GIC: Introduce API to get interrupt ID

Acknowledging interrupt shall return a raw value from the interrupt
controller in which the actual interrupt ID may be encoded. Add a
platform API to extract the actual interrupt ID from the raw value
obtained from interrupt controller.

Document the new function. Also clarify the semantics of interrupt
acknowledge.

Change-Id: I818dad7be47661658b16f9807877d259eb127405
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

2 files changed, 24 insertions, 5 deletions

diff --git a/docs/platform-interrupt-controller-API.rst b/docs/platform-interrupt-controller-API.rst
index 795c08562..c14f00530 100644
--- a/docs/platform-interrupt-controller-API.rst
+++ b/docs/platform-interrupt-controller-API.rst
@@ -292,6 +292,22 @@ inserts to order memory updates before updating mask, then writes to the GIC
 *Priority Mask Register*, and make sure memory updates are visible before
 potential trigger due to mask update.
 
+Function: unsigned int plat_ic_get_interrupt_id(unsigned int raw); [optional]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : unsigned int
+    Return   : unsigned int
+
+This API should extract and return the interrupt number from the raw value
+obtained by the acknowledging the interrupt (read using
+``plat_ic_acknowledge_interrupt()``). If the interrupt ID is invalid, this API
+should return ``INTR_ID_UNAVAILABLE``.
+
+In case of ARM standard platforms using GIC, the implementation of the API
+masks out the interrupt ID field from the acknowledged value from GIC.
+
 ----
 
 *Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.*
diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index f0a8aaf30..af933fdbc 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -2479,14 +2479,17 @@ Function : plat\_ic\_acknowledge\_interrupt() [mandatory]
     Return   : uint32_t
 
 This API is used by the CPU to indicate to the platform IC that processing of
-the highest pending interrupt has begun. It should return the id of the
-interrupt which is being processed.
+the highest pending interrupt has begun. It should return the raw, unmodified
+value obtained from the interrupt controller when acknowledging an interrupt.
+The actual interrupt number shall be extracted from this raw value using the API
+`plat_ic_get_interrupt_id()`__.
+
+.. __: platform-interrupt-controller-API.rst#function-unsigned-int-plat-ic-get-interrupt-id-unsigned-int-raw-optional
 
 This function in ARM standard platforms using GICv2, reads the *Interrupt
 Acknowledge Register* (``GICC_IAR``). This changes the state of the highest
 priority pending interrupt from pending to active in the interrupt controller.
-It returns the value read from the ``GICC_IAR``. This value is the id of the
-interrupt whose state has been changed.
+It returns the value read from the ``GICC_IAR``, unmodified.
 
 In the case of ARM standard platforms using GICv3, if the API is invoked
 from EL3, the function reads the system register ``ICC_IAR0_EL1``, *Interrupt
@@ -2494,7 +2497,7 @@ Acknowledge Register group 0*. If the API is invoked from S-EL1, the function
 reads the system register ``ICC_IAR1_EL1``, *Interrupt Acknowledge Register
 group 1*. The read changes the state of the highest pending interrupt from
 pending to active in the interrupt controller. The value read is returned
-and is the id of the interrupt whose state has been changed.
+unmodified.
 
 The TSP uses this API to start processing of the secure physical timer
 interrupt.