This is the optional troubleshooting appendix to my first Tesla P40 post.

If you only care about the build that eventually worked, you can skip this one. The short version is: I could not get my Dell Precision T7820 to reliably initialize a Tesla P40 as a usable CUDA device, so I eventually stopped fighting it and moved to an AM4 consumer platform instead.

But if you are here because you searched for something like:

  • Tesla P40 detected by lspci but not nvidia-smi
  • P40 fallen off the bus
  • NVIDIA driver cannot initialize P40
  • Dell T7820 P40 Above 4G
  • P40 detected but CUDA not working
  • P40 BAR allocation failure

…then welcome. I feel your pain. This post is for you.

This was the weird middle ground where the GPU was not simply invisible, but it also was not usable. Linux could see the card on the PCIe bus. The system knew a Tesla P40 was installed. But the NVIDIA driver could not reliably bring it online.

The Hardware

My original plan was to build a cheap local AI inference server around used NVIDIA Tesla P40s. I bought two P40s for about $250 each. Each card has 24GB of VRAM, so the appeal was obvious: lots of CUDA-compatible VRAM for not much money.

The first host system was a Dell Precision T7820 workstation:

  • Dell Precision T7820
  • Xeon Silver 4112
  • 32GB DDR4
  • 500GB SSD
  • 950W PSU
  • AMD Radeon Pro WX3100 display GPU
  • NVIDIA Tesla P40 compute GPU
  • Goal: eventually run two P40s

On paper, this seemed like a reasonable platform. The T7820 is a real workstation chassis. It has a large power supply. It has room for full-length cards. Dell documented support for high-power GPUs. It already had a small display GPU, which mattered because the P40 has no display outputs.

This was not supposed to be the hard part.

The First Symptom

The T7820 booted fine without the P40 installed.

When I installed the P40, things got strange quickly.

The machine would power on, but display behavior became unreliable. In some configurations, I could still get video from the WX3100. In others, I could not. The workstation seemed confused about which GPU was supposed to be the display device, even though the P40 has no display outputs and should have been treated as a compute card.

Eventually, I got far enough to boot Linux with the P40 installed.

That is when the real troubleshooting started.

Linux Could See the Card

The first good sign was that lspci could see the P40.

The card showed up as an NVIDIA GP102GL device, which is what I expected for a Tesla P40.

lspci | grep -i nvidia

showed the card on the PCIe bus.

A more detailed inspection:

lspci -nn -s <bus-id>

identified it correctly as:

NVIDIA Corporation GP102GL [Tesla P40] [10de:1b38]

That was encouraging, but it also created a trap. It is easy to see that output and think, “Great, the system detects the GPU, so this is just a driver problem.”

A PCIe device can enumerate successfully and still fail later when the driver tries to initialize it, map its resources, enable bus mastering, or bring up the card’s firmware and runtime state.

That is exactly the kind of failure I appeared to be dealing with.

nvidia-smi Did Not Work

The real test was:

nvidia-smi

If that showed a Tesla P40, that would mean it initialized it as a valid CUDA device. But that failed.

Depending on the OS, driver version, BIOS settings, and boot flags I had tried at that particular moment, the exact failure varied. But the broad pattern was consistent:

  • Linux could see the P40.
  • The NVIDIA driver could not reliably initialize it.
  • nvidia-smi could not show a healthy CUDA device.
  • In some cases, the card appeared to drop off the PCIe bus when the NVIDIA driver tried to touch it.

That last part was the most frustrating, because it felt so close to working.

The Nouveau Problem

One of the first things to check on Linux is whether the open-source nouveau driver has bound to the card.

lspci -k -s <bus-id>

For NVIDIA CUDA workloads, I wanted to see the proprietary NVIDIA driver bound to the device, not nouveau.

I checked whether nouveau was loaded:

lsmod | grep nouveau

And I used the usual blacklist approach:

sudo nano /etc/modprobe.d/blacklist-nouveau.conf

With contents like:

blacklist nouveau
options nouveau modeset=0

Then:

sudo update-initramfs -u
sudo reboot

This is standard NVIDIA-on-Linux hygiene. It is worth doing, but in my case it did not solve the problem.

The Driver Version Tour

I tried a lot of NVIDIA driver versions.

This was partly because the P40 is old enough that driver support matters, and partly because Ubuntu’s recommendations varied depending on which Ubuntu release I was testing.

I tried branches including:

  • 580
  • 570
  • 535
  • 470
  • older branches during troubleshooting

The 580 branch was especially relevant because NVIDIA has indicated that the 580 series is the last support branch for Pascal-era professional cards.

