[Mesa-dev] draw: Replace varray and vcache by vsplit

[Keith Whitwell]

On Fri, 2010-08-13 at 07:46 -0700, Chia-I Wu wrote:
> On Fri, Aug 13, 2010 at 10:14 PM, Keith Whitwell <keithw at vmware.com> wrote:
> > On Fri, 2010-08-13 at 07:04 -0700, Chia-I Wu wrote:
> >> Hi,
> >>
> >> There are two primitive transformations in gallium draw module.  In
> >> varray, primitives are "split"ted.  When a primitive has more vertices
> >> than the middle end can handle, varray splits the primitive and calls
> >> the middle end multiple times.
> >>
> >> In vcache, primitives are "decompose"d.  More advanced primitives are
> >> decomposed into one of point, line(_adj), or triangle(_adj).
> >> Similarly, vcache may call the middle end multiple times to flush its
> >> internal buffer.  In some cases, vcache passes the primitves through
> >> without decomposing nor splitting, as can be seen in vcache_check_run.
> >>
> >> The issue with vcache is that it has to decompose a primitive
> >> differently depending on the provoking convention, as explained in
> >>
> >>   http://lists.freedesktop.org/archives/mesa-dev/2010-August/001797.html
> >>
> >> It becomes a problem when GS is active.
> >>
> >> My proposal is to make vcache split instead of decompose.  Because
> >> varray only splits and vcache has a pass-through path, the rest of the
> >> workflow already has to support all primitive types.  Switching from
> >> decompose to split does not require a big change to the rest of the
> >> workflow.
> >>
> >> But then vcache will look a lot like varray, only with indexed
> >> primitive support.  It leads me to a new frontend that replaces both
> >> varray and vcache: vsplit
> >>
> >>  http://cgit.freedesktop.org/~olv/mesa/log/?h=draw-vsplit
> >>
> >> vsplit is based on varray.  It uses some code from vcache to support
> >> indexed primitives.  When vcache decomposes, there are flags being set
> >> to indicate that if the stipple counter should be reset or if some
> >> edge of a triangle should be omitted in unfilled mode.  The segments
> >> of a splitted primitive have flags for similar purposes too:
> >>
> >>   DRAW_SPLIT_AFTER   More segments to come after this one
> >>   DRAW_SPLIT_BEFORE  There are preceding segments
> >>
> >> These flags are set by vsplit and the middle ends pass them to the
> >> other stages.  Therefore, the run methods of middle ends are augmented
> >> to take the flags.
> >>
> >> To summarize, vsplit
> >>
> >>  - fixes GS when (flatshade && flatshade_first) is on
> >>  - never sends more vertices than the middle end claims to handle
> >>  - is faster than vcache: split instead of decompose, no get_elt
> >>    calls
> >>  - no longer uses the higher bits of draw_elts for stipple/edge flags
> >>
> >> Suggestions?
> >
> >
> > Hi - I haven't looked at the patches yet, but a couple of questions:
> >
> > How does this interact with the draw_pipe_* code - which requires
> > decomposed primitives?
> draw_pipe.c decomposes the primitives.  It is there before because it
> has to support varray and vcache_check_run which do not decompose.

OK.

> > How does this cope with indexed rendering where the vertex buffers
> > themselves are too large (for hardware or some other entity)?  Eg.
> > imagine the hardware could cope with up to 64k vertices, and you have a
> > drawelements call randomly referencing vertices in range 0..128k ?
> Vertex fetching happens in the middle end so the range of the indices
> is not a problem.  Though vsplit guarantees that it never calls the
> middle end with more vertices than the middle end claims to support
> (as returned by draw_pt_middle_end::prepare).  The limit is usually
> decidied by the size of the buffer for vertex emitting.

I guess I'm wondering how it does this.  If the middle end says it
supports 64k vertices, and the vertex element looks like

  [0, 128k, 64k, 32k, 96k, 16k, 1, ... ]

what gets sent?  (Sorry, I still haven't looked at the code, you could
well have addressed this).

Keith