Re: [PATCH] hwmon: (core) Add trace events to _attr_show/store functions

[Nicolin Chen]

On Tue, Oct 09, 2018 at 03:49:45PM -0400, Steven Rostedt wrote:
> > > > Trace events are useful for people who collect data from the
> > > > ftrace output. This patch adds initial trace events for the
> > > > hwmon core. To call hwmon_attr_base() for aligned attr index
> > > > numbers, this patch also moves the function upward.
> > > > 
> > > > Ftrace outputs:
> > > >  ...: hwmon_attr_show_string: index=2, attr_name=in2_label, val=VDD_5V
> > > >  ...: hwmon_attr_show: index=2, attr_name=in2_input, val=5112
> > > >  ...: hwmon_attr_show: index=2, attr_name=curr2_input, val=440
> > > > 
> > > > Note that the _attr_show and _attr_store functions are tied
> > > > to the _with_info API. So a hwmon driver requiring the trace
> > > > events feature should use _with_info API to register a hwmon
> > > > device.  
> > > 
> > > Hmm, this doesn't really explain why these trace events are needed.
> > > They look to be attached to sysfs reads. What's the purpose of tracing
> > > these?  
> > 
> > Our power folks analyse Ftrace outputs of cpufreq, thermal and
> > hwmon (power/voltage/current) so as to get the relationship of
> > them. The reason why we specifically need trace events is that
> > it's convenient and timestamped, and because both cpufreq and
> > thermal already have trace events.
> > 
> > I could add this to make the commit message more convincing if
> > you'd prefer that.
> 
> I'm not against the patch, but yes, having this in the change log is
> helpful.

I will add it in v2.

> > > > @@ -193,6 +206,7 @@ static ssize_t hwmon_attr_show_string(struct device *dev,
> > > >  				      char *buf)
> > > >  {
> > > >  	struct hwmon_device_attribute *hattr = to_hwmon_attr(devattr);
> > > > +	enum hwmon_sensor_types type = hattr->type;
> > > >  	const char *s;
> > > >  	int ret;
> > > >  
> > > > @@ -201,6 +215,9 @@ static ssize_t hwmon_attr_show_string(struct device *dev,
> > > >  	if (ret < 0)
> > > >  		return ret;
> > > >  
> > > > +	trace_hwmon_attr_show_string(hattr->index + hwmon_attr_base(type),  
> > > 
> > > Also, the other to tracepoints use hattr->type, here you create a
> > > separate variable. Is that just to keep within the 80 char limit?  
> > 
> > Yes. It looks clearer to me than wrapping the line here, since
> > complier usually optimize the local variable away.
> 
> Or you can just break the 80 character limit ;-)

Personally a checkpatch warning bugs me more than having an extra
line :)

Thank you
Nicolin