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A comment on LibreRPi – open source replacements for RPi firmware. Parent comment

Re:LibreRPi – open source replacements for RPi firmware by pjc50 on Thursday September 15, 2022 @04:37AM (#32848963)
I'm not aware of the specific details, but one of the usual critical things is clock and PLL configuration. It looks like the PLLs that generate all the internal clock frequencies, including the 48MHz for USB, are connected to the VPU. It may be the case that relevant hardware simply isn't connected to the ARM.

(From Broadcom's point of view, the VPU is the "real" computer with all the processing power, and the ARM is tacked on the side)

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Re:LibreRPi – open source replacements for RPi firmware by ggm on Thursday September 15, 2022 @04:40AM (#32848976)
This is so true, and so amusing. It almost makes me wonder if the microinstructions of the VPU could be where we run.. and use this ARM chip to do things like AES offload!

Thanks for a very useful observation. Clocking is fundamental to a lot of things, buses like the clock to be done right so I can believe booting requires good clocking and some bus initialisation.

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Re:LibreRPi – open source replacements for RPi firmware by pjc50 on Thursday September 15, 2022 @07:46AM (#32850307)
Turns out there is some pretty good architecture documentation: https://docs.broadcom.com/doc/12358545 .. but that only describes the 3D capabilities, not the whole register map.
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Re:LibreRPi – open source replacements for RPi firmware by monocasa on Thursday September 15, 2022 @12:50PM (#32854824)
That doesn't really cover the processor under discussion here, only the shader cores called the QPUs.

This work and the processor under discussion is another core called the VPU that unfortunately gets confused because with the default firmware it runs the majority of the OpenGL driver stack, controlling the QPUs.

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Re:LibreRPi – open source replacements for RPi firmware by q-big on Thursday September 15, 2022 @08:08AM (#32850506)
> Turns out there is some pretty good architecture documentation: https://docs.broadcom.com/doc/12358545 .. but that only describes the 3D capabilities, not the whole register map.

This documentation is for the VideoCore IV. The BCM2711 SOC that is used in the Raspberry Pi 4 has a VideoCore VI.

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