event: Add documentation

Add documentation for events, including the event command.

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/develop/event.rst

@@ -0,0 +1,66 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Events
+======
+
+U-Boot supports a way for various events to be handled by interested
+subsystems. This provide a generic way to handle 'hooks' like setting up the
+CPUs after driver model is active, or reading a partition table after a new
+block device is probed.
+
+Rather than using weak functions and direct calls across subsystemss, it is
+often easier to use an event.
+
+An event consists of a type (e.g. EVT_DM_POST_INIT) and some optional data,
+in `union event_data`. An event spy can be creasted to watch for events of a
+particular type. When the event is created, it is sent to each spy in turn.
+
+
+Declaring a spy
+---------------
+
+To declare a spy, use something like this::
+
+    static int snow_setup_cpus(void *ctx, struct event *event)
+    {
+        /* do something */
+        return 0;
+    }
+    EVENT_SPY(EVT_DM_POST_INIT, snow_setup_cpus);
+
+Your function is called when EVT_DM_POST_INIT is emitted, i.e. after driver
+model is inited (in SPL, or in U-Boot proper before and after relocation).
+
+
+Debugging
+---------
+
+To assist with debugging events, enable `CONFIG_EVENT_DEBUG` and
+`CONFIG_CMD_EVENT`. The :doc:`../usage/event` command can then be used to
+provide a spy list.
+
+It is also possible to list spy information from the U-Boot executable,, using
+the `event_dump.py` script::
+
+    $ scripts/event_dump.py /tmp/b/sandbox/u-boot
+    Event type            Id                              Source location
+    --------------------  ------------------------------  ------------------------------
+    EVT_MISC_INIT_F       f:sandbox_misc_init_f           arch/sandbox/cpu/start.c:125
+
+This shows each event spy in U-Boot, along with the event type, function name
+(or ID) and source location.
+
+Note that if `CONFIG_EVENT_DEBUG` is not enabled, the event ID is missing, so
+the function is shown instead (with an `f:` prefix as above). Since the ID is
+generally the same as the function name, this does not matter much.
+
+The event type is decoded by the symbol used by U-Boot for the event linker
+list. Symbols have the form::
+
+    _u_boot_list_2_evspy_info_2_EVT_MISC_INIT_F
+
+so the event type can be read from the end. To manually list spy information
+in an image, use $(CROSS_COMPILE)nm::
+
+    nm u-boot |grep evspy |grep list
+    00000000002d6300 D _u_boot_list_2_evspy_info_2_EVT_MISC_INIT_F

doc/develop/index.rst

@@ -17,6 +17,7 @@ Implementation
    distro
    driver-model/index
    environment
+   event
    global_data
    logging
    makefiles