hw: Add native bootrom

[fischeti]

Adds native bootrom to the cluster instead of fetching from externally. Also adds additional scratch registers to the peripherals which can be used to write the entry point of the binary.

The current bootrom is very simple currently, and just fetches from the start of the DRAM region:

_start:
    lui   t0, %hi(_l3_base) // Load the L3 base address
    addi  t0, t0, %lo(_l3_base)
    jalr  zero, t0, 0  // Jump to the L3 base address

The desired bootrom would look more like this:

_snitch_park:
    # Set trap vector
    la      t0, _snitch_resume
    csrw    mtvec, t0
    # Enable software and cluster interrupts
    csrsi   mstatus, MSTATUS_MIE # CSR set (uimm) 
    lui     t0, 0x80  # (1 << 19) cluster interrupts
    addi    t0, t0, 8 # (1 << 3) software interrupts
    csrw    mie, t0
    wfi

_snitch_resume:
    auipc   t0, 0
    li      t1, -3736 # Jump to scratch register 1 at offset 0x188
    add     t0, t0, t1
    lw      t0, 0(t0)
    jalr    ra, 0(t0)
    j       _snitch_park

where the cores are parked, and after getting an interrupt they start fetching from one of the scratch register (which is written by a host core e.g.).

This was also my first approach, but getting this to run in the testbench was not trivial to do (at that time). It requires an external interrupt and writing the entrypoint to one of the configuration registers, which is possible with an AXI driver connected to the narrow AXI interface. However, verilator4 does not support timing constructs, so AXI drivers cannot be used at all, which is why I reverted back to a very simple bootrom.

Now, we upgraded to verilator 5, which would theoretically allow such a thing. The question now is if this is needed, or if the intended use case is that the user will anyway write his custom bootrom that will overwrite the default one. Afaik, this is also the approach that we did for previous tapeouts (i.e. Occamy).

TODO

• Add bootrom to rtl target and prerequisites
• Credit initial authors before merging

Co-authored-by: Milos Hirsl <[email protected]>
Co-authored-by: Thierry Dubochet <[email protected]>