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

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Chunyan Zhang <zhang.chunyan@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.

> +#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?

> +
> +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