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).


To assist with debugging events, enable CONFIG_EVENT_DEBUG and CONFIG_CMD_EVENT. The event command 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 script:

$ scripts/ /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:


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