gstreamer/docs/random/ds/negotiation

Properties:

  "template_caps"
    The union of all possible caps that a pad might have in any
    condition.  Is a class property, and doesn't change over the
    life of an object.  Set by the element during class init.

  "internal_caps"
    The caps that the element will allow on a pad, given its current
    state and the external_caps of all the other pads of the element.
    Set by the element.

  "external_caps"
    The caps representing what the core will allow on a pad.  This is
    a combination of the peer's allowed caps, and filtering caps, and
    any in-process negotiation restrictions.  Set by the core.

  "negotiated_caps"
    The format that the element will use.  Set by the core.

  "filter_caps"
    The caps intersected with everything else during negotiation.  Set
    by the application.

  "negotiable"
    A boolean that indicates that the pad is ready to negotiate.  Set
    by the element.



Negotiation steps, from the element perspective:

 - During object initialization, an element must set template_caps
   and allowed_caps for each pad.

 - In response to changes in the internal state of the element or in
   the peer_allowed_caps of any other pad owned by the element, the
   element should update allowed_caps of a pad.

 - Pads are initially created with negotiable as FALSE.  When an
   element wants to indicate that negotiation is allowable, it should
   set negotiable to TRUE.

 - In response to a negotiate signal, an element should copy the caps
   in "allowed_caps" to "negotiated_caps", and extract any information
   needed to handle the format.  At this point, "allowed_caps" is
   guaranteed to be fixed.


Negotiation steps, from the core perspective:

 





