PCI Device Discovery Issue - One Page Summary

ZOTAC ZBOX-CI320NANO with Coreboot MMIO Patch

Problem Statement

PCIe WiFi card (Intel 3160 at 01:00.0) fails when MMIO patch is applied to Coreboot, but identical Bay Trail hardware (Swanky Chromebook) works correctly with the same patch.

Root Cause: PCIe Bus Discovery Method

ZBOX (Broken)

[0.300966] PCI: Discovered peer bus 01
[0.301081] pci_bus 0000:01: root bus resource [mem 0x00000000-0xfffffffff]

Linux treats bus 01 as independent "peer bus" → unlimited address range → conflict

Swanky (Working)

[0.237285] pci 0000:00:1c.0: PCI bridge to [bus 01]
[0.345008] pci_bus 0000:01: resource 1 [mem 0xfd900000-0xfd9fffff]

Linux discovers bus 01 through bridge → bounded window → no conflict

Why This Matters

With proper bridge discovery (Swanky):

• Bus 01 inherits bounded window: [mem 0xfd900000-0xfd9fffff]
• WiFi at 0xFD900000 is within its own bus's window
• No conflict with bus 00's MMIO window 0xF0000000-0xFDFFFFFF
• ✅ WiFi works

With peer bus discovery (ZBOX):

• Bus 01 gets unlimited range: [mem 0x00000000-0xfffffffff]
• Overlaps with bus 00's MMIO window 0xF0000000-0xFDFFFFFF
• WiFi at 0xFDB00000 claimed by BOTH bus 00 and bus 01
• Linux: "Conflict! Move WiFi above 4GB" → 0x100000000
• Device becomes inaccessible at new address (reads return 0xFFFFFFFF)
• ❌ WiFi broken (probe fails with error -5)

Technical Details

What Coreboot Does (Both Systems):

• ✅ Correctly allocates bridge window at 0xFDB00000-0xFDBFFFFF
• ✅ Assigns WiFi device to 0xFDB00000 within bridge window
• ✅ Sets bridge memory window register (offset 0x20)

What ACPI Tells Linux (Different!):

• Swanky ACPI: Bridge 00:1c.0 properly declared with subordinate bus 01
• ZBOX ACPI: Bus 01 declared as separate PCI root (peer bus)

Why "Peer Bus" Discovery Happens

ZBOX's ACPI tables likely contain one or more of:

1. Separate PCI root device for bus 01 in DSDT

   Device (PCI1) {                    // Should NOT exist
       Name (_HID, EISAID("PNP0A08"))
       Name (_BBN, 0x01)              // Base bus 01
   }
   

2. Missing/incorrect bridge device definition for 00:1c.2

3. MCFG table listing bus 01 as separate segment

4. Missing subordinate bus number in bridge configuration

Why MMIO Patch Triggers the Issue

WITHOUT MMIO Patch:

• ACPI doesn't define 0xF0000000-0xFDFFFFFF window
• All devices fail to claim BARs (no compatible window)
• Linux reassigns everything to 0x80000000 range
• Peer bus bug is masked by full reassignment
• ✅ Works by accident

WITH MMIO Patch:

• ACPI defines 0xF0000000-0xFDFFFFFF window for bus 00
• Bus 00 devices claim their BARs successfully
• Peer bus bug is exposed: bus 01's unlimited range conflicts
• WiFi relocated above 4GB → broken
• ❌ Fails

The Fix

Primary Solution: Correct ZBOX's ACPI tables

1. Remove any separate PCI root device for bus 01
2. Ensure bridge 00:1c.2 is properly declared with subordinate bus
3. Define bridge window in _CRS method

Expected result after fix:

pci 0000:00:1c.2: PCI bridge to [bus 01]
pci 0000:00:1c.2:   bridge window [mem 0xfdb00000-0xfdbfffff]
pci_bus 0000:01: resource 1 [mem 0xfdb00000-0xfdbfffff]

Alternative Workaround: Allocate PCIe bridge in 0x80-0xDF range

• Avoids the 0xF0-0xFD "danger zone" entirely
• Works even with peer bus bug (no overlap to conflict)

Verification Steps

1. Extract ACPI tables: sudo cat /sys/firmware/acpi/tables/DSDT > dsdt.bin
2. Decompile: iasl -d dsdt.bin
3. Search for: grep -c "PNP0A08" dsdt.dsl (should be 1, not 2+)
4. Look for bridge definitions: grep "Device.*1C\|Device (RP" dsdt.dsl

Key Takeaway

The MMIO patch itself is not the problem. It exposes a pre-existing ACPI bug where ZBOX declares bus 01 as a peer bus instead of subordinate to its parent PCIe bridge. Swanky's ACPI is correct, so it works. Fix ZBOX's ACPI to match Swanky's bridge hierarchy.

Issue: ACPI configuration bug
Impact: WiFi non-functional with MMIO patch
Fix Location: Coreboot ACPI table generation (DSDT)
Status: Workaround available (no MMIO patch), permanent fix requires ACPI update