Inner workings of Raspberry Pi userland graphics driver (not firmware or kernel part)

Question

I'm trying to understand the userland part of the Raspberry Pi graphics driver code from https://github.com/raspberrypi/userland

My understanding so far is: - a firmware blob runs in the GPU and offers an OpenGL-like interface which, on lower levels, is based on message (byte-array) passing on top of one of multiple 28-bit-word FIFOs called VCHIQ (the other VCHIQ queues are irrelevant for graphics) - on the CPU part, OpenGL calls are turned into messages to the GPU. Access to the low-level facility (either the message queue or VCHIQ -- I haven't found that part yet in the code) requires a Linux kernel module, but no high-level logic happens in there. - the GPU part is closed, but that's okay for my purposes. The (ARM) CPU part is, AFAIK, open

My ultimate goal is to get communication with the GPU working on bare metal (without Linux), but with the closed firmware blob intact. As a first goal, I want to understand how an OpenGL call is actually passed to the GPU. Anything beyond that is not part of this question.

However, I'm stuck at finding the actual code for this. The OpenGL calls use RPC_CALL* and in turn RPC_DO, which calls khronos_server_lock_func_table(). However, that function seems to be missing from the code, and to my surprise, I couldn't find anything useful about it on Google.

My questions: - am I still on the ARM CPU side, or did I move to GPU land without noticing? If the latter is the case, where did I cross that line? - Assuming I'm still on the CPU side -- where is the code for that function? Is it open at all, or do we actually have closed parts left around on the CPU side here? All sources on the web seem to indicate that the code for the CPU is 100% open. - at which point does the implementation of the C OpenGL functions actually send a message to the GPU? I'm somewhat expecting a call to the kernel functionality that represents VCHIQ to be happening at some point, probably implemented as a device file.

Answer 1

I don't fully understand how do you intend to access the GPU without using Linux, and I am not that familiar with the technicalities, but some time ago I've been digging into the GPU for my private project so I'll tell you what I know.

The GPU is VideoCore IV and its documentation is available on Broadcom's website.

Also, on the Raspberry Pi Wiki you can see on the picture on the left that VCHIQ is in the kernel driver, so you might look for the implementation details in the kernel's source code.

Maybe this might be of some help too: VideoCore IV Programmer's Manual. About the document:

This is a independent documentation project based on a combination of static analysis and trial and error on real hardware. This work is 100% independent from and not sanctioned by or connected with Broadcom or its agents. No Broadcom documents or materials were used beyond those publicly available.

As for the software itself, The Khronos Group provides OpenGL ES and OpenVG implementation, but it's not open source. You can get the documentation from their website, but I doubt you'll find anything on such low level.

Hope it helps.