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

[Chunyan Zhang]

On Tue, Jun 7, 2016 at 6:04 PM, Alexander Shishkin
<alexander.shishkin at linux.intel.com> wrote:
> Chunyan Zhang <zhang.chunyan at linaro.org> writes:
>
>> 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.

Sorry, didn't get you, what did you mean by "The value of trace points" here?

>
>> +#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.

If I understand correctly, STM core can assign many channels for one
stm_source, the number we want to be allocated is specified by
'stm_source_data.nr_chans', for this patchset it's 1 for the time
being, STM_FTRACE_CHAN is the index in the range of channels allocated
to this stm_source by STM core.

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

My idea was stm_ftrace.c is part of STM driver, this patch is a
process of traces from Ftrace subsystem.  These two parts are small so
far, since this serial only added function trace exporting to STM, but
I want to add more functionalities to export more kinds of traces
which Ftrace subsystem supports to STM in the feature, my thought is
dividing into two parts like this serial did is convenient to add
other feature to each part.

>
>> +
>> +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