PCI Express Expansion Chassis Attachment Failure on Windows: Troubleshooting Guide

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Connecting a PCI Express (PCIe) expansion chassis to a computer running Windows can sometimes lead to complications. Users may encounter situations where devices housed within the expansion chassis are not correctly recognized or fail to start properly. These issues often manifest as specific error codes displayed in the Windows Device Manager, indicating underlying resource allocation or system configuration problems. This guide provides detailed steps to troubleshoot and resolve common attachment failures using registry modifications.

Before proceeding with any registry changes, it is strongly recommended to back up your registry or create a system restore point. Modifying the registry incorrectly can lead to severe system instability or prevent Windows from starting.

Symptoms of Attachment Failure¶

When a PCI Express expansion chassis with connected devices fails to integrate correctly with your Windows system, you may observe one or both of the following error messages associated with the affected devices in Device Manager:

Error 1: “This device cannot find enough free resources that it can use. (Code 12)”¶

This error indicates that Windows Plug and Play (PnP) has detected the device but cannot allocate the necessary system resources (such as memory ranges, I/O ports, or interrupt lines) for it to function. For devices within an expansion chassis, this often points to a problem with how the system or the PCIe bridge within the chassis is managing resource allocation for devices downstream.

Error 2: “The device is not working properly because Windows cannot load the drivers required for this device. (Code 31)”¶

While this error typically suggests a driver issue, when it occurs with devices in an expansion chassis alongside enumeration problems, it can be a symptom of a deeper system resource conflict, particularly related to the assignment of PCI bus numbers. The system may fail to initialize the device correctly, preventing the driver from loading, because the device’s location on the PCI hierarchy cannot be properly established.

Understanding the root causes behind these errors is key to applying the correct workaround. Both issues stem from how the Windows operating system interacts with the complex PCI Express topology presented by an expansion chassis, especially one containing switches and multiple layers of devices.

Causes of These Issues¶

The underlying reasons for these attachment failures are related to how the PCI and PCI Express specifications handle resource management and device enumeration, and how the Windows operating system interprets the state of these devices during boot and Plug and Play operations.

Cause of Error 1 (Code 12)¶

The Code 12 error, specifically indicating a lack of resources, is frequently linked to the initial state of the PCI Express bridge devices found within the expansion chassis. A PCI Express bridge is a component that connects different parts of the PCIe hierarchy, translating transactions and managing traffic. These bridges are crucial for allowing devices connected “behind” them (further down the tree) to communicate with the system.

PCI bridges, including those in a PCIe hierarchy, require configuration space and often resource windows (memory and I/O ranges) to map the resources of downstream devices into the system’s address space. According to the PCI and PCIe specifications, bridge devices have configuration registers that define these resource windows, including base and limit addresses. By default, upon system startup or reset, the initial values of the limit registers for these bridge resource windows are often set to a value less than the initial values of the base registers.

This specific initial state (limit < base) is interpreted by some operating systems, including Windows, as a signal that the bridge’s resource window is disabled or not yet configured. Consequently, the Plug and Play subsystem in Windows may not recognize that the bridge requires resources or needs to have its windows configured to enumerate and allocate resources for the devices connected downstream of it. Without proper resource window configuration on the bridge, the devices behind it cannot be correctly enumerated or assigned their own resources. This leads to the Code 12 error, as Windows cannot find the necessary resources for the device, effectively because the path to those resources via the bridge is seen as closed.

This issue highlights a potential interaction problem between the hardware’s default power-on state and the operating system’s enumeration logic, particularly in complex PCIe topologies involving multiple bridges or switches within an expansion chassis.

Cause of Error 2 (Code 31)¶

The Code 31 error in this context, often linked to devices not starting or drivers not loading, is commonly caused by an exhaustion of available Peripheral Component Interconnect (PCI) bus numbers. Every device in a PCI or PCI Express hierarchy is assigned a unique identifier based on its bus, device, and function number (BDF or Bus:Device.Function). Bus numbers are critical for uniquely addressing devices in the system’s hardware topology.

System BIOS firmware plays a significant role in configuring the PCI/PCIe hierarchy during the power-on self-test (POST). The BIOS allocates ranges of bus numbers to different segments of the hierarchy. When a standard computer system starts, the BIOS assigns a certain range of bus numbers to the integrated or on-board PCI/PCIe devices and available expansion slots.

