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    SubjectRe: [tip:perf/core] tracing: Add DEFINE_EVENT(), DEFINE_SINGLE_EVENT() support to docbook
    On Wed, 2009-12-02 at 11:27 -0500, Mathieu Desnoyers wrote:

    > A few questions about the semantic:
    > Is "declare" here always only used as a declaration ? (e.g. only in
    > headers, never impacted by CREATE_TRACE_POINT ?)

    Well yes it is impacted by CREATE_TRACE_POINT, but so is DECLARE_TRACE
    for that matter ;-)

    The difference is that DECLARE_EVENT_CLASS will at most (with
    CREATE_TRACE_POINT) only create the functions that can be used by other
    events. It does not create an event itself. That is, it's not much
    different than making a "static inline function" except that function
    will not be static nor will it be inline ;-)

    > Is "define" here always mapping to a definition ? (e.g. to be used in a
    > C file to define the class or event handling stub)

    The DEFINE_* will create something that can be hooked to the trace
    points in other C files.

    > I feel that your DEFINE_EVENT_CLASS might actually be doing a bit more
    > than just "defining", it would actually also perform the declaration.
    > Same goes for "DEFINE_EVENT". So can you tell us a bit more about that
    > is the context of templates ?

    Well, the macros used by these are totally off the wall anyway :-) So
    any name we come up with will not match what the rest of the kernel does
    regardless. But we need to give something that is close.

    I'm liking more the:

    that comes the closest to other semantics in the kernel. That is (once

    DECLARE_EVENT_CLASS - makes only the class. It does create helper
    functions, but if there's no DEFINE_EVENT that uses them, then they are
    just wasting space.

    The DEFINE_EVENT will create the trace points in the C file that has
    CREATE_TRACE_POINTS defined. But it requires the helper functions
    created by a previous DECLARE_EVENT_CLASS.

    DEFINE_EVENT_CLASS will do both create a EVENT_CLASS template, as well
    as a EVENT that uses the class. The name of the class is a separate
    namespace as the event. Here both the class and the event have the same
    name, but other events can use this class by referencing the name.


    DEFINE_EVENT(x, y, ...);

    The DEFINE_EVENT_CLASS will create a class x and an event x, then the
    DEFINE_EVENT will create another event y that uses the same class x.

    Actually, with the above, we may not need to have DECLARE_EVENT_CLASS()
    at all, because why declare a class if you don't have an event to use
    it? But then again, you may not want the name of the class also a name
    of an event.

    -- Steve

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