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Subject: Re: [rfc] Describe events in a structured way via sysfs
From: Johannes Berg <johannes@sipsolutions.net>
To: Ingo Molnar <mingo@elte.hu>
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Date: Thu, 24 Jun 2010 18:14:13 +0200
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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.

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.

johannes