However, connecting a complex PCI Express expansion chassis, especially one that contains a complex switch (which acts like multiple interconnected bridges) and supports a deep hierarchy of devices (many devices connected downstream through several layers of switches), can quickly consume a large number of bus numbers. Each bridge and switch requires bus numbers to define the buses that exist behind them. If the range of bus numbers pre-allocated or made available by the computer’s BIOS is insufficient to accommodate the entire structure of the expansion chassis and all its devices, the operating system will eventually run out of unique bus numbers to assign.

When Windows attempts to enumerate the devices in the expansion chassis but cannot assign a unique bus number to a device or a segment of the hierarchy, it cannot fully configure the device’s address or load its driver, as the device’s location and identity within the system’s bus topology are undefined. This failure to establish the device’s presence and allocate fundamental resources like a bus number results in the device not starting correctly, often manifesting as a Code 31 error.

This scenario is more likely to occur with older or less flexible BIOS implementations that allocate a fixed or limited range of bus numbers, insufficient for modern, highly expandable PCIe configurations.

Workarounds Using Registry Modification¶

Microsoft provides workarounds for these specific issues by modifying the Windows registry. These modifications alter the behavior of the Windows Plug and Play subsystem regarding the enumeration and resource allocation for PCI devices, specifically addressing the scenarios described above. These workarounds involve adding or modifying a HackFlags registry entry under the PCI key.

WARNING: Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk. Ensure you have a backup or a system restore point before proceeding.

Workaround for Error 1 (Code 12) - Modifying Bridge Resource Handling¶

This workaround involves setting a specific flag in the registry that likely influences how Windows handles the initial state of PCI Express bridge resource windows, potentially causing the system to enumerate and allocate resources for bridges even if their registers initially indicate a disabled state.

Follow these steps:

1. Click Start, type regedit in the search box, and then click regedit in the Programs list.
2. If prompted by User Account Control, click Yes or provide administrator credentials.
3. Navigate to the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci
4. Check if the HackFlags registry entry exists in the right-hand pane.
5. If HackFlags is not present:
   a. Right-click on the Pci key.
   b. Point to New, and then click DWORD (32-bit) Value.
   c. Type HackFlags and press ENTER.
   d. Right-click HackFlags, and then click Modify.
   e. In the Value data box, type 400.
   f. Under Base, click Hexadecimal.
   g. Click OK.
   h. Close Registry Editor.
6. If HackFlags is present:
   a. Right-click HackFlags, and then click Modify.
   b. In the Value data box, type 400. Note: If there are other flags already set (the existing value is not 0), you will need to add 400 to the existing value in hexadecimal. For example, if the existing value is 100 (hex), you would change it to 500 (hex). However, based on Microsoft’s documentation for these specific workarounds, they are typically applied individually or combined by summing their values, suggesting these are distinct flags. In many cases, the key might not exist or have a default value of 0. For this specific workaround for Error 1, the target value is 400 (hex).
   c. Under Base, click Hexadecimal.
   d. Click OK.
   e. Close Registry Editor.

After applying this registry change, restart your computer for the changes to take effect. This modification specifically targets the issue causing Code 12 errors by altering how Windows handles the resource windows of PCIe bridges.

Workaround for Error 2 (Code 31) - Expanding Bus Number Allocation¶

This workaround involves setting a different flag in the registry that likely influences how Windows allocates PCI bus numbers, potentially increasing the range or changing the allocation strategy to accommodate larger and deeper PCIe hierarchies presented by expansion chassis.

Follow these steps:

1. Click Start, type regedit in the search box, and then click regedit in the Programs list.
2. If prompted by User Account Control, click Yes or provide administrator credentials.
3. Navigate to the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci
4. Check if the HackFlags registry entry exists in the right-hand pane.
5. If HackFlags is not present:
   a. Right-click on the Pci key.
   b. Point to New, and then click DWORD (32-bit) Value.
   c. Type HackFlags and press ENTER.
   d. Right-click HackFlags, and then click Modify.
   e. In the Value data box, type 200.
   f. Under Base, click Hexadecimal.
   g. Click OK.
   h. Close Registry Editor.
6. If HackFlags is present:
   a. Right-click HackFlags, and then click Modify.
   b. In the Value data box, type 200. As with the previous workaround, if other flags are set, you might need to calculate the sum. For this specific workaround for Error 2, the target value is 200 (hex).
   c. Under Base, click Hexadecimal.
   d. Click OK.
   e. Close Registry Editor.

