event: Add basic support for events

Add a way to create and dispatch events without needing to allocate
memory. Also add a way to 'spy' on events, thus allowing 'hooks' to be
created.

Use a linker list for static events, which we can use to replace functions
like arch_cpu_init_f(). Allow an EVENT_DEBUG option which makes it
easier to see what is going on at runtime, but uses more code space.

Dynamic events allow the creation of a spy at runtime. This is not always
necessary, but can be enabled with EVENT_DYNAMIC if needed.

A 'test' event is the only option for now.

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

1 files changed, 188 insertions, 0 deletions

diff --git a/include/event.h b/include/event.h
new file mode 100644
index 0000000000..effd58c704
--- /dev/null
+++ b/include/event.h
@@ -0,0 +1,188 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Events provide a general-purpose way to react to / subscribe to changes
+ * within U-Boot
+ *
+ * Copyright 2021 Google LLC
+ * Written by Simon Glass <sjg@chromium.org>
+ */
+
+#ifndef __event_h
+#define __event_h
+
+/**
+ * enum event_t - Types of events supported by U-Boot
+ *
+ * @EVT_DM_PRE_PROBE: Device is about to be probed
+ */
+enum event_t {
+	EVT_NONE,
+	EVT_TEST,
+
+	EVT_COUNT
+};
+
+union event_data {
+	/**
+	 * struct event_data_test  - test data
+	 *
+	 * @signal: A value to update the state with
+	 */
+	struct event_data_test {
+		int signal;
+	} test;
+};
+
+/**
+ * struct event - an event that can be sent and received
+ *
+ * @type: Event type
+ * @data: Data for this particular event
+ */
+struct event {
+	enum event_t type;
+	union event_data data;
+};
+
+/** Function type for event handlers */
+typedef int (*event_handler_t)(void *ctx, struct event *event);
+
+/**
+ * struct evspy_info - information about an event spy
+ *
+ * @func: Function to call when the event is activated (must be first)
+ * @type: Event type
+ * @id: Event id string
+ */
+struct evspy_info {
+	event_handler_t func;
+	enum event_t type;
+#if CONFIG_IS_ENABLED(EVENT_DEBUG)
+	const char *id;
+#endif
+};
+
+/* Declare a new event spy */
+#if CONFIG_IS_ENABLED(EVENT_DEBUG)
+#define _ESPY_REC(_type, _func)   { _func, _type, #_func, }
+#else
+#define _ESPY_REC(_type, _func)   { _func, _type, }
+#endif
+
+static inline const char *event_spy_id(struct evspy_info *spy)
+{
+#if CONFIG_IS_ENABLED(EVENT_DEBUG)
+	return spy->id;
+#else
+	return "?";
+#endif
+}
+
+/*
+ * It seems that LTO will drop list entries if it decides they are not used,
+ * although the conditions that cause this are unclear.
+ *
+ * The example found is the following:
+ *
+ * static int sandbox_misc_init_f(void *ctx, struct event *event)
+ * {
+ *    return sandbox_early_getopt_check();
+ * }
+ * EVENT_SPY(EVT_MISC_INIT_F, sandbox_misc_init_f);
+ *
+ * where EVENT_SPY uses ll_entry_declare()
+ *
+ * In this case, LTO decides to drop the sandbox_misc_init_f() function
+ * (which is fine) but then drops the linker-list entry too. This means
+ * that the code no longer works, in this case sandbox no-longer checks its
+ * command-line arguments properly.
+ *
+ * Without LTO, the KEEP() command in the .lds file is enough to keep the
+ * entry around. But with LTO it seems that the entry has already been
+ * dropped before the link script is considered.
+ *
+ * The only solution I can think of is to mark linker-list entries as 'used'
+ * using an attribute. This should be safe, since we don't actually want to drop
+ * any of these. However this does slightly limit LTO's optimisation choices.
+ */
+#define EVENT_SPY(_type, _func) \
+	static __attribute__((used)) ll_entry_declare(struct evspy_info, \
+						      _type, evspy_info) = \
+	_ESPY_REC(_type, _func)
+
+/**
+ * event_register - register a new spy
+ *
+ * @id: Spy ID
+ * @type: Event type to subscribe to
+ * @func: Function to call when the event is sent
+ * @ctx: Context to pass to the function
+ * @return 0 if OK, -ve on error
+ */
+int event_register(const char *id, enum event_t type, event_handler_t func,
+		   void *ctx);
+
+#if CONFIG_IS_ENABLED(EVENT)
+/**
+ * event_notify() - notify spies about an event
+ *
+ * It is possible to pass in union event_data here but that may not be
+ * convenient if the data is elsewhere, or is one of the members of the union.
+ * So this uses a void * for @data, with a separate @size.
+ *
+ * @type: Event type
+ * @data: Event data to be sent (e.g. union_event_data)
+ * @size: Size of data in bytes
+ * @return 0 if OK, -ve on error
+ */
+int event_notify(enum event_t type, void *data, int size);
+
+/**
+ * event_notify_null() - notify spies about an event
+ *
+ * Data is NULL and the size is 0
+ *
+ * @type: Event type
+ * @return 0 if OK, -ve on error
+ */
+int event_notify_null(enum event_t type);
+#else
+static inline int event_notify(enum event_t type, void *data, int size)
+{
+	return 0;
+}
+
+static inline int event_notify_null(enum event_t type)
+{
+	return 0;
+}
+#endif
+
+#if CONFIG_IS_ENABLED(EVENT_DYNAMIC)
+/**
+ * event_uninit() - Clean up dynamic events
+ *
+ * This removes all dynamic event handlers
+ */
+int event_uninit(void);
+
+/**
+ * event_uninit() - Set up dynamic events
+ *
+ * Init a list of dynamic event handlers, so that these can be added as
+ * needed
+ */
+int event_init(void);
+#else
+static inline int event_uninit(void)
+{
+	return 0;
+}
+
+static inline int event_init(void)
+{
+	return 0;
+}
+#endif
+
+#endif