The usual install flow looked roughly like:

ubuntu-drivers devices

Then either:

sudo ubuntu-drivers install

or a specific branch:

sudo apt install nvidia-driver-580

Followed by:

sudo reboot
nvidia-smi

I repeated this basic loop more times than I want to admit.

Driver changes did change the symptoms in some cases, but they did not produce a working P40.

Ubuntu Version Hopping

I also tried multiple operating system versions:

  • Ubuntu Desktop 26.04
  • Ubuntu Server 24.04
  • Ubuntu 22.04
  • Windows 11

I started with Ubuntu 26.04 because it was current and I wanted a clean Linux baseline. But because the P40 is older, and because NVIDIA driver behavior can be sensitive to kernel and distribution versions, I worked backward.

Ubuntu 24.04 was a sensible next stop, and Ubuntu 22.04 was the conservative choice.

The fact that I could not get stable behavior across these versions pushed me further away from “this is a software issue” and toward “this platform/card combination is not initializing correctly.”

Above 4G Decoding and BAR Allocation

The Tesla P40 has a large BAR request.

BAR stands for Base Address Register. In PCIe terms, a device uses BARs to request address space from the system. The system firmware and OS need to allocate address ranges for the device’s registers and memory windows.

This is where BIOS settings like Dell’s “Memory Map I/O Above 4G” become important.

Without Above 4G decoding, the system may try to allocate PCIe device resources below the 4GB address boundary. That can be a problem with large GPUs and multiple PCIe devices.

In the Dell BIOS, I enabled the relevant setting:

Memory Map I/O Above 4G: Enabled

That should have helped, but it did not fix the problem.

The P40 still appeared to have resource allocation trouble. The system appeared unable to allocate or use the resources in a way that allowed the NVIDIA driver to initialize the card.

This was one of the biggest signs that I might be dealing with a platform/firmware problem rather than a simple software issue.

PCIe Slot Experiments

I also experimented with physical slot placement.

The T7820 slot layout is not as flexible in practice as “big workstation with lots of slots” makes it sound. Between the WX3100 display GPU, the full-length P40, slot spacing, and which slots were connected in which configuration, there were only so many reasonable combinations.

I tried the P40 in the available high-power/full-length slots and moved the WX3100 around where possible.

Some configurations would not give me display output. Some configurations detected the card but still failed to initialize it. Some were physically impractical.

I tried changing PCIe generation settings in the BIOS.

Options included forcing different PCIe speeds:

  • Auto
  • Gen 3
  • Gen 2
  • Gen 1

If Gen 3 was unstable for some reason, perhaps forcing Gen 2 or even Gen 1 would allow the card to initialize.

I also disabled settings that could plausibly interfere with PCIe stability or resource allocation:

  • ASPM disabled
  • Thunderbolt disabled
  • Secure Boot disabled
  • Legacy boot disabled/off

None of this got the P40 to a healthy nvidia-smi state.

Kernel Boot Flags

I spent a lot of time with Linux kernel boot parameters.

The goal was to try to get Linux to override the BIOS’ PCIe resource allocation and firmware-provided PCIe information.

The flags I tried included combinations of:

pci=realloc
pci=nocrs
pci=nommconf

I also tried related variants during troubleshooting.

The most relevant ones were:

pci=realloc

This tells Linux to try reallocating PCI resources. The hope was that if the firmware did a poor job assigning PCIe resources, the kernel could clean things up.

pci=nocrs

This tells Linux to ignore the ACPI _CRS resource settings from firmware for PCI host bridges. This can sometimes help when firmware reports bad or overly restrictive PCI resource windows.

pci=nommconf

This disables PCIe MMCONFIG access. In my case, this was relevant because the system showed signs of an ECAM/MMCONFIG-related resource conflict.

To test these, I edited GRUB:

sudo nano /etc/default/grub

And modified:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash ..."

Then:

sudo update-grub
sudo reboot

After each boot, I checked:

cat /proc/cmdline
lspci
lspci -vv -s <bus-id>
dmesg | grep -i -E "nvidia|pci|bar|mmio|resource|fallen"
nvidia-smi

Some flags changed the failure mode, and they did lead to successful BAR request allocation, which was progress. However, none made the T7820 a working P40 host.

The most suspicious clue was an apparent ECAM/MMCONFIG resource conflict around the PCIe configuration space. I saw indications involving an ECAM region around:

0x80000000-0x8fffffff

That made pci=nommconf worth trying, but it still did not produce a stable CUDA device.