After applying this registry change, restart your computer for the changes to take effect. This modification is aimed at resolving Code 31 errors that occur because the system runs out of available PCI bus numbers when a complex expansion chassis is attached.

Enable the Workarounds for Error 1 and Error 2 Simultaneously¶

In some cases, both issues (resource window handling and bus number exhaustion) might contribute to the failure. You can enable both workarounds concurrently by setting the HackFlags value to the sum of the individual values in hexadecimal. The value for the Error 1 workaround is 400 (hex), and the value for the Error 2 workaround is 200 (hex). Enabling both requires setting the HackFlags value to 400h + 200h = 600h.

WARNING: Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk. Ensure you have a backup or a system restore point before proceeding.

Follow these steps:

1. Click Start, type regedit in the search box, and then click regedit in the Programs list.
2. If prompted by User Account Control, click Yes or provide administrator credentials.
3. Navigate to the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci
4. Check if the HackFlags registry entry exists in the right-hand pane.
5. If HackFlags is not present:
   a. Right-click on the Pci key.
   b. Point to New, and then click DWORD (32-bit) Value.
   c. Type HackFlags and press ENTER.
   d. Right-click HackFlags, and then click Modify.
   e. In the Value data box, type 600.
   f. Under Base, click Hexadecimal.
   g. Click OK.
   h. Close Registry Editor.
6. If HackFlags is present:
   a. Right-click HackFlags, and then click Modify.
   b. In the Value data box, type 600. If the existing HackFlags value is not 0, and you intend to apply *only these two specific workarounds (200h and 400h), you should set the value to 600h. However, if other HackFlags are already legitimately set by other fixes or system configurations, you should add 600h to the existing value.* For the purpose of enabling only these two workarounds described, the value should be set to 600h.
   c. Under Base, click Hexadecimal.
   d. Click OK.
   e. Close Registry Editor.

After applying this registry change, restart your computer for the changes to take effect. Setting the HackFlags to 600h attempts to address both the bridge resource window initialization issue (Error 1) and the PCI bus number exhaustion issue (Error 2) simultaneously.

Here is a quick summary of the HackFlags values for these workarounds:

Issue Addressed	Error Code	HackFlags (Hexadecimal)
Bridge Resource Window Handling	Code 12	400
PCI Bus Number Allocation	Code 31	200
Both Issues Combined	Code 12 & 31	600

These registry modifications are specific workarounds provided by Microsoft for these known issues related to complex PCI Express topologies. They signal the operating system to deviate from its default Plug and Play behavior in ways that accommodate the specific characteristics of these expansion chassis problems.

General Troubleshooting Considerations¶

Before resorting to registry modifications, consider some general troubleshooting steps:

• Check Physical Connections: Ensure the expansion chassis is securely connected to the host computer (e.g., via a Thunderbolt or dedicated PCIe cable) and powered on correctly. Verify that devices inside the chassis are properly seated in their slots.
• Update Drivers: Ensure the latest drivers for the devices within the expansion chassis are installed. Also, check for driver updates for your motherboard’s chipset and Thunderbolt controller (if applicable).
• Update BIOS/Firmware: Outdated system BIOS or chassis firmware can sometimes contribute to resource allocation or enumeration problems. Check the manufacturer’s website for updates. A newer BIOS might allocate a larger bus number range or handle PCIe bridge states more robustly.
• Simplify the Setup: If possible, try connecting fewer devices to the chassis to see if the issue is related to the sheer number of devices or a conflict with a specific device.

While these general steps can resolve many hardware detection issues, the registry workarounds detailed above are specifically designed to address the deeper PnP and resource allocation problems that manifest as Code 12 and Code 31 errors with complex PCIe expansion systems.

Conclusion¶

Attaching a PCI Express expansion chassis to a Windows computer can sometimes present challenges, resulting in devices failing to enumerate or start due to resource allocation conflicts or bus number limitations. The Code 12 and Code 31 errors are common indicators of these underlying issues. By understanding the specific causes related to PCIe bridge resource window handling and PCI bus number exhaustion, administrators and users can apply targeted workarounds. The registry modifications involving the HackFlags value in the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP\Pci key provide a method to adjust Windows’s behavior to accommodate these complex hardware configurations. Always remember the importance of backing up your system before making registry changes and proceed with caution.

Have you encountered these issues with PCI Express expansion chassis? Did these registry workarounds help resolve your problem? Share your experiences and any additional tips in the comments section below.