95 lines
3.2 KiB
ReStructuredText
95 lines
3.2 KiB
ReStructuredText
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.. _context_services:
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Context Services
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################
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Concepts
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********
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Each kernel context has an associated *context type*, which indicates whether
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the context is a task, a fiber, or the kernel's interrupt handling context.
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Task and fiber contexts also have an associated *context identifier* value,
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which is used to uniquely identify these contexts.
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Each task and fiber context may also support a 32-bit *custom data* value.
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This value is accessible only by the task or fiber itself, and can be used
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by the application for any purpose. The default custom data value for a
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task or fiber is zero.
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.. note::
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The custom data value is not available to ISRs, which operate in the shared
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kernel interrupt handling context.
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Purpose
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*******
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Use the kernel context services when writing code that needs to operate
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differently when executed by different contexts.
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Usage
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*****
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Configuring Custom Data Support
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===============================
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Use the :option:`CONTEXT_CUSTOM_DATA` configuration option
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to enable support for context custom data. By default, custom data
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support is disabled.
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Example: Performing Context-Specific Processing
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===============================================
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This code shows how a routine can use a context's custom data value
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to limit the number of times a context may call the routine.
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Counting is not performed when the routine is called by an ISR, which does not
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have a custom data value.
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.. note::
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Obviously, only a single routine can use this technique
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since it monopolizes the use of the custom data value.
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.. code-block:: c
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#define CALL_LIMIT 7
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int call_tracking_routine(void)
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{
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uint32_t call_count;
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if (context_type_get() != NANO_CTX_ISR) {
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call_count = (uint32_t)context_custom_data_get();
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if (call_count == CALL_LIMIT)
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return -1;
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call_count++;
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context_custom_data_set((void *)call_count);
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}
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/* do rest of routine's processing */
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...
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}
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APIs
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****
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The following kernel context APIs are provided by :file:`microkernel.h`
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and by :file:`nanokernel.h`:
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+-------------------------------------+---------------------------------------+
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| Call | Description |
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+=====================================+=======================================+
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| :c:func:`context_self_get()` | Gets context identifier of currently |
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| | executing task or fiber. |
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+-------------------------------------+---------------------------------------+
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| :c:func:`context_type_get()` | Gets type of currently executing |
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| | context (i.e. task, fiber, or ISR). |
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+-------------------------------------+---------------------------------------+
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| :c:func:`context_custom_data_set()` | Writes custom data for currently |
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| | executing task or fiber. |
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+-------------------------------------+---------------------------------------+
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| :c:func:`context_custom_data_get()` | Reads custom data for currently |
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| | executing task or fiber. |
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+-------------------------------------+---------------------------------------+
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