<ds> I'd like to split it up into manageable "projects"
<ds> hopefully some of which we can recruit others to do
<Company> i'd like a decision about if we drop threads
<ds> in what sense?
<ds> threads being not required?
<Company> threads being not part of the API
<ds> or threads being not used
<Company> so no thread safety considerations
<ds> I would like threads to not be part of the API
<ds> requiring apps/plugins to be written thread-safely is insane
<Company> the question is this: If someone calls gst_element_set_state (element, state), does he have to marshal it into the correct thread himself or not?
<ds> no
<ds> reason being: we already have to deal with subthreads
<ds> we == the core
<Company> then you'll have a lot of problems
<Company> i'd like to just drop subthreads and be done with it
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<ds> and run everything in one thread?
<Company> yes
<Company> schedulers are free to use threads though
<ds> this is obviously a retreat
<Company> right
<ds> but perhaps a strategic one
<Company> otherwise you have to guarantee that you get no races
<ds> I've seen this as a viable option for some time
<Company> there's stuff that just does not work as is currently
<ds> it always seemed difficult to convince other people that it's the right thing to do
<ds> for example, wtay
<Company> like setting xwindow ids, changing states or app-events
<Company> like seeks
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<Company> all of this has the typical thread issues currently: it sometimes works and sometimes crashes the app
<Company> and that's not acceptable to me
<ds> let's do it
<Company> if you look at my 0.10 code, I've tried to have GstObjects belong to a "context"
<ds> this all fits into the idea that I think we should be a lot more like gtk architecturally
<ds> for example, gtk doesn't have anything like caps negotiation, where elements call each other recursively
<Company> and have functions ensure they're in the right context or otherwise marshal
<Company> yes it does
<ds> ?
<Company> size requests and allocations
<ds> but that's more like how I want it -- setting a property and firing a signal telling others that it's updated
<ds> as i understand it
<Company> though gtk has the bonus of being a tree which we don't have, we have a directed, maybe cyclic graph
<Company> and for caps, the graph isn't even directed
<Company> gtk is far too simple for gst
<ds> true
<Company> i can't even come up with an analogy to a video transcoder
<ds> but we do some relatively simple stuff in a complicated way
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<Company> like?
<ds> caps negotiation
<Company> that's not simple
<ds> passing around the data should be
<ds> was it you or thomasvs that wanted to split core headers into app headers and plugin headers?
<Company> I definitely wanted that
<Company> app/plugin/registry to be exact
<ds> registry?
<ds> for functions that access "internal" stuff?
<Company> stuff an app normally doesn't touch
<Company> so we can do something like declare the "app" stuff supported for a longer time
<ds> oooh, good call
<ds> I'm sold
<Company> so, now that threads are gone, what are the biggies I want in 0.10?
<Company> 1) non-blocking main-loop integrated processing
<ds> that's rather important if you don't have threads :)
<Company> right
<Company> but it works in my gst-01.0 stuff already
<Company> minus the elements that need to be ported of cause
<Company> which aren't that many fortunately
<Company> though some are hard (like gnomevfs), because they have no non-blocking API
<ds> a subclass of GstElement that fires up a helper thread?
<ds> (dirty, but it works)
<Company> probably
<ds> - autopluggers would be easier to write if there was a core method
<ds>   to do what plugidentities do.
<Company> no clue how to implement that
<ds> there's also a note about bins being more involved with negotiation
<Company> that's the next biggie: 2) fix the negotiation process to allow good autoplugging
<ds> that was something I thought about last time around
<Company> bins?
<ds> that sounds like something I'd work on
<ds> GstBin
<Company> the only reason bhins still exist for me is to group elements
<ds> so that a bin subclass could override how its children are negotiated
<ds> much like GtkBin, actually
<ds> and allocation
<Company> hum
<Company> how do you solve that issue with links that are only partly inside the bin and partly inside another one?
<ds> that would be up to the parent (grandparent?)
<Company> so all links must be in the same toplevel bin?
<ds> yes
<ds> isn't that the case currently?
<ds> probably not checked
<Company> pfff
<Company> currently you can do anything
<ds> heh
<ds> what about state changes?
<Company> dunno
<Company> it's not really important for me
<Company> apart from the EOS stuff
<ds> I'd like states to be split into "what state does the app want me to be in" and "what state am I in"
<ds> with the latter being under control of the element
<Company> yeah, that'd be nice
<ds> that way, you don't have multiple people trying to control the state
<Company> we definitely need to fix EOS
<Company> i'm not sure how we should handle errors
<ds> element errors?
<Company> yeah, if we want to explicitly unset errors or not
<Company> currently on error the element goes to paused
<ds> it's easier with actual_state
<Company> in theory i just set it to PLAYING again and continue?
<ds> the actual_state goes to PAUSED and fires a signal
<Company> dunno, maybe actual_state should go to ERROR
<ds> yeah
<Company> that'd require gst_element_unset_error though
<ds> with actual_state, we can add states without thinking too hard
<Company> but if you can just unset it, why not just do it and continue
<ds> the app may want to be involved
<ds> especially to get the error message
<ds> do we want a separate api section for autopluggers?
<Company> why would we want that?
<ds> or make it part of registry
<Company> maybe s/registry/internal/ or some other name
<ds> because autopluggers may want to access internal information that we wouldn't expose to apps or plugins
<ds> could you add gst/gstcompat.c to your arch repo?
<Company> hm
<Company> what internal info would an autoplugger need
<ds> depends
<ds> whatever replaces plugidentities
<Company> a simple one (decoder) just needs caps and ranks
<ds> I meant for negotiation information
<Company> hm
<Company> no, i don't want a seperate section for it
<Company> just put it in "internal", "extended" or whatever
<ds> ok
<ds> do we want to plan for the app api to be ABI compatible past 0.10?
<ds> er, that's silly
<ds> er, kinda silly
<Company> API compat
<Company> you could achieve ABI compat if you expose all objects just as typedefs
<Company> lemme reboot into linux so i can add gstcompat.c
<Company> brb
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<ds> so do you still support caps registration?
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<ds> registering media types, fields, descriptions, and default fixate functions?
<ds> and validation
<Company> yes
<Company> that's an important thing we need
<Company> otherwise people mess up by either carelessly doing stuff wrong or not understanding what they're supposed to do
<ds> whose responsibility is it to register these?
<ds> libgstplugins?
<ds> a plugin?
<ds> do we want this info in the registry so that a plugin can automatically be loaded to provide registration for video/x-raw-rgb?
<Company> haven't really thought about that part yet
<ds> it's somewhat similar to typefind functions
<Company> yeah
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<Company> i'd probably have a seperate Factory for caps info
<Company> and have the system load them when you use the caps info stuff
<ds> should caps info be manditory?
<ds> have you synced your repo?
<Company> i'm trying to, but some commit was interrupted last time...
<ds> how does one sync a local repository to fd.o?
<ds> rsync?
<Company> tla archive-mirror
<ds> ah
<Company> see rhythmbox.org => developers for how it works
<Company> and _never_ press ctrl-c in arch
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<ds> is there a reason why a pad might know (and set) allowed_caps, but not be ready to negotiate?
<Company> i don't know a good one
<ds> know a bad one?
<Company> "arch-mirror: unable to acquire revision lock (internal error in archive-pfs.c(pfs_lock_revision))"
<Company> stupidly ordered code
<Company> no, don't know one
<ds> here's a good question:
<ds> should the core negotiate links in a pipeline that are negotiatable, even though some other links are not negotiatable?
<ds> this ends up making us negotiate some things multiple times
<ds> filesrc ! mad ! audioconvert ! audioscale ! osssink
<Company> the core doesn't know if links are negotiable
<ds> it will in the future :)
<Company> sinesrc ! identity ! osssink
<Company> how does it know if sinesrc ! identity is negotiable?
<ds> because the allowed_caps on the two pads intersect
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<ds> I'm pretty sure that I don't want gst_pad_link() to return negotiation errors
<ds> ever
<Company> you can't avoid that
<ds> I want to separate "linkable" from "negotiable"
<Company> ah, gst_pad_link
<ds> so you can link videotestsrc ! identity ! osssink
<Company> i confused it with GstPadLink
<Company> yeah, i was thinking about that, too
<ds> vts ! osssink won't link, since the pad templates don't intersect
<Company> http://web.ics.purdue.edu/~kuliniew/wp/archives/2004/08/07/error-handling/
<Company> :)
<ds> right now the vts ! id ! osssink pipeline fails to link
<Company> yeah
<Company> and goom ! xvimagesink only fails when xvimagesink is at least READY
<Company> (assuming your xv can't do RGB)