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

[Ingo Molnar <mingo@elte.hu>]

* Johannes Berg <johannes@sipsolutions.net> 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.

Thanks,

	Ingo