Re: hwmon: trace event support?

[Nicolin Chen <nicoleotsuka@gmail.com>]

Thanks for the quick response.

On Wed, Oct 03, 2018 at 05:42:23PM -0700, Guenter Roeck wrote:
> > Ftrace data. Similar to tz->poll_queue in thermal_core, hwmon core
> > could also have a work queue polling the registered sensor inputs
> > (by default disabled; enabled only if users configure poll_delay)
> > so that the power data can be generated to Ftrace outputs as well.
> > 
> > To add this, the hwmon core needs an interface that can get sensor
> > inputs as drivers like ina3221 don't report any values back to the
> > core but directly expose them via sysfs ABI nodes.
> > 
> > I noticed the hwmon_device_register_with_info() could be actually
> > a good API to use since it has defined different sensor types and
> > more importantly the ops->read() interface, but the ina3221 driver
> > is very compact that there would be very little gain from this API
> > at this moment. However, maybe having trace event support would be
> 
> "very little gain" is a false assumption. Its size would most likely
> be reduced by 20% or more, mostly because all the sysfs handling
> is done by the core if one uses the _info API.

Didn't intend to deny the improvement; probably should have used a
more positive word instead of "little" :)

> > a good reason to apply this API.
> > 
> > What do you think about it? Any concern or better solution?
> 
> I would not object to adding trace support into the hwmon subsystem.
> However, it should be tied to the new API. I would resist patches
> adding trace support to individual hwmon drivers unless the new API
> is used and additional driver specific trace support is warranted.

Yes, my idea is to implement it with the _info API inside the hwmon
core. What do you think about the mentioned solution? Would you be
in favor of a polling work queue?

"----------. Similar to tz->poll_queue in thermal_core, hwmon core
 could also have a work queue polling the registered sensor inputs
 (by default disabled; enabled only if users configure poll_delay)
 so that the power data can be generated to Ftrace outputs as well."

> Note that this also applies to hwmon drivers registering through
> thermal. The thermal subsystem calls the _info API but misuses it
> to avoid a warning generated when using the old API. Of course,
> I have no influence over the hwmon code in the thermal subsystem,
> so whatever is done there is essentially wild-wild-west.

I saw they have some obvious code in the hwmon core. If you want,
we can keep the polling work queue and trace events away from it,
which sounds plausible to me considering that thermal subsystem
has its own polling work queue and trace events for sensor data.

Thank you
Nicolin