cyclic: Add documentation

Add documentation for the cyclic function infrastructure, including the
cyclic command.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/develop/cyclic.rst

@@ -0,0 +1,50 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Cyclic functions
+================
+
+The cyclic function execution infrastruture provides a way to periodically
+execute code, e.g. every 100ms. Examples for such functions might be LED
+blinking etc. The functions that are hooked into this cyclic list should
+be small timewise as otherwise the execution of the other code that relies
+on a high frequent polling (e.g. UART rx char ready check) might be
+delayed too much. To detect cyclic functions with a too long execution
+time, the Kconfig option `CONFIG_CYCLIC_MAX_CPU_TIME_US` is introduced,
+which configures the max allowed time for such a cyclic function. If it's
+execution time exceeds this time, this cyclic function will get removed
+from the cyclic list.
+
+Registering a cyclic function
+-----------------------------
+
+To register a cyclic function, use something like this::
+
+    static void cyclic_demo(void *ctx)
+    {
+        /* Just a small dummy delay here */
+        udelay(10);
+    }
+    
+    int board_init(void)
+    {
+        struct cyclic_info *cyclic;
+        
+        /* Register demo cyclic function */
+        cyclic = cyclic_register(cyclic_demo, 10 * 1000, "cyclic_demo", NULL);
+        if (!cyclic)
+        printf("Registering of cyclic_demo failed\n");
+        
+        return 0;
+    }
+
+This will register the function `cyclic_demo()` to be periodically
+executed all 10ms.
+
+How is this cyclic functionality integrated /  executed?
+--------------------------------------------------------
+
+The cyclic infrastructure integrates the main function responsible for
+calling all registered cyclic functions cyclic_run() into the common
+WATCHDOG_RESET macro. This guarantees that cyclic_run() is executed
+very often, which is necessary for the cyclic functions to get scheduled
+and executed at their configured periods.

doc/develop/index.rst

@@ -27,6 +27,7 @@ Implementation
    ci_testing
    commands
    config_binding
+   cyclic
    devicetree/index
    distro
    driver-model/index