Counters

Description

Kernel counters can be used to keep track of how many times a particular event has occurred. Usually this event is an external signal of some sort. The most common use of counters is in the implementation of clocks, but they can be useful with other event sources as well. Application code can attach alarms to counters, causing a function to be called when some number of events have occurred.

A new counter is initialized by a call to cyg_counter_create. The first argument is used to return a handle to the new counter which can be used for subsequent operations. The second argument allows the application to provide the memory needed for the object, thus eliminating any need for dynamic memory allocation within the kernel. If a counter is no longer required and does not have any alarms attached then cyg_counter_delete can be used to release the resources, allowing the cyg_counter data structure to be re-used.

Initializing a counter does not automatically attach it to any source of events. Instead some other code needs to call cyg_counter_tick whenever a suitable event occurs, which will cause the counter to be incremented and may cause alarms to trigger. The current value associated with the counter can be retrieved using cyg_counter_current_value and modified with cyg_counter_set_value. Typically the latter function is only used during initialization, for example to set a clock to wallclock time, but it can be used to reset a counter if necessary. However cyg_counter_set_value will never trigger any alarms. A newly initialized counter has a starting value of 0.

The kernel provides two different implementations of counters. The default is CYGIMP_KERNEL_COUNTERS_SINGLE_LIST which stores all alarms attached to the counter on a single list. This is simple and usually efficient. However when a tick occurs the kernel code has to traverse this list, typically at DSR level, so if there are a significant number of alarms attached to a single counter this will affect the system's dispatch latency. The alternative implementation, CYGIMP_KERNEL_COUNTERS_MULTI_LIST, stores each alarm in one of an array of lists such that at most one of the lists needs to be searched per clock tick. This involves extra code and data, but can improve real-time responsiveness in some circumstances. Another configuration option that is relevant here is CYGIMP_KERNEL_COUNTERS_SORT_LIST, which is disabled by default. This provides a trade off between doing work whenever a new alarm is added to a counter and doing work whenever a tick occurs. It is application-dependent which of these is more appropriate.

Valid contexts

cyg_counter_create is typically called during system initialization but may also be called in thread context. Similarly cyg_counter_delete may be called during initialization or in thread context. cyg_counter_current_value, cyg_counter_set_value and cyg_counter_tick may be called during initialization or from thread or DSR context. In fact, cyg_counter_tick is usually called from inside a DSR in response to an external event of some sort.