Foundry/barswap/k80_quirk

K80 16/16 — map all 16 GK210 dies at boot on the BTC-S37

What this does: makes all 16 GK210 dies of an 8× Tesla K80 machine (192 GB VRAM) map into MMIO simultaneously, at boot, automatically — on a cheap BTC-S37 mining board whose firmware + 36-bit CPU otherwise let only one 16 GB GPU BAR map at a time.

How: a single built-in Linux kernel quirk (k80-strap-quirk.c). No barswap, no runtime tool, no VBIOS flash, no userspace. Drop the kernel in, reboot, done.

Status: ✅ WORKING — 16/16 verified on the reference 8× K80 machine, card 8 included.

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The problem

• Each K80 die advertises a 16 GB BAR1 (a CPU window into its VRAM). 16 dies × 16 GB = 256 GB of MMIO.
• The BTC-S37's soldered Pentium 2117U (Ivy Bridge) has 36-bit physical addressing → a 64 GB ceiling. 256 GB can't be placed. The kernel's PCI allocator fails to place the big BARs and disables the prefetchable window of the bridge above the last card (Prefetchable memory behind bridge: [disabled]).
• Once that bridge window is disabled, that card's dies stop decoding entirely — they read 0xffffffff and can't be reached by any runtime tool. This stranded the 8th card ("card 8": dies 20/21 on root port 00:1c.7).

The fix is to make every BAR1 256 MB instead of 16 GB: 16 × 256 MB = 4 GB, fits trivially. BAR1 size is a strap (PSTRAPS, BAR0 + 0x101000, field [17:14], value 2 = 256 MB, override bit 31), not silicon — so it's settable. But the strap must be applied before the kernel sizes the bridge windows, or card 8 is already lost.

The solution — one kernel quirk, two parts

k80-strap-quirk.c is built into the kernel (obj-y) and does:

1. quirk_gk210_shrink_bar1 — a DECLARE_PCI_FIXUP_HEADER that rewrites the kernel's view of each die's BAR1 to 256 MB before resource assignment (modeled on the in-tree quirk_s3_64M). The allocator never attempts the 16 GB placement → never disables any bridge → all 16 dies get a 256 MB BAR1 and every bridge stays alive, card 8 included.
   • Must be HEADER, not EARLY: pci_fixup_early runs before pci_read_bases() (BAR0/resources unpopulated — an EARLY version reports "BAR0 unassigned"); HEADER runs after sizing, before assignment.
2. k80_strap_silicon — a late_initcall that, after assignment (when BAR0 is reachable), writes the real PSTRAPS strap on each die so the silicon decode is 256 MB too. Required because the two dies on a card share a bridge window — an unstrapped 16 GB die would alias over its sibling. Part 1 keeps the bridges alive so this reaches all 16.

What was ruled out (so you don't repeat it)

• barswap time-multiplexing — the original approach; works but fragile and only ever one die in the slot. The strap shrink makes all 16 map at once → barswap unneeded.
• VBIOS strap edit + nvflash — flashes fine (Kepler is unsigned), but this board does not run GPU option ROMs at POST, so the VBIOS devinit never executes before BAR sizing. (See ../vbios_* / the dump tooling.)
• pci_fixup_early quirk — too early; BAR0 unassigned at that point.
• Runtime setpci/k80_bar_resize — works for 14/16, but can't reach card 8 post-boot (its bridge is already disabled). Kept as tools/strap_all.py (manual fallback / diagnostics).

Build

Reproducible: pinned Ubuntu kernel source 6.8.0-106.106 (from Launchpad) + the target machine's exact config + this quirk → kernel 6.8.12-k80strap (6.8.12 = the stable level Ubuntu's source rebases to).

Cross-compile, anywhere with Docker (incl. Apple Silicon — builds x86-64 at native speed):

./build.sh /path/to/config-6.8.0-106-generic    # a local copy of the target's /boot config
K80_HOST=user@k80-host ./build.sh               # or fetch it from the target over ssh
# output: out/linux-image-*.deb

To build natively on an x86-64 Ubuntu box instead, run the steps in Dockerfile (steps 1-5) directly; the result is the same quirk from the same pinned source.

Install (the appliance path)

The kernel is fully self-contained — drop the .deb into an identical box (BTC-S37 + K80 + Ubuntu 24.04, Secure Boot off) and reboot. The .deb is NOT in the repo (it is a Linux kernel binary): build it first with either recipe above and copy it into prebuilt/ (gitignored).

scp prebuilt/linux-image-6.8.12-k80strap_*.deb <box>:~/
ssh <box> 'sudo dpkg -i ~/linux-image-6.8.12-k80strap_*.deb && sudo update-grub'
# make it the default boot (after first verifying with grub-reboot one-shot):
ssh <box> 'sudo sed -i "s/^GRUB_DEFAULT=.*/GRUB_DEFAULT=\"Advanced options for Ubuntu>Ubuntu, with Linux 6.8.12-k80strap\"/" /etc/default/grub && sudo update-grub'
sudo reboot

Prereqs: Secure Boot off; grub cmdline keeps pci=nommconf; BIOS iGPU aperture 128 MB (frees 32-bit MMIO for the BAR0s); nvidiafb blacklisted — see the gotcha below, this one is mandatory.

