FABRIC-3.md: pci_init() DTB path item was wrong -- re-scope, don't build the ECAM version

Investigated before writing code, per this document's own discipline, and
found the original item 5 text's assumption doesn't hold: it framed this
as "swap the ACPI MCFG lookup for a DTB one, same flat-ECAM access
underneath." Confirmed against three primary sources that this is wrong:

- bcm2712-rpi-5-b.dts: RP1 sits under pcie2 ("brcm,bcm2712-pcie"),
  reg = <0x10 0x00120000 0x00 0x9310> -- a ~37KB window, far too small
  for a flat 256MB ECAM region.
- pcie-brcmstb.c (the real Linux driver for this compatible string):
  brcm_pcie_map_bus() computes a standard ECAM-shaped offset but accesses
  it through an IDX_ADDR/DATA_ADDR indirect index+window pair, not a
  direct MMIO read. A real third config-access mechanism, not a bigger
  version of the DTB-lookup task.

Re-scoped item 5 (§IV.3) to record the real shape: pci.c's cfg_read32/
cfg_write32 family dispatches today via #ifdef ARCH_AMD64 vs. everything
else (flat ECAM) -- but QEMU aarch64 (flat ECAM, real ACPI MCFG) and Pi 5
aarch64 (Broadcom indirect windowing) are the same build, so this needs a
runtime dispatch change to a file all three architectures currently boot
through, not a small addition. Also flagged: pci_init()'s vmm_map_range()
for the ECAM window is amd64-only today (aarch64/riscv64 rely on UEFI's
identity map); native boot has no UEFI identity map at all, so any PCIe
work here needs its own explicit mapping regardless of access mechanism.

Also corrected §V.3 item 5's now-stale "shares the same code" cross-
reference -- the Mars's JH7110 PCIe controller needs its own primary-
source check, not an assumed-shared implementation with RP1's.

Not implemented this pass -- recording the real shape is the deliverable;
building the runtime-dispatch version is a materially larger, separate
task. Doc-only change; 3-arch boot verified per the project's standing
rule, proving only that nothing regressed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019YcT3H2PQeyujrzjqS3Var

