linux-kernel - Re: [PATCH] hwmon: (core) Add trace events to _attr

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Date:   Tue, 9 Oct 2018 15:49:45 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     jdelvare@...e.com, linux@...ck-us.net, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH] hwmon: (core) Add trace events to _attr_show/store
 functions

On Tue, 9 Oct 2018 12:11:11 -0700
Nicolin Chen <nicoleotsuka@...il.com> wrote:

> On Tue, Oct 09, 2018 at 02:57:21PM -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.


> 
> > > @@ -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 ;-)

-- Steve