console: Implement pre-console buffer

Allow redirection of console output prior to console initialisation to a
temporary buffer.

To enable this functionality, the board (or arch) must define:
 - CONFIG_PRE_CONSOLE_BUFFER - Enable pre-console buffer
 - CONFIG_PRE_CON_BUF_ADDR - Base address of pre-console buffer
 - CONFIG_PRE_CON_BUF_SZ - Size of pre-console buffer (in bytes)

The pre-console buffer will buffer the last CONFIG_PRE_CON_BUF_SZ bytes
Any earlier characters are silently dropped.

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>
Change-Id: I3c4caad276b9e981ebea0a0fb79d85ee3a3bcb7d
Reviewed-on: http://gerrit.chromium.org/gerrit/8686
Reviewed-by: Che-Liang Chiou <clchiou@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>

1 files changed, 14 insertions, 0 deletions

diff --git a/README b/README
index 9cef0b2604..cfc6b59116 100644
--- a/README
+++ b/README
@@ -570,6 +570,20 @@ The following options need to be configured:
 		must be defined, to setup the maximum idle timeout for
 		the SMC.
 
+- Pre-Console Buffer:
+                Prior to the console being initialised (i.e. serial UART
+                initialised etc) all console output is silently discarded.
+                Defining CONFIG_PRE_CONSOLE_BUFFER will cause U-Boot to
+                buffer any console messages prior to the console being
+                initialised to a buffer of size CONFIG_PRE_CON_BUF_SZ
+                bytes located at CONFIG_PRE_CON_BUF_ADDR. The buffer is
+                a circular buffer, so if more than CONFIG_PRE_CON_BUF_SZ
+                bytes are output before the console is  initialised, the
+                earlier bytes are discarded.
+
+                'Sane' compilers will generate smaller code if
+                CONFIG_PRE_CON_BUF_SZ is a power of 2
+
 - Boot Delay:	CONFIG_BOOTDELAY - in seconds
 		Delay before automatically booting the default image;
 		set to -1 to disable autoboot.