FABRIC-3.md

@@ -405,9 +405,43 @@ blocker for free.
    devicetree excerpt that confirmed it. Verified 3-arch boot to `ok>` (compile-only — these
    two functions have no caller yet; that's the entry-stub/framebuffer-driver items above,
    still open).
-5. **`pci_init()` DTB path**: a devicetree-based alternative for RP1 discovery, since
-   `boot_info->acpi_table` will be `NULL` on this path and RP1 is PCIe-attached, not directly
-   memory-mapped.
+5. **`pci_init()` DTB path — RE-SCOPED 2026-09-04, this item's own original text was wrong.**
+   Investigated before writing any code (per this document's own discipline) rather than
+   building the originally-assumed "swap the ACPI MCFG lookup for a DTB one, same flat-ECAM
+   access underneath" version — that assumption does not hold, confirmed against three primary
+   sources, not guessed:
+   - `bcm2712-rpi-5-b.dts` (real board file): RP1 is attached under `pcie2`
+     (`pcie2: pcie@1000120000`, `compatible = "brcm,bcm2712-pcie"`) via
+     `rp1_target: &pcie2 { status = "okay"; };`.
+   - `pcie2`'s own `reg` is `<0x10 0x00120000 0x00 0x9310>` — a ~37 KB window, far too small to
+     be a flat 256 MB ECAM region (256 buses × 1 MB) the way `pci.c`'s existing `ecam_*`
+     functions assume.
+   - The real Linux driver for `"brcm,bcm2712-pcie"` (`drivers/pci/controller/pcie-brcmstb.c`)
+     confirms why: `brcm_pcie_map_bus()` computes a standard ECAM-shaped offset
+     (`PCIE_ECAM_OFFSET(bus, devfn, 0)`) but does **not** read/write it directly — it writes
+     that offset to an `IDX_ADDR` index register, then accesses the config data through a small
+     windowed `DATA_ADDR` region. Root-complex-local accesses (`devfn == 0` on the root bus)
+     skip the indirection and hit `base + PCIE_ECAM_REG(where)` directly; every downstream
+     device access (RP1 included) goes through the index/data window.
+   - This is a real **third config-access mechanism**, not a bigger version of "find the ECAM
+     base in the DTB instead of ACPI." `pci.c`'s `cfg_read32`/`cfg_write32` family
+     (`pci.c:188–232`) today dispatches exactly two ways, selected by `#ifdef ARCH_AMD64` vs.
+     everything else (flat `ecam_*`) — and that `#ifdef` selection is itself now wrong for this
+     case: QEMU aarch64 (flat ECAM via ACPI MCFG, real and working) and Pi 5 aarch64
+     (Broadcom indirect windowing) are the **same build** (`ARCH=aarch64`), so the dispatch
+     must become a runtime choice, not a compile-time one — a real change to a shared file all
+     three architectures currently boot clean through, not a small addition.
+   - Secondary finding, not yet acted on: `pci_init()`'s `vmm_map_range()` call for the ECAM
+     window is inside `#ifdef ARCH_AMD64` only (`pci.c:306–318`) — aarch64/riscv64 rely on
+     UEFI's own identity map instead (per this file's own header comment). Native boot has no
+     UEFI identity map at all, so **any** PCIe work on this path — regardless of access
+     mechanism — needs its own explicit `vmm_map_range()` call, not just a new config-access
+     branch.
+   - **Not implemented this pass.** The re-scoped shape (add a third, runtime-selected
+     Broadcom-indirect access mode to `pci.c`, plus explicit VMM mapping for the native path)
+     is a materially larger, higher-blast-radius change than items 1–4 — it touches a file all
+     three architectures share and boot through today. Recording the real shape here is this
+     pass's own deliverable; building it is future work.
 6. **`config.txt` contents, decided from research**: `kernel=kernel_2712.img` (or `kernel8.img`
    with `os_check=0` if the Pi-5-specific name isn't used), `arm_64bit=1`, pointing at
    `bcm2712-rpi-5-b.dtb`.
@@ -515,10 +549,15 @@ it doesn't. This is a real punch-list item, not a hypothetical.
    pass — unlike the Pi 5's mailbox interface (well-documented, reused across many Pi bare-
    metal projects), no equivalent research done yet for JH7110's own display controller.
    Flagged here rather than assumed simple.
-5. **`pci_init()` DTB path**: shares the same new code §IV.3 item 5 scopes for the Pi 5's RP1
-   — one implementation, two consumers (M.2 here, RP1 there), assuming the underlying DTB PCI
-   binding shape is similar enough (ECAM-based, most likely, but not yet confirmed for JH7110
-   specifically).
+5. **`pci_init()` DTB path — the "shares §IV.3 item 5's code" framing is now wrong, corrected
+   2026-09-04.** §IV.3 item 5's own investigation found the Pi 5's RP1 sits behind a
+   Broadcom-specific indirect config-access window (`"brcm,bcm2712-pcie"`, confirmed against
+   `pcie-brcmstb.c`), not flat ECAM — a SoC-specific mechanism with no reason to assume it
+   matches JH7110's own PCIe controller. The "one implementation, two consumers" plan this line
+   originally stated no longer holds; the Mars's M.2 E-Key slot needs its **own** primary-source
+   check of JH7110's actual PCIe controller (compatible string, `reg` window size, and whether
+   its Linux driver uses flat ECAM or its own indirect scheme) before any implementation
+   decision, same discipline §IV.3 item 5 itself just applied. Not yet done.
 6. **Boot image packaging**: `vf2-imager`/`mkimage`-based FIT image (§V.1) — confirm this
    tooling's actual invocation once building the first real image, not just cited from
    VisionFive 2 research.

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