Make platform-specific Feather-Trace depend on !CONFIG_DEBUG_RODATA

Feather-Trace rewrites instructions in the kernel's .text segment.
This segment may be write-protected if CONFIG_DEBUG_RODATA is selected.
In this case, fall back to the default flag-based Feather-Trace
implementation. In the future, we could either adopt the ftrace method
of rewriting .text addresses using non-.text mappings or we could
consider replacing Feather-Trace with ftrace altogether.

For now, this patch avoids unexpected runtime errors.

2 files changed, 10 insertions, 1 deletions

diff --git a/litmus/Kconfig b/litmus/Kconfig
index 1c0a96d54992..f3aa7478faf7 100644
--- a/litmus/Kconfig
+++ b/litmus/Kconfig
@@ -80,6 +80,15 @@ config FEATHER_TRACE
           Feather-Trace basic tracing infrastructure. Includes device file
           driver and instrumentation point support.
 
+          There are actually two implementations of Feather-Trace.
+          1) A slower, but portable, default implementation.
+          2) Architecture-specific implementations that rewrite kernel .text at runtime.
+
+          If enabled, Feather-Trace will be based on 2) if available (currently only for x86).
+          However, if DEBUG_RODATA=y, then Feather-Trace will choose option 1) in any case
+          to avoid problems with write-protected .text pages.
+
+          Bottom line: to avoid increased overheads, choose DEBUG_RODATA=n.
 
 config SCHED_TASK_TRACE
         bool "Trace real-time tasks"
diff --git a/litmus/ft_event.c b/litmus/ft_event.c
index 58124fe0ad90..399a07becca5 100644
--- a/litmus/ft_event.c
+++ b/litmus/ft_event.c
@@ -2,7 +2,7 @@
 
 #include <litmus/feather_trace.h>
 
-#ifndef CONFIG_ARCH_HAS_FEATHER_TRACE
+#if !defined(CONFIG_ARCH_HAS_FEATHER_TRACE) || defined(CONFIG_DEBUG_RODATA)
 /* provide dummy implementation */
 
 int ft_events[MAX_EVENTS];