Docs: Clarify IRQ/FIQ handler API in Interrupt Design Guide

The API description currently states that interrupt handlers
(interrupt_type_handler_t pointers) SHOULD return a pointer
to the target cpu_context.

This patch rewords the description of the interrupts handlers to state
that it MUST return a pointer to the target security context. It also
warns about potential portability issue. Specifically, this pointer is
not used on AArch64 and calls to context library APIs must be used to
ask BL31 to return to a specific context. However, this could change in
the future and interrupt handlers must always return the pointer.

Change-Id: I3f82a046de4d7a5b51a8cbebe7eb2a00dbbdb4f0
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>

docs/interrupt-framework-design.md

@@ -335,9 +335,9 @@ during the registration of a handler for an interrupt type.
 This component declares the following prototype for a handler of an interrupt type.
 
         typedef uint64_t (*interrupt_type_handler_t)(uint32_t id,
-					     uint32_t flags,
-					     void *handle,
-					     void *cookie);
+                                                     uint32_t flags,
+                                                     void *handle,
+                                                     void *cookie);
 
 The `id` is parameter is reserved and could be used in the future for passing
 the interrupt id of the highest pending interrupt only if there is a foolproof
@@ -358,10 +358,16 @@ The `handle` parameter points to the `cpu_context` structure of the current CPU
 for the security state specified in the `flags` parameter.
 
 Once the handler routine completes, execution will return to either the secure
-or non-secure state. The handler routine should return a pointer to
-`cpu_context` structure of the current CPU for the target security state. It
-should treat all error conditions as critical errors and take appropriate action
-within its implementation e.g. use assertion failures.
+or non-secure state. The handler routine must return a pointer to
+`cpu_context` structure of the current CPU for the target security state. On
+AArch64, this return value is currently ignored by the caller as the
+appropriate `cpu_context` to be used is expected to be set by the handler
+via the context management library APIs.
+A portable interrupt handler implementation must set the target context both in
+the structure pointed to by the returned pointer and via the context management
+library APIs. The handler should treat all error conditions as critical errors
+and take appropriate action within its implementation e.g. use assertion
+failures.
 
 The runtime firmware provides the following API for registering a handler for a
 particular type of interrupt. A Secure Payload Dispatcher service should use
@@ -370,8 +376,8 @@ interrupts. This API also requires the caller to specify the routing model for
 the type of interrupt.
 
     int32_t register_interrupt_type_handler(uint32_t type,
-					interrupt_type_handler handler,
-					uint64_t flags);
+                                            interrupt_type_handler handler,
+                                            uint64_t flags);
 
 
 The `type` parameter can be one of the three interrupt types listed above i.e.
@@ -962,13 +968,13 @@ as the resume SMC FID. It is important to note that `TSP_FID_RESUME` is a
 secure software sequence for issuing a `standard` SMC would look like this,
 assuming `P.STATE.I=0` in the non secure state :
 
-	int rc;
-	rc = smc(TSP_STD_SMC_FID, ...); 	/* Issue a Standard SMC call */
-        /* The pending non-secure interrupt is handled by the interrupt handler
-           and returns back here. */
-	while (rc == SMC_PREEMPTED) {		/* Check if the SMC call is preempted */
-	    rc = smc(TSP_FID_RESUME);		/* Issue resume SMC call */
-	}
+    int rc;
+    rc = smc(TSP_STD_SMC_FID, ...);     /* Issue a Standard SMC call */
+    /* The pending non-secure interrupt is handled by the interrupt handler
+       and returns back here. */
+    while (rc == SMC_PREEMPTED) {       /* Check if the SMC call is preempted */
+        rc = smc(TSP_FID_RESUME);       /* Issue resume SMC call */
+    }
 
 The `TSP_STD_SMC_FID` is any `standard` SMC function identifier and the smc()
 function invokes a SMC call with the required arguments. The pending non-secure