Emphasize that TF only supports 4 KB granule size

At the moment, various parts of the Trusted Firmware code assume
that the granule size used is 4 KB. For example, the linker scripts
enforce 4 KB alignment restrictions on some sections.

However, the ARMv8-A architecture allows 16 KB and 64 KB granule
sizes as well. Some other parts of the TF code, particularly the
architectural code and definitions, have been implemented with
this in mind and cater for all 3 cases.

This discrepancy creates some confusion as to what is effectively
supported in TF. This patch adds some code comments and clarification
in the documentation to make this limitation clearer.

Change-Id: I1f202369b240d8bed9d43d57ecd2a548c86c8598
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

docs/porting-guide.rst

@@ -60,11 +60,16 @@ A platform port must enable the Memory Management Unit (MMU) as well as the
 instruction and data caches for each BL stage. Setting up the translation
 tables is the responsibility of the platform port because memory maps differ
 across platforms. A memory translation library (see ``lib/xlat_tables/``) is
-provided to help in this setup. Note that although this library supports
-non-identity mappings, this is intended only for re-mapping peripheral physical
-addresses and allows platforms with high I/O addresses to reduce their virtual
-address space. All other addresses corresponding to code and data must currently
-use an identity mapping.
+provided to help in this setup.
+
+Note that although this library supports non-identity mappings, this is intended
+only for re-mapping peripheral physical addresses and allows platforms with high
+I/O addresses to reduce their virtual address space. All other addresses
+corresponding to code and data must currently use an identity mapping.
+
+Also, the only translation granule size supported in Trusted Firmware is 4KB, as
+various parts of the code assume that is the case. It is not possible to switch
+to 16 KB or 64 KB granule sizes at the moment.
 
 In ARM standard platforms, each BL stage configures the MMU in the
 platform-specific architecture setup function, ``blX_plat_arch_setup()``, and uses

include/lib/xlat_tables/xlat_tables_defs.h

@@ -48,7 +48,11 @@
 
 #define TABLE_ADDR_MASK		ULL(0x0000FFFFFFFFF000)
 
-#define PAGE_SIZE_SHIFT		FOUR_KB_SHIFT /* 4, 16 or 64 KB */
+/*
+ * The ARMv8-A architecture allows translation granule sizes of 4KB, 16KB or
+ * 64KB. However, TF only supports the 4KB case at the moment.
+ */
+#define PAGE_SIZE_SHIFT		FOUR_KB_SHIFT
 #define PAGE_SIZE		(U(1) << PAGE_SIZE_SHIFT)
 #define PAGE_SIZE_MASK		(PAGE_SIZE - 1)
 #define IS_PAGE_ALIGNED(addr)	(((addr) & PAGE_SIZE_MASK) == 0)