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When troubleshooting PCIe performance issues, one of the first steps is to check the PCIe link speed and lane width to ensure that a device is operating at its full potential. Using lspci, we can examine whether a device is running at the expected speed and width or if it has been downgraded.

📌 In this guide, you’ll learn:
✅ How to check the PCIe link speed and width using lspci
✅ How to interpret the output and identify link downgrades
✅ Common reasons for PCIe speed degradation
✅ How to troubleshoot and fix link speed issues

🛑 1. Checking PCIe Link Speed Using lspci

To check the PCIe link speed and width for a specific device, use:

lspci -vvvs c9:00.0 | grep -i speed

📌 Breaking down the command:

• lspci – Lists all PCI devices.
• -vvv – Displays verbose device details.
• -s c9:00.0 – Specifies the PCI device (bus c9, device 00.0).
• | grep -i speed – Filters the output to show only speed-related information.

🔍 2. Understanding the Output

Example Output:

LnkCap: Port #0, Speed 16GT/s, Width x4, ASPM L0s L1, Exit Latency L0 <512ns, L1 <16us
LnkSta: Speed 8GT/s (downgraded), Width x2 (downgraded)
LnkCtl2: Target Link Speed: 16GT/s, EnterCompliance- SpeedDis-

📌 Breaking down the output:

⚠️ 3. Why Is PCIe Link Speed Downgraded?

If your PCIe device is running at a lower speed or fewer lanes than expected, it could be due to several factors:

🔹 1. PCIe Slot Limitations

• Some motherboards do not support full-speed PCIe on all slots.
• Solution: Check your motherboard manual and install the device in a higher-speed slot.

🔹 2. BIOS/UEFI Settings

• Some BIOS settings limit PCIe speeds to reduce power consumption.
• Solution:
  1. Reboot and enter BIOS/UEFI (DEL, F2, or F12 during boot).
  2. Look for "PCIe Speed Settings" and set it to the highest available speed (e.g., Gen4 or Auto).

🔹 3. PCIe Power Management (ASPM)

• Active State Power Management (ASPM) can reduce link speeds to save power.

Solution: Disable ASPM in Linux:

echo performance | sudo tee /sys/module/pcie_aspm/parameters/policy

🔹 4. Firmware or Driver Issues

• Some devices may require specific firmware updates or drivers to run at full speed.

Solution: Update the kernel and drivers:

sudo apt update && sudo apt upgrade -y

🔹 5. Faulty or Poor-Quality PCIe Cables/Risers

• If using a PCIe riser, a poor-quality riser can cause lane width reductions.
• Solution: Try reconnecting the device or using a higher-quality riser.

🔹 6. Thermal Throttling

• Some PCIe devices reduce speed to prevent overheating.

Solution: Check the device temperature:

sensors

🛠️ 4. Fixing PCIe Link Downgrade Issues

🔹 Step 1: Check Your Motherboard Manual

Ensure the device is installed in a full-speed slot.

🔹 Step 2: Verify PCIe Power Settings

Disable PCIe power-saving mode:

sudo nano /etc/default/grub

Find:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

Change to:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash pcie_aspm=off"

Save the file and update GRUB:

sudo update-grub
sudo reboot

🔹 Step 3: Reset the PCIe Device

Reinitialize the PCIe device without rebooting:

echo 1 | sudo tee /sys/bus/pci/devices/0000:c9:00.0/remove
echo 1 | sudo tee /sys/bus/pci/rescan

🔹 Step 4: Manually Set PCIe Link Speed

If the issue persists, force the link speed:

sudo setpci -s c9:00.0 CAP_EXP+10.B=4

📌 This forces the link speed to Gen4 (16GT/s).

📊 5. Summary

💬 Join the Discussion!

Have you ever faced PCIe performance issues?
What troubleshooting steps did you use to fix them?

💬 Share your experience in the comments below! 🚀

👉 If you're troubleshooting Linux performance, check out: How to Check and Monitor Hard Drive Health Using SMART in Linux