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diff --git a/services/std_svc/sdei/sdei_state.c b/services/std_svc/sdei/sdei_state.c
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+/*
+ * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <assert.h>
+#include <stdbool.h>
+
+#include <lib/cassert.h>
+
+#include "sdei_private.h"
+
+/* Aliases for SDEI handler states: 'R'unning, 'E'nabled, and re'G'istered */
+#define r_ 0U
+#define R_ (1u << SDEI_STATF_RUNNING)
+
+#define e_ 0U
+#define E_ (1u << SDEI_STATF_ENABLED)
+
+#define g_ 0U
+#define G_ (1u << SDEI_STATF_REGISTERED)
+
+/* All possible composite handler states */
+#define reg_ (r_ | e_ | g_)
+#define reG_ (r_ | e_ | G_)
+#define rEg_ (r_ | E_ | g_)
+#define rEG_ (r_ | E_ | G_)
+#define Reg_ (R_ | e_ | g_)
+#define ReG_ (R_ | e_ | G_)
+#define REg_ (R_ | E_ | g_)
+#define REG_ (R_ | E_ | G_)
+
+#define MAX_STATES (REG_ + 1u)
+
+/* Invalid state */
+#define SDEI_STATE_INVALID ((sdei_state_t) (-1))
+
+/* No change in state */
+#define SDEI_STATE_NOP ((sdei_state_t) (-2))
+
+#define X___ SDEI_STATE_INVALID
+#define NOP_ SDEI_STATE_NOP
+
+/* Ensure special states don't overlap with valid ones */
+CASSERT(X___ > REG_, sdei_state_overlap_invalid);
+CASSERT(NOP_ > REG_, sdei_state_overlap_nop);
+
+/*
+ * SDEI handler state machine: refer to sections 6.1 and 6.1.2 of the SDEI v1.0
+ * specification (ARM DEN0054A).
+ *
+ * Not all calls contribute to handler state transition. This table is also used
+ * to validate whether a call is permissible at a given handler state:
+ *
+ * - X___ denotes a forbidden transition;
+ * - NOP_ denotes a permitted transition, but there's no change in state;
+ * - Otherwise, XXX_ gives the new state.
+ *
+ * DISP[atch] is a transition added for the implementation, but is not mentioned
+ * in the spec.
+ *
+ * Those calls that the spec mentions as can be made any time don't picture in
+ * this table.
+ */
+
+static const sdei_state_t sdei_state_table[MAX_STATES][DO_MAX] = {
+/*
+ * Action: REG REL ENA DISA UREG ROUT CTX COMP COMPR DISP
+ * Notes: [3] [1] [3] [3][4] [2]
+ */
+ /* Handler unregistered, disabled, and not running. This is the default state. */
+/* 0 */ [reg_] = { reG_, NOP_, X___, X___, X___, X___, X___, X___, X___, X___, },
+
+ /* Handler unregistered and running */
+/* 4 */ [Reg_] = { X___, X___, X___, X___, X___, X___, NOP_, reg_, reg_, X___, },
+
+ /* Handler registered */
+/* 1 */ [reG_] = { X___, X___, rEG_, NOP_, reg_, NOP_, X___, X___, X___, X___, },
+
+ /* Handler registered and running */
+/* 5 */ [ReG_] = { X___, X___, REG_, NOP_, Reg_, X___, NOP_, reG_, reG_, X___, },
+
+ /* Handler registered and enabled */
+/* 3 */ [rEG_] = { X___, X___, NOP_, reG_, reg_, X___, X___, X___, X___, REG_, },
+
+ /* Handler registered, enabled, and running */
+/* 7 */ [REG_] = { X___, X___, NOP_, ReG_, Reg_, X___, NOP_, rEG_, rEG_, X___, },
+
+ /*
+ * Invalid states: no valid transition would leave the handler in these
+ * states; and no transition from these states is possible either.
+ */
+
+ /*
+ * Handler can't be enabled without being registered. I.e., XEg is
+ * impossible.
+ */
+/* 2 */ [rEg_] = { X___, X___, X___, X___, X___, X___, X___, X___, X___, X___, },
+/* 6 */ [REg_] = { X___, X___, X___, X___, X___, X___, X___, X___, X___, X___, },
+};
+
+/*
+ * [1] Unregister will always also disable the event, so the new state will have
+ * Xeg.
+ * [2] Event is considered for dispatch only when it's both registered and
+ * enabled.
+ * [3] Never causes change in state.
+ * [4] Only allowed when running.
+ */
+
+/*
+ * Given an action, transition the state of an event by looking up the state
+ * table above:
+ *
+ * - Return false for invalid transition;
+ * - Return true for valid transition that causes no change in state;
+ * - Otherwise, update state and return true.
+ *
+ * This function assumes that the caller holds necessary locks. If the
+ * transition has constrains other than the state table describes, the caller is
+ * expected to restore the previous state. See sdei_event_register() for
+ * example.
+ */
+bool can_sdei_state_trans(sdei_entry_t *se, sdei_action_t act)
+{
+ sdei_state_t next;
+
+ assert(act < DO_MAX);
+ if (se->state >= MAX_STATES) {
+ WARN(" event state invalid: %x\n", se->state);
+ return false;
+ }
+
+ next = sdei_state_table[se->state][act];
+ switch (next) {
+ case SDEI_STATE_INVALID:
+ return false;
+
+ case SDEI_STATE_NOP:
+ return true;
+
+ default:
+ /* Valid transition. Update state. */
+ SDEI_LOG(" event state 0x%x => 0x%x\n", se->state, next);
+ se->state = next;
+
+ return true;
+ }
+}