h" |
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164 | |
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165 | Install Clock Tick Interrupt Service Routine |
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166 | -------------------------------------------- |
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167 | |
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168 | The clock driver must provide a function to install the clock tick interrupt |
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169 | service routine via ``Clock_driver_support_install_isr()``. |
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170 | .. code:: c |
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171 | |
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172 | #include <bsp/irq.h> |
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173 | #include <bsp/fatal.h> |
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174 | static void some_support_install_isr( rtems_interrupt_handler isr ) |
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175 | { |
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176 | rtems_status_code sc; |
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177 | sc = rtems_interrupt_handler_install( |
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178 | SOME_IRQ, |
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179 | "Clock", |
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180 | RTEMS_INTERRUPT_UNIQUE, |
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181 | isr, |
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182 | NULL |
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183 | ); |
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184 | if ( sc != RTEMS_SUCCESSFUL ) { |
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185 | bsp_fatal( SOME_FATAL_IRQ_INSTALL ); |
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186 | } |
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187 | } |
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188 | #define Clock_driver_support_install_isr( isr, old ) \\ |
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189 | some_support_install_isr( isr ) |
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190 | #include "../../../shared/clockdrv_shell.h" |
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191 | |
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192 | Support At Tick |
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193 | --------------- |
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194 | |
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195 | The hardware specific support at tick is specified by``Clock_driver_support_at_tick()``. |
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196 | .. code:: c |
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197 | |
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198 | static void some_support_at_tick( void ) |
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199 | { |
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200 | /* Clear interrupt \*/ |
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201 | } |
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202 | #define Clock_driver_support_at_tick() \\ |
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203 | some_support_at_tick() |
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204 | #include "../../../shared/clockdrv_shell.h" |
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205 | |
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206 | System Shutdown Support |
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207 | ----------------------- |
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208 | |
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209 | The :dfn:`Clock Driver Shell` provides the routine ``Clock_exit()`` that is |
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210 | scheduled to be run during system shutdown via the ``atexit()`` routine. |
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211 | The hardware specific shutdown support is specified by``Clock_driver_support_shutdown_hardware()`` which is used by``Clock_exit()``. It should disable the clock tick source if it was |
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212 | enabled. This can be used to prevent clock ticks after the system is shutdown. |
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213 | .. code:: c |
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214 | |
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215 | static void some_support_shutdown_hardware( void ) |
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216 | { |
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217 | /* Shutdown hardware \*/ |
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218 | } |
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219 | #define Clock_driver_support_shutdown_hardware() \\ |
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220 | some_support_shutdown_hardware() |
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221 | #include "../../../shared/clockdrv_shell.h" |
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222 | |
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223 | Multiple Clock Driver Ticks Per Clock Tick |
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224 | ------------------------------------------ |
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225 | |
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226 | In case the hardware needs more than one clock driver tick per clock tick (e.g. |
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227 | due to a limited range of the hardware timer), then this can be specified with |
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228 | the optional ``#define CLOCK_DRIVER_ISRS_PER_TICK`` and ``#define |
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229 | CLOCK_DRIVER_ISRS_PER_TICK_VALUE`` defines. This is currently used only for x86 |
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230 | and it hopefully remains that way. |
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231 | .. code:: c |
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232 | |
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233 | /* Enable multiple clock driver ticks per clock tick \*/ |
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234 | #define CLOCK_DRIVER_ISRS_PER_TICK 1 |
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235 | /* Specifiy the clock driver ticks per clock tick value \*/ |
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236 | #define CLOCK_DRIVER_ISRS_PER_TICK_VALUE 123 |
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237 | #include "../../../shared/clockdrv_shell.h" |
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238 | |
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239 | Clock Driver Ticks Counter |
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240 | -------------------------- |
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241 | |
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242 | The :dfn:`Clock Driver Shell` provide a global variable that is simply a count |
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243 | of the number of clock driver interrupt service routines that have occurred. |
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244 | This information is valuable when debugging a system. This variable is |
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245 | declared as follows: |
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246 | .. code:: c |
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247 | |
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248 | volatile uint32_t Clock_driver_ticks; |
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249 | |
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250 | .. COMMENT: COPYRIGHT (c) 1988-2002. |
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251 | |
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252 | .. COMMENT: On-Line Applications Research Corporation (OAR). |
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253 | |
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254 | .. COMMENT: All rights reserved. |
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255 | |
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