[Mesa-dev] [RFC] mapi: a generic dispatcher to be used by OpenVG (and OpenGL)

[Chia-I Wu]

Hi all,

I've been working on and off for a while on a dispatcher builder called mapi
(multiple-api, in contrary to gl-api).  The code is available at

  http://cgit.freedesktop.org/~olv/mesa/log/?h=mapi

The motivation is to build a dispatcher for OpenVG.  I will give an overview
for mapi in this mail, and see if it could be merged and how.

To create a dispatcher for an API with mapi, one needs to create a text file
that describes the function names, parameters, and return values of the API.
The text file is parsed by mapi_abi.py, a script provided by mapi, to generate
a header file.  A libglapi.a-like dispatcher can then be created by compiling
mapi with the generated header file.

Unlike libglapi.a, which must be used with a set of accompanying headers such
as glapidispatch.h and glapioffsets.h, the mapi derived dispatcher has a
simpler interface.  It consists of functions to create and fill an opaque
dispatch table, and to make a dispatch table current.  But that is all for now.

Internally, mapi consists of

 - current dispatch/context management
 - execmem allocation
 - thread support
 - public entry points (C or ASM)
 - dynamic entry point generation

The first three are directly copied from glapi, slightly modified so that they
do not depend on core mesa headers.  The handling of C entry points is just
like how it is done in glapi.  The handling of ASM and dynamic entry points
points is different though.  In mapi, adding support for arch-specific entry
points is straightforward.  For example,

  http://cgit.freedesktop.org/~olv/mesa/commit/?h=mapi&id=c2eb929ad643adba9db01e02b06e489100f8c769

allows mapi to generate ASM and dynamic entry points on x86-64 with TLS.  It is
simple, and all changes live in a file of its own.  This example uses inline
assembly.  It is not a requirement.  But the build rule needs to be adjusted if
a .S file is to be used.

There are 8 commits in the branch.  The current state is

 - mapi works as discussed above
 - other than generic C entry points, it can generate x86 and x86-64 assembly
   entry points
 - there is an OpenVG dispatcher created with mapi
 - glapi is modified to use the first three components of mapi (in fact, they
   are copied from glapi)
 - dispatchers now live under subdirectories of src/mapi/.

I'd like make glapi use all internal components of mapi.  Actually, there is
another branch that does that

  http://cgit.freedesktop.org/~olv/mesa/log/?h=mapi-replaces-glapi

As can been seen, not much is left in glapi.  The problem with the branch is
that mapi does not support as many architectures and toolchains as glapi does
with regard to ASM entry points.

My plan is to merge mapi branch first, likely after Kristian merges his gles
work.  It allows us to verify mapi and the new way to build glapi.  Once mapi
supports all interesting archs and toolchains (help needed!), we can then merge
mapi-replaces-glapi branch.

Any thought or comment?

-- 
olv at LunarG.com