Re: [rfc] Describe events in a structured way via sysfs

From: Ingo Molnar
Date: Thu Jun 24 2010 - 13:34:06 EST

* Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:

> On Thu, 2010-06-24 at 11:36 +0200, Ingo Molnar wrote:
> > That's probably best achieved via a TRACE_EVENT() variant, by passing in the
> > sysfs location.
> >
> > It might even make sense to make this a part of TRACE_EVENT() itself and make
> > 'NULL' the current default, non-sysfs-enumerated behavior. That way we can
> > gradually (and non-intrusively) find all the right sysfs places for events.
> No, this doesn't work. A lot of events are multi-instance. Say you have an
> event for each USB device. This event would have to show up in many places
> in sysfs, and each trace_foo() invocation needs to get the struct device
> pointer, not just the TRACE_EVENT() definition. Additionally, to
> create/destroy the sysfs pieces we need something like init_trace_foo(dev)
> and destroy_trace_foo(dev) be called when the sysfs points for the device
> should be created/destroyed.

Yes - but even this could be expressed via TRACE_EVENT(): by giving it a
device-specific function pointer and then instantiating individual events from
a single, central place in sysfs.

That is the place where we already know where it ends up in sysfs, and where
the event-specific function can match up whether that particular node belongs
to it and whether an additional event directory should be created for that
particular sysfs node.

> The TRACE_EVENT() just defines the template, but such multi-instance events
> really should be standardised in terms of their struct device (or maybe
> kobject).
> I think that needs some TRACE_DEVICE_EVENT macro that creates the required
> inlines etc, and including the init/destroy that are called when the event
> should show up in sysfs.
> There's no way you can have the event show up in sysfs at the right spot
> with _just_ a TRACE_EVENT macro, since at define time in the header file you
> don't even have a valid struct device pointer.

That would be another possible way to do it - to explicitly create the events
directory. It looks a bit simpler as we wouldnt have to touch TRACE_EVENT()
and because it directly expresses the 'this node has an events directory'
property at the place where we create the device node.


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