Preliminary in-tree documentation for console-log.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 60 insertions, 0 deletions

diff --git a/doc/console-log.txt b/doc/console-log.txt
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+SkiBoot Console Log
+-------------------
+
+Skiboot maintains a circular textual log buffer in memory.
+
+It can be accessed using any debugging method that can peek at
+memory contents. While the debug_descriptor does hold the location
+of the memory console, we're pretty keen on keeping its location
+static.
+
+Events are logged in the following format:
+[timebase,log_level] message
+
+You should use the new prlog() call for any log message and set the
+log level/priority appropriately.
+
+printf() is mapped to PR_PRINTF and should be phased out and replaced
+with prlog() calls.
+
+See timebase.h for full timebase explanation.
+
+Log level from skiboot.h:
+#define PR_EMERG        0
+#define PR_ALERT        1
+#define PR_CRIT         2
+#define PR_ERR          3
+#define PR_WARNING      4
+#define PR_NOTICE       5
+#define PR_PRINTF       PR_NOTICE
+#define PR_INFO         6
+#define PR_DEBUG        7
+#define PR_TRACE        8
+#define PR_INSANE       9
+
+The console_log_levels byte in the debug_descriptor controls what
+messages are written to any console drivers (e.g. fsp, uart) and
+what level is just written to the in memory console (or not at all).
+
+This enables (advanced) users to vary what level of output they want
+at runtime in the memory console and through console drivers (fsp/uart)
+
+You can vary two things by poking in the debug descriptor:
+a) what log level is printed at all
+  e.g. only turn on PR_TRACE at specific points during runtime
+b) what log level goes out the fsp/uart console
+   defaults to PR_PRINTF
+    
+We use two 4bit numbers (1 byte) for this in debug descriptor (saving
+some space, not needlessly wasting space that we may want in future).
+    
+The default is 0x75 (7=PR_DEBUG to in memory console, 5=PR_PRINTF to drivers
+    
+If you write 0x77 you will get debug info on uart/fsp console as
+well as in memory. If you write 0x95 you get PR_INSANE in memory but
+still only PR_NOTICE through drivers.
+    
+People who write something like 0x1f will get a very quiet boot indeed.
+ 
+
+