[RFC PATCH 2/4] trace: Introduce an output interface from ftrace to STM

On Tue, Jun 7, 2016 at 6:04 PM, Alexander Shishkin
[off-list ref] wrote:

Chunyan Zhang [off-list ref] writes:

quoted

This patch is introducing a new function to print Ftrace messages
to STM buffer when the traces happen.  In order to reduce the
effect on timing overhead as much as possible, only the current
function and its parent ip address will be recorded into STM in
this patch.  This idea was first introduced by Philippe Langlais
at ST-Microelectronics a long time ago[1].

So why is this useful? The value of trace points is in their payload.

From what I understand, we have no way to log as much information as

Ftrace has done if we want to control the overhead of writing STM in a
limited/acceptable range.  Exporting information to STM is much more
time-consuming than writing ring buffer, the thought was simply that
export as much useful information as possible while taking a limited
bytes.

Thanks,
Chunyan

quoted

+#define STM_FTRACE_CHAN 0

This is why we have the stm master/channel allocation policy, which
should already assign a channel to your stm_source device when it is
linked to an stm device.

Also, why is this a separate compilation unit from the stm_ftrace.o?

quoted

+
+void ftrace_stm_func(unsigned long ip, unsigned long parent_ip)
+{
+     unsigned long ip_array[2] = {ip, parent_ip};
+
+     stm_ftrace_write((char *)ip_array, sizeof(unsigned long) * 2,
+                      STM_FTRACE_CHAN);
+}
+EXPORT_SYMBOL_GPL(ftrace_stm_func);

diff --git a/kernel/trace/trace_output_stm.h b/kernel/trace/trace_output_stm.h
new file mode 100644
index 0000000..fc3f989
--- /dev/null
+++ b/kernel/trace/trace_output_stm.h

@@ -0,0 +1,14 @@
+#ifndef __TRACE_OUTPUT_STM_H
+#define __TRACE_OUTPUT_STM_H
+
+#include <linux/module.h>
+
+#ifdef CONFIG_STM_FTRACE
+extern void stm_ftrace_write(const char *buf, unsigned int len,
+                          unsigned int chan);
+extern void ftrace_stm_func(unsigned long ip, unsigned long parent_ip);
+#else
+static inline void ftrace_stm_func(unsigned long ip, unsigned long parent_ip) {}
+#endif
+
+#endif /* __TRACE_OUTPUT_STM_H */
--

1.9.1