[Raspberry Pi] Consider using the GPU for precise GPIO timings, rather than DMA

[Wallacoloo]

Gert showed that it was possible to dump the value of over 50 million 16-bit values per second into the GPIOs using the Pi's GPU. His application was to have each 16 bits present one pixel (RGB565) and use a resistor DAC to send it to a VGA LCD, along with clock signals generated by the GPU, all with accurate enough timings for video (in order to correctly place each pixel, that requires timing accurate to about 20 ns).

Unfortunately, his code, at the time of writing, is only supplied as a binary blob and the documentation of the GPU features he used is nonexistent.

But there are other efforts to demystify the Pi's GPU. There is, for example, this tutorial for writing GPU code on the Pi. It may be possible to commandeer one GPU core permanently and use it to precisely drive the GPIO pins (Not sure how much control we have over scheduling).

Useful documents:
Herman Hermitage Unofficial Videocore Docs
Official Broadcom Videocore Documentation

[Wallacoloo]

The Thread Control section of this document seems to indicate that once a program is allocated space onto one of the GPU cores (called a "QPU"), it has full control of its scheduling - it can only lose control via an explicit Thread Switch signal.

I don't know how memory writes are coordinated. But I imagine there's a way to write to arbitrary memory locations (i.e. the GPIO bank). Docs say the QPUs allow for "I/O mapped into the register space". Edit: this appears to indicate that the GPIO section IS accessible from the Videocore (0x7E000000 - 0x7EFFFFFF, IO).

It looks like there's been some successful work getting the LLVM compiler to output code that can run on the QPUs.

There's also some source files for initializing & using the QPUs on bare metal

[Wallacoloo]

Started this thread on the Raspberry Pi forums. jamesh suggests using the scalar cores.

Reading the HHH (Herman H. Hermitage) docs hint that the scalar cores are the same thing as the VPUs ("Scalar/Vector Processor")

[niksoley]

It sounds good, how is it evolving? Is there still the issue of losing steps at high speeds?

[Wallacoloo]

@niksoley This project's been dead for a while now. I never even started work on the GPU stuff. Feel free to fork & develop it if you like though.