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Re: Pinouts for the P4 connector in 3/80, maybe others?

On 2023-05-6, at 02:00, Romain Dolbeau <romain%dolbeau.org@localhost> wrote:

TL;DR: looking for pinouts of the P4 connector in 3/80, possibly other
sun3x (3400 CPU board) and early sun 4 (4300 CPU board)

And also, how does (if it does) the system detect the framebuffer on
said P4 device (SBus is easy with OpenBoot/OPenFirmware, not sure what
the 3/80 does, haven't tried booting mine n 20 years and the PSU seems
long dead, idly wondering how to resurrect it with a modern display).

TL;DR: the code tries bus accesses at known addresses and moves on if there is a failure or the read doesn’t match a known value.
The system almost certainly tries to detect the frame buffers it knows about by performing read probe at the known physical address of specific registers of the board(s) in question. 

As an example, let’s follow how a bwtwo might get detected. First, the pre-compiled device list is consulted. For bwtwo on a sun3x, that’s here:

That instructs the device driver to attempt to probe the board at physical base address 0x50300000. At runtime that will result in ca->ca_paddr being set to 0x50300000 in this function, which will then attempt to peek a 32 bit value at that address to see if it reads back as a known constant for any of the various bwtwo/cg4 frame buffers that respond at this address.


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