MANDATORY: blacklist nvidiafb. The legacy in-kernel NVIDIA framebuffer driver (nvidiafb, nothing to do with nvidia.ko/compute) auto-binds to any NVIDIA display chip it finds. On the stock kernel the K80 BARs are unassigned so it bails harmlessly — but once this quirk maps the dies, nvidiafb probes one as a CRT framebuffer, faults reading its clocks (nvGetClocks), and hard-hangs the box mid-boot (network comes up, then a silent wedge — no oops reaches disk). The console runs on the Intel iGPU (i915), so nvidiafb has no legitimate role here. Ubuntu ships a blacklist-framebuffer.conf that covers it; make sure it's active and belt-and-suspenders the cmdline:

ssh <box> 'sudo mv -n /etc/modprobe.d/blacklist-framebuffer.conf.OFF /etc/modprobe.d/blacklist-framebuffer.conf 2>/dev/null; \
  grep -q modprobe.blacklist=nvidiafb /etc/default/grub || sudo sed -i "s/\(GRUB_CMDLINE_LINUX_DEFAULT=\"[^\"]*\)\"/\1 modprobe.blacklist=nvidiafb\"/" /etc/default/grub; \
  sudo update-initramfs -u -k all && sudo update-grub'

Symptom if you skip it: quirk kernel hangs at boot while the stock kernel boots fine, and journalctl -b -1 -k | grep nvidiafb shows nvGetClocks in a call trace.

A clone box needs the .deb + the nvidiafb blacklist above. Rebuild only for different hardware (a NIC / storage controller not in this config) — the recipe above handles that.

MANDATORY for CUDA compute: disable MSI (NVreg_EnableMSI=0). Mapping the dies (this quirk) and NVML/nvidia-smi work fine, but the nvidia driver request_irq() fails (-22) for the PLX-switched x1 slots on this board — the IRQ routing can't do MSI. CUDA needs interrupts for kernel completion, so cuInit returns 101 and every CUDA app silently falls back to CPU. Force legacy INTx and it works:

echo "options nvidia NVreg_EnableMSI=0" | sudo tee /etc/modprobe.d/nvidia-msi.conf
sudo rmmod nvidia_uvm nvidia_drm nvidia_modeset nvidia; sudo modprobe --ignore-install nvidia

Tell if you skip it: dmesg | grep request_irq shows -22, and a driver-API cuInit(0) returns 101 (CUDA_ERROR_INVALID_DEVICE).

**MANDATORY for sustained CUDA: disable runtime D3 (NVreg_DynamicPowerManagement=0

• persistence mode).** With MSI fixed, a brief CUDA run works — but under a multi-minute load the die goes idle between ops, drops to D3cold, and this board can't wake it: Unable to change power state from D3cold to D0, Device inaccessible → Xid 79 "GPU has fallen off the bus" → the job hangs. It looks thermal but isn't (temps stay ~60–70 °C). Keep the GPU in D0:

echo "options nvidia NVreg_EnableMSI=0 NVreg_DynamicPowerManagement=0" | sudo tee /etc/modprobe.d/nvidia-msi.conf
sudo rmmod nvidia_uvm nvidia_drm nvidia_modeset nvidia; sudo modprobe --ignore-install nvidia
sudo nvidia-smi -pm 1   # persistence mode, before running any CUDA

Tell if you skip it: brief CUDA works, sustained decode dies with dmesg | grep -i "fallen off\|D3cold". Recovery needs a reboot (the die is off the bus).

Verify

sudo python3 tools/check_fixup.py     # 16/16 dies BAR1=256MB + card-8 bridge alive
sudo dmesg | grep k80_strap           # 16x "BAR1 ->256MB" + 16x "silicon BAR1 ->256MB"
sudo python3 tools/bridge_diag.py     # bridge-window state, working card vs card 8

Expected: 16/16 dies with a 256 MB BAR1 assigned, card-8 bridge pref=[0x3.._-0x3.._] (not <DISABLED>), and every die's PSTRAPS field=2.

Files

k80-strap-quirk.c          the kernel quirk (HEADER fixup + late_initcall strap)  <- the whole solution
Dockerfile + build.sh      reproducible cross-compile (any Docker host); fetches the
                           target's /boot kernel config at build time (never committed)
prebuilt/*.deb             build output drop location (gitignored; build it, see Build)
tools/check_fixup.py       verify 16/16 + card-8 bridge
tools/bridge_diag.py       dump bridge windows (working card vs holdout)
tools/strap_all.py         manual silicon strap (the late_initcall does this now)
tools/strap_test.py        per-die PSTRAPS reachability probe
archive/                   the loadable-module attempt that did NOT work (history)

Hardware (fixed — the point is the cheap hardware)

BTC-S37 mining board · Intel Pentium 2117U (Ivy Bridge, 36-bit) · 8× Tesla K80 = 16 GK210 dies, 192 GB · PCIe x1 links · AMI BIOS · Ubuntu 24.04, kernel 6.8.