bl2-el3: Add documentation for BL2 at EL3

Update firmware-design.rst, porting-guide.rst and user-guide.rst
with the information about BL2 at EL3. Firmware-design.rst is
also update to explain how to test this feauture with FVP.

Change-Id: I86d64bc64594e13eb041cea9cefa3f7f3fa745bd
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>

docs/firmware-design.rst

@@ -418,6 +418,63 @@ BL2 execution continues as follows:
 
 #. BL1 passes control to BL31 at the specified entrypoint at EL3.
 
+Running BL2 at EL3 execution level
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Some platforms have a non-TF Boot ROM that expects the next boot stage
+to execute at EL3. On these platforms, TF BL1 is a waste of memory
+as its only purpose is to ensure TF BL2 is entered at S-EL1. To avoid
+this waste, a special mode enables BL2 to execute at EL3, which allows
+a non-TF Boot ROM to load and jump directly to BL2. This mode is selected
+when the build flag BL2_AT_EL3 is enabled. The main differences in this
+mode are:
+
+#. BL2 includes the reset code and the mailbox mechanism to differentiate
+   cold boot and warm boot. It runs at EL3 doing the arch
+   initialization required for EL3.
+
+#. BL2 does not receive the meminfo information from BL1 anymore. This
+   information can be passed by the Boot ROM or be internal to the
+   BL2 image.
+
+#. Since BL2 executes at EL3, BL2 jumps directly to the next image,
+   instead of invoking the RUN_IMAGE SMC call.
+
+
+We assume 3 different types of BootROM support on the platform:
+
+#. The Boot ROM always jumps to the same address, for both cold
+   and warm boot. In this case, we will need to keep a resident part
+   of BL2 whose memory cannot be reclaimed by any other image. The
+   linker script defines the symbols __TEXT_RESIDENT_START__ and
+   __TEXT_RESIDENT_END__ that allows the platform to configure
+   correctly the memory map.
+#. The platform has some mechanism to indicate the jump address to the
+   Boot ROM. Platform code can then program the jump address with
+   psci_warmboot_entrypoint during cold boot.
+#. The platform has some mechanism to program the reset address using
+   the PROGRAMMABLE_RESET_ADDRESS feature. Platform code can then
+   program the reset address with psci_warmboot_entrypoint during
+   cold boot, bypassing the boot ROM for warm boot.
+
+In the last 2 cases, no part of BL2 needs to remain resident at
+runtime. In the first 2 cases, we expect the Boot ROM to be able to
+differentiate between warm and cold boot, to avoid loading BL2 again
+during warm boot.
+
+This functionality can be tested with FVP loading the image directly
+in memory and changing the address where the system jumps at reset.
+For example:
+
+	-C cluster0.cpu0.RVBAR=0x4014000
+	--data cluster0.cpu0=bl2.bin@0x4014000
+
+With this configuration, FVP is like a platform of the first case,
+where the Boot ROM jumps always to the same address. For simplification,
+BL32 is loaded in DRAM in this case, to avoid other images reclaiming
+BL2 memory.
+
+
 AArch64 BL31
 ~~~~~~~~~~~~
 

docs/porting-guide.rst

@@ -1624,6 +1624,70 @@ element in the boot sequence. If there are no more boot sources then it
 must return 0, otherwise it must return 1. The default implementation
 of this always returns 0.
 
+Boot Loader Stage 2 (BL2) at EL3
+--------------------------------
+
+When the platform has a non-TF Boot ROM it is desirable to jump
+directly to BL2 instead of TF BL1. In this case BL2 is expected to
+execute at EL3 instead of executing at EL1. Refer to the `Firmware
+Design`_ for more information.
+
+All mandatory functions of BL2 must be implemented, except the functions
+bl2\_early\_platform\_setup and bl2\_el3\_plat\_arch\_setup, because
+their work is done now by bl2\_el3\_early\_platform\_setup and
+bl2\_el3\_plat\_arch\_setup. These functions should generally implement
+the bl1\_plat\_xxx() and bl2\_plat\_xxx() functionality combined.
+
+
+Function : bl2\_el3\_early\_platform\_setup() [mandatory]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+	Argument : u_register_t, u_register_t, u_register_t, u_register_t
+	Return   : void
+
+This function executes with the MMU and data caches disabled. It is only called
+by the primary CPU. This function receives four parameters which can be used
+by the platform to pass any needed information from the Boot ROM to BL2.
+
+On ARM standard platforms, this function does the following:
+
+-  Initializes a UART (PL011 console), which enables access to the ``printf``
+   family of functions in BL2.
+
+-  Initializes the storage abstraction layer used to load further bootloader
+   images. It is necessary to do this early on platforms with a SCP\_BL2 image,
+   since the later ``bl2_platform_setup`` must be done after SCP\_BL2 is loaded.
+
+- Initializes the private variables that define the memory layout used.
+
+Function : bl2\_el3\_plat\_arch\_setup() [mandatory]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+	Argument : void
+	Return   : void
+
+This function executes with the MMU and data caches disabled. It is only called
+by the primary CPU.
+
+The purpose of this function is to perform any architectural initialization
+that varies across platforms.
+
+On ARM standard platforms, this function enables the MMU.
+
+Function : bl2\_el3\_plat\_prepare\_exit() [optional]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+	Argument : void
+	Return   : void
+
+This function is called prior to exiting BL2 and run the next image.
+It should be used to perform platform specific clean up or bookkeeping
+operations before transferring control to the next image. This function
+runs with MMU disabled.
+
 FWU Boot Loader Stage 2 (BL2U)
 ------------------------------
 

docs/user-guide.rst

@@ -245,6 +245,9 @@ Common build options
    BL2U image. In this case, the BL2U in the ARM Trusted Firmware will not
    be built.
 
+- ``BL2_AT_EL3``: This is an optional build option that enables the use of
+   BL2 at EL3 execution level.
+
 -  ``BL31``: This is an optional build option which specifies the path to
    BL31 image for the ``fip`` target. In this case, the BL31 in the ARM
    Trusted Firmware will not be built.