build: Define build option for hardware-assisted coherency

The boolean build option HW_ASSISTED_COHERENCY is introduced to enable
various optimizations in ARM Trusted Software, when built for such
systems. It's set to 0 by default.

Change-Id: I638390da6e1718fe024dcf5b402e07084f1eb014
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

Makefile

@@ -385,6 +385,7 @@ $(eval $(call assert_boolean,ENABLE_PSCI_STAT))
 $(eval $(call assert_boolean,ENABLE_RUNTIME_INSTRUMENTATION))
 $(eval $(call assert_boolean,ERROR_DEPRECATED))
 $(eval $(call assert_boolean,GENERATE_COT))
+$(eval $(call assert_boolean,HW_ASSISTED_COHERENCY))
 $(eval $(call assert_boolean,LOAD_IMAGE_V2))
 $(eval $(call assert_boolean,NS_TIMER_SWITCH))
 $(eval $(call assert_boolean,PL011_GENERIC_UART))
@@ -419,6 +420,7 @@ $(eval $(call add_define,ENABLE_PMF))
 $(eval $(call add_define,ENABLE_PSCI_STAT))
 $(eval $(call add_define,ENABLE_RUNTIME_INSTRUMENTATION))
 $(eval $(call add_define,ERROR_DEPRECATED))
+$(eval $(call add_define,HW_ASSISTED_COHERENCY))
 $(eval $(call add_define,LOAD_IMAGE_V2))
 $(eval $(call add_define,LOG_LEVEL))
 $(eval $(call add_define,NS_TIMER_SWITCH))

docs/user-guide.md

@@ -328,6 +328,15 @@ performed.
 *   `HANDLE_EA_EL3_FIRST`: When defined External Aborts and SError Interrupts
     will be always trapped in EL3 i.e. in BL31 at runtime.
 
+*   `HW_ASSISTED_COHERENCY`: On most ARM systems to-date, platform-specific
+    software operations are required for CPUs to enter and exit coherency.
+    However, there exists newer systems where CPUs' entry to and exit from
+    coherency is managed in hardware. Such systems require software to only
+    initiate the operations, and the rest is managed in hardware, minimizing
+    active software management. In such systems, this boolean option enables ARM
+    Trusted Firmware to carry out build and run-time optimizations during boot
+    and power management operations. This option defaults to 0.
+
 *   `LOAD_IMAGE_V2`: Boolean option to enable support for new version (v2) of
     image loading, which provides more flexibility and scalability around what
     images are loaded and executed during boot. Default is 0.

make_helpers/defaults.mk

@@ -105,6 +105,10 @@ FWU_FIP_NAME			:= fwu_fip.bin
 # For Chain of Trust
 GENERATE_COT			:= 0
 
+# Whether system coherency is managed in hardware, without explicit software
+# operations.
+HW_ASSISTED_COHERENCY		:= 0
+
 # Flag to enable new version of image loading
 LOAD_IMAGE_V2			:= 0