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77 lines
4 KiB
Text
77 lines
4 KiB
Text
The point of the metadata is to provide some context for each buffer. In
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the case of audio data, for instance, it would provide the samplerate, bit
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depth, and channel count.
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The trick is that there may be multiple types of metadata ganged onto a
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single buffer. This is why they're going to be a GList. This does mean
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extra overhead in all cases, but I think it's minimal. The GList type
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uses a chunk allocater so we're not wasting too much memory or time when
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adding to the list.
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The trick is dealing with these structs as they pass through a pipeline,
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since they have potentially different mutability properties. For
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instance, if you've got a mp3 decoder connected to a tee, which sends the
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buffers off to both the decoder and a spectrum analyzer (and then a
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visualization element). The preferred setup would be where every time a
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audio/raw metadata comes down the pipe (indicating a potential change in
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audio format), the audiosink and spectrum would just save off pointers.
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So when exactly does this metadata go away (deallocated)? Well, that
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means metadata has to be refcounted. But that gets rather hairy. OK, in
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the simple case you create a metadata struct, it comes with refcount set
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to 1. You pass it through, it stays one, eventually someone drops the
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last reference on the buffer it's tied to, you free the metadata too.
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Easy. What if you tee? You could go through and for every metadata in
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the buffer, increment the refcount by the same as the buffer. So in the
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above case (tee'd), the audiosink and spectrum would get the buffer with a
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refcount of 2, and it'd have a metadata with refcount 2. Do they ref it
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each themselves, then unref the buffer? Or do they remove the metadata?
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Removing the metadata would require a buffer CoW, which would suck, so
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yes, they'd just ref the metadata.
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But.... what if they're all in different threads? Then we're off into
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the magical world of mutexes. Everything with a refcount in a threaded
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world must be mutexed, else you can do atomic increment and atomic
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dec&test. Can this be done from C easily? Perhaps it needs to be found
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from kernel includes via autoconf?
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The goal in designing the way metadata will be defined and used is to keep
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it as simple as possible. The basis for accomplishing this is the fact
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that in order to actually use (rather than just pass) the metadata, you
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have to know what the fields are, which means you have to have compiled in
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support for that metadata at build time. Therefore, if you're using
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metadata, you must have build-time access to the necessary include file
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that defines it.
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So, given that you've got an include file, it would be nice if the whole
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thing could be contained there. This would limit the need to be linked
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against something, or have load-time requirements as to that has to be
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loaded before you are.
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Given that really all metadata is is a region of memory of a given size
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with a certain signature, this isn't all that hard. First you lay out the
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struct that defines the metadata. Then you set up #defines that expand to
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the size of the struct in question, as well as the four-cc code that
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defines the type.
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The work is done by a few #defines, a la the #defines used in all Gtk
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objects. The first is a NEW() method that allocates the memory for the
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metadata and fills in all the normal fields (type, size, utility
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functions). Because of the way it's defined (as a #define, no less),
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you'll have to invoke it as META_NEW(meta), since it can't return()
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anything.
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Another #define will check to make sure a meta is indeed that type by
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verifying the type code and size. Theoretically, meta types can overlap
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with the same fourcc code, as long as they have different sizes. But I
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probably ought to have a global public registry so people writing things
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don't conflict. MSFT got that right, at least.
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So, a hairy problem is what to do when there are utility functions
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associated with one of these things. One option is to not bother with
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them. This is very likely a possible solution, since metadata is supposed
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to be flat memory of a given size. Not much to do to either free or copy
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it, is there?
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