Implement log framework

This patch gives users control over logging messages printed from the C
code using the LOG macros defined in debug.h Users now have the ability
to reduce the log_level at run time using the tf_log_set_max_level()
function. The default prefix string can be defined by platform by
overriding the `plat_log_get_prefix()` platform API which is also
introduced in this patch.

The new log framework results in saving of some RO data. For example,
when BL1 is built for FVP with LOG_LEVEL=LOG_LEVEL_VERBOSE, resulted
in saving 384 bytes of RO data and increase of 8 bytes of RW data. The
framework also adds about 108 bytes of code to the release build of FVP.

Fixes ARM-software/tf-issues#462

Change-Id: I476013d9c3deedfdd4c8b0b0f125665ba6250554
Co-authored-by: Eleanor Bonnici <Eleanor.bonnici@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/docs/porting-guide.rst b/docs/porting-guide.rst
index c8d61ed1a..979856669 100644
--- a/docs/porting-guide.rst
+++ b/docs/porting-guide.rst
@@ -1014,6 +1014,21 @@ This function flushes to main memory all the image params that are passed to
 next image. This function is currently invoked in BL2 to flush this information
 to the next BL image, when LOAD\_IMAGE\_V2 is enabled.
 
+Function : plat\_log\_get\_prefix()
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    Argument : unsigned int
+    Return   : const char *
+
+This function defines the prefix string corresponding to the `log_level` to be
+prepended to all the log output from ARM Trusted Firmware. The `log_level`
+(argument) will correspond to one of the standard log levels defined in
+debug.h. The platform can override the common implementation to define a
+different prefix string for the log output.  The implementation should be
+robust to future changes that increase the number of log levels.
+
 Modifications specific to a Boot Loader stage
 ---------------------------------------------