An eagle eye view into the Mesa source tree

Recap

My last post introduced Mesa’s loader as the module that takes care of auto-selecting the right driver for our hardware. If the loader fails to find a suitable hardware driver it will fall back to a software driver, but we can also force this situation ourselves, which may come in handy in some scenarios. We also took a quick look at the glxinfo tool that we can use to query the capabilities and features exposed by the selected driver.

The topic of today focuses on providing a quick overview of the Mesa source code tree, which will help us identify the parts of the code that are relevant to our interests depending on the driver and/or the feature we intend to work on.

Browsing the source code

First off, there is already some documentation on this topic available on the Mesa 3D website that is a good place to start. Since that already gives some insight on what goes into each part of the repository I’ll focus on complementing that information with a little bit more of detail for some of the most important parts I have interacted with so far:

  • In src/egl/ we have the implementation of the EGL standard. If you are working on EGL-specific features, tracking down an EGL-specific problem or you are simply curious about how EGL links into the GL implementation, this is the place you want to visit. This includes the EGL implementations for the X11, DRM and Wayland platforms.
  • In src/glx/ we have the OpenGL bits relating specifically to X11 platforms, known as GLX. So if you are working on the GLX layer, this is the place to go. Here there is all the stuff that takes care of interacting with the XServer, the client-side DRI implementation, etc.
  • src/glsl/ contains a critical aspect of Mesa: the GLSL compiler used by all Mesa drivers. It includes a GLSL parser, the definition of the Mesa IR, also referred to as GLSL IR, used to represent shader programs internally, the shader linker and various optimization passes that operate on the Mesa IR. The resulting Mesa IR produced by the GLSL compiler is then consumed by the various drivers which transform it into native GPU code that can be loaded and run in the hardware.
  • src/mesa/main/ contains the core Mesa elements. This includes hardware-independent views of core objects like textures, buffers, vertex array objects, the OpenGL context, etc as well as basic infrastructure, like linked lists.
  • src/mesa/drivers/ contains the actual classic drivers (not Gallium). DRI drivers in particular go into src/mesa/drivers/dri. For example the Intel i965 driver goes into src/mesa/drivers/dri/i965. The code here is, for the most part, very specific to the underlying hardware platforms.
  • src/mesa/swrast*/ and src/mesa/tnl*/ provide software implementations for things like rasterization or vertex transforms. Used by some software drivers and also by some hardware drivers to implement certain features for which they don’t have hardware support or for which hardware support is not yet available in the driver. For example, the i965 driver implements operations on the accumulation and selection buffers in software via these modules.
  • src/mesa/vbo/ is another important module. Across its various versions, OpenGL has specified many ways in which a program can tell OpenGL about its vertex data, from using functions of the glVertex*() family inside glBegin()/glEnd() blocks, to things like vertex arrays, vertex array objects, display lists, etc… The drivers, however, do not need to deal with all this, Mesa makes it so that they always receive their vertex data as collection of vertex arrays, significantly reducing complexity on the side of the driver implementator. This is the module that takes care of managing all this, so no matter what type of drawing you GL program is doing or how it specifies its vertex data, it will always go through this module before it reaches the driver.
  • src/loader/, as we have seen in my previous post, contains the Mesa driver loader, which provides the logic necessary to decide which Mesa driver is the right one to use for a specific hardware so that Mesa’s libGL.so can auto-select the right driver when loaded.
  • src/gallium/ contains the Gallium3D framework implementation. If, like me, you only work on a classic driver, you don’t need to care about the contents of this at all. If you are working on Gallium drivers however, this is the place where you will find the various Gallium drivers in development (inside src/gallium/drivers/), like the various Gallium ATI/AMD drivers, Nouveau or the LLVM based software driver (llvmpipe) and the Gallium state trackers.

So with this in mind, one should have enough information to know where to start looking for something specific:

  • If are interested in how vertex data provided to OpenGL is manipulated and uploaded to the GPU, the vbo module is probably the right place to look.
  • If we are looking to work on a specific aspect of a concrete hardware driver, we should go to the corresponding directory in src/mesa/drivers/ if it is a classic driver, or src/gallium/drivers if it is a Gallium driver.
  • If we want to know about how Mesa, the framework, abstracts various OpenGL concepts like textures, vertex array objects, shader programs, etc. we should look into src/mesa/main/.
  • If we are interested in the platform specific support, be it EGL or GLX, we want to look into src/egl or src/glx.
  • If we are interested in the GLSL implementation, which involves anything from the compiler to the intermediary IR and the various optimization passes, we need to look into src/glsl/.

Coming up next

So now that we have an eagle view of the contents of the Mesa repository let’s see how we can prepare a development environment so we can start hacking on
some stuff. I’ll cover this in my next post.

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