Local C library documentation updates

- Update porting guide to describe where files live and how to get
  FreeBSD source code.
- Update change-log to describe relocation and new functions.

Change-Id: Id8f30cc7bafdd1064b3a5c5aae958c5aa3fb79f3

2 files changed, 45 insertions, 1 deletions

diff --git a/docs/change-log.md b/docs/change-log.md
index e813cb1..08d1854 100644
--- a/docs/change-log.md
+++ b/docs/change-log.md
@@ -64,7 +64,6 @@ Detailed changes since last release
     -   Entry into standby states is not supported.
     -   The api is only supported on the AEMv8 Base FVP.
 
-
 *   The PSCI AFFINITY_INFO api has undergone limited testing on the AEMv8 Base
     FVP to allow experimental use.
 
@@ -77,6 +76,12 @@ Detailed changes since last release
     undergone change. A pointer to these structures is returned instead of their
     copy.
 
+*   Required C library and runtime header files are now included locally in ARM
+    Trusted Firmware instead of depending on the toolchain standard include
+    paths. The local implementation has been cleaned up and reduced in scope.
+    Implementations for `putchar()` and `strchr()` were added to the local C
+    library.
+
 
 ARM Trusted Firmware - version 0.2
 ==================================
diff --git a/docs/porting-guide.md b/docs/porting-guide.md
index aa1451f..11ff3cc 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -13,6 +13,7 @@ Contents
     *   Boot Loader stage 2 (BL2)
     *   Boot Loader stage 3-1 (BL3-1)
     *   PSCI implementation (in BL3-1)
+4. C Library
 
 - - - - - - - - - - - - - - - - - -
 
@@ -943,12 +944,50 @@ provided in the description of the `plat_get_aff_count()` and
 `plat_get_aff_state()` functions above.
 
 
+4.  C Library
+-------------
+
+To avoid subtle toolchain behavioral dependencies, the header files provided
+by the compiler are not used. The software is built with the `-nostdinc` flag
+to ensure no headers are included from the toolchain inadvertently. Instead the
+required headers are included in the ARM Trusted Firmware source tree. The
+library only contains those C library definitions required by the local
+implementation. If more functionality is required, the needed library functions
+will need to be added to the local implementation.
+
+Versions of [FreeBSD] headers can be found in `include/stdlib`. Some of these
+headers have been cut down in order to simplify the implementation. In order to
+minimize changes to the header files, the [FreeBSD] layout has been maintained.
+The generic C library definitions can be found in `include/stdlib` with more
+system and machine specific declarations in `include/stdlib/sys` and
+`include/stdlib/machine`.
+
+The local C library implementations can be found in `lib/stdlib`. In order to
+extend the C library these files may need to be modified. It is recommended to
+use a release version of [FreeBSD] as a starting point.
+
+The C library header files in the [FreeBSD] source tree are located in the
+`include` and `sys/sys` directories. [FreeBSD] machine specific definitions
+can be found in the `sys/<machine-type>` directories. These files define things
+like 'the size of a pointer' and 'the range of an integer'. Since an AArch64
+port for [FreeBSD] does not yet exist, the machine specific definitions are
+based on existing machine types with similar properties (for example SPARC64).
+
+Where possible, C library function implementations were taken from [FreeBSD]
+as found in the `lib/libc` directory.
+
+A copy of the [FreeBSD] sources can be downloaded with `git`.
+
+    git clone git://github.com/freebsd/freebsd.git -b origin/release/9.2.0
+
+
 - - - - - - - - - - - - - - - - - - - - - - - - - -
 
 _Copyright (c) 2013, ARM Limited and Contributors. All rights reserved._
 
 
 [User Guide]: user-guide.md
+[FreeBSD]:    http://www.freebsd.org
 
 [../plat/common/aarch64/platform_helpers.S]: ../plat/common/aarch64/platform_helpers.S
 [../plat/fvp/platform.h]:                    ../plat/fvp/platform.h