September, 2009

Sep 09

libgoo & gst-goo

Back in 2007 I started to work integrating OpenMAX IL components into the GStreamer platform.

OpenMAX is a set of programming interfaces, in C language, for portable multimedia processing. Specifically the Integration Layer (IL) defines the interface to communicate with multimedia codecs implemented by hardware or software.

Texas Instrument started to work on an implementation of the OpenMAX IL for their DSP accelerated codecs for OMAP platform.

A quick and rough view of the software architecture implemented to achieve this processing is more or less exposed in the next diagram:

| OpenMAX IL          |
| libdspbridge        |
| Kernel (DSP Bridge) |

The DSP Bridge driver is a Linux Kernel device driver designed to supply a direct link between the GPP program and the assigned DSP node. Basically the features offered by the driver are:

  • Messaging: Ability to exchange fixed size control messages with DSP
  • Dynamic memory management: Ability to dynamically map files to DSP address space
  • Dynamic loading: Ability to dynamically load new nodes on DSP at run time
  • Power Management: Static and dynamic power management for DSP

The libdspbridge is part of the user-space utilities of the DSP bridge, which purpose is to provide a simple programming interface to the GPP  programs for the driver services.

In the DSP side, using the C/C++ compiler for the C64x+ and the libraries contained in the user-space utilities, it is possible to compile a DSP program  and package it as a DSP node, ready to be controlled by the DSP bridge driver. But right now TI provides a set of out-of-the-box DSP multimedia codecs for non-commercial purposes. These nodes are contained in the tiopenmax package.

So, as I said before, my job was to wrap up the OpenMAX IL components delivered by TI as a GStreamer plug-in. In that way a lot of available multimedia consumers could use the hardware accelerated codecs. But also, our team did the test of the delivered OpenMAX components.

After trying several approaches we came to the conclusion that we need a new layer of software which will provide us

  • Facilitate a great testing coverage of the components without the burden of the upper framework (GStreamer in this case).
  • Improve the code reuse.
  • Use an object oriented programming through GObject.
  • Facilitate the bug’s workaround for each component and maintenance of those workarounds.
  • A playground for experimenting with features such as (OpenMAX specific) tunneling and the (TI specific) DSP Audio Software Framework (DASF).

For those reasons we started to develop an intermediate layer called GOO (GObject OpenMAX).

| GStreamer / gst-goo |
| libgoo              |
| OpenMAX             |

libgoo is a C language library that wraps OpenMAX using GObject. The follow diagram shows part of its class hierarchy.

                           | GooComponent |
+---------------+ +---------------+ +---------------+ +---------------+
| GooTiAudioEnc | | GooTiAudioDec | | GooTiVideoDec | | GooTiVideoEnc |
+---------------+ +---------------+ +---------------+ +---------------+
        |                 |                 |                 |
 +-------------+   +-------------+  +---------------+ +---------------+
 | GooTiAACEnc |   | GooTiAACDec |  | GooTiMpeg4Dec | | GooTiMpeg4Enc |
 +-------------+   +-------------+  +---------------+ +---------------+

At the top there is GooComponent which represents any OpenMAX component. If the OMX IL implementation is neat and clean, there shouldn’t need to add subclasses underneath it, just parametrize it, and should be ready to use as any other OMX IL component. But reality, as usual, is quite different: Every implementation is different from each other; and to make it worst, each component in a same implementation might behave differently, and that was the case of the TI implementation.

Finally, over libgoo there is gst-goo, the set of GStreamer elements which use the libgoo components. GstGoo also sketched some proof of concepts such as ghost buffers (to be used with the OpenMAX interop profile), and dasfsink and dasfsrc (TI specific).

In those days, before I move to the GStreamer team, an old fellow, Felipe Contreras, worked on gomx, which is the precedent of libgoo, before he got an opportunity in Nokia and started to code on GstOpenMAX. An interesting issue at this point is that Felipec is pushing boldly for a new set of GStreamer elements which ditched OpenMAX and talks directly to the kernel’s DSP bridge: gst-dsp.

What’s the future of libgoo and GstGoo? I couldn’t say. Since I moved to Igalia, I left its development. I’ve heard about a couple companies showed some kind of interest on it, sadly, the current developers are very constrained by the TI workload.