Looking at PCIe State

One of the more useful commands during this process was:

lspci -vv -s <bus-id>

I looked for things like:

Region 0:
Region 1:
Region 3:
Memory at ...
I/O ports at ...
BusMaster
Memory+
LnkCap
LnkSta

The exact output varied, but the broad thing I cared about was whether the card had memory regions assigned and whether it looked enabled as a PCIe device. The card was present but did not look fully enabled.

Power Delivery Rabbit Hole

Because the P40 is a 250W datacenter card with an EPS-style power connector, power delivery was a major suspect.

This is also an area where you need to be careful. PCIe 8-pin and EPS 8-pin connectors are not interchangeable, even though they look similar. They have different pinouts. The P40 expects EPS-style power. You cannot just jam a normal PCIe GPU power cable into it and assume everything is fine.

My setup involved adapters that converted:

2x PCIe 8-pin -> 1x EPS 8-pin

Each P40 should be fed by two PCIe GPU power connectors adapted into one EPS connector for the card.

The Dell made this harder because the T7820 PSU does not behave like a normal modular ATX PSU. Dell distributes power through proprietary motherboard and power distribution connections. I needed Dell-specific cabling to get from the system’s proprietary connectors to the PCIe power connectors needed by the adapter.

I also checked continuity and the cable wiring. I tried to verify that the card was receiving power correctly.

I also tried an external PSU test, powering the P40 separately while the Dell hosted it on the PCIe slot.

None of this fixed the issue. This did not absolutely prove that power wasn’t the issue, but it made that a less likely culprit.

The External Windows Test

At one point, the working theory became: maybe the P40s are just bad, so I tested the cards one at a time in my Windows 11 gaming machine.

That machine was AMD AM4-based with a more normal consumer motherboard and power setup. The result was important: the cards could be managed by NVIDIA drivers in that system.

That strongly suggested that I did not simply have two dead P40s. It became much more likely that the Dell platform was probably the problem.

The VBIOS / NVFlash Rabbit Hole

I also poked at the VBIOS of the GPUs. Some used datacenter cards have odd firmware histories.

I tried nvflash to read the firmware and see what version they had. This proved extremely difficult.

One of the notable failures was an error along the lines of:

Falcon in HALT or STOP state

I also tried accessing the ROM through sysfs, but ran into permission/availability issues there as well. I eventually was able to read the firmware from my gaming machine, and determined that they had official, vanilla firmware.

The Error Pattern

After all of this, the pattern looked like this:

  1. The T7820 worked normally without the P40.
  2. The P40 was physically installed and powered.
  3. Linux could enumerate the P40 on the PCIe bus.
  4. The device ID matched a Tesla P40 / GP102GL.
  5. BIOS settings like Above 4G were enabled.
  6. Different Ubuntu versions did not solve it.
  7. Different NVIDIA drivers did not solve it.
  8. Nouveau blacklisting did not solve it.
  9. Kernel PCIe flags did not solve it.
  10. Power troubleshooting did not solve it.
  11. The P40s worked in a separate AM4 system.
  12. The T7820 still could not reliably initialize the card as a CUDA device.

What I Think Was Happening

My best read is that the T7820 had a PCIe resource allocation or platform firmware problem with this P40 configuration. The card could be discovered on the bus, but the system could not provide the resource mapping and initialization environment the NVIDIA driver needed.

The frustrating thing is that none of this troubleshooting produced one clean smoking gun. It was more like a pile of individually plausible issues all pointing in the same direction: this was not a good host for my P40 build.

Why I Stopped

At some point, troubleshooting has an opportunity cost. I was not trying to prove I could do this with the Dell Precision T7820, I was trying to build a working local AI inference server.

The T7820 had already consumed a long weekend of reboots, driver installs, BIOS changes, power checks, kernel flags, OS reinstalls, and increasingly cursed forum-search energy.

The P40s worked in my AM4 gaming machine.

Why am I torturing myself?

Instead of continuing to fight a proprietary workstation platform, I decided to pivot and build around commodity AM4 hardware: a normal consumer motherboard, normal ATX power supply, normal BIOS behavior, and no Dell-specific weirdness.

That felt backwards at first. The whole reason I bought the T7820 was because it seemed more appropriate for enterprise GPU hardware. But in practice, the boring consumer platform gave me more control.

Takeaways

The biggest lesson is this:

lspci is not enough. A GPU can be visible on the PCIe bus and still be unusable. For CUDA workloads, the real test is whether the NVIDIA driver can initialize the card and whether nvidia-smi can see it as a healthy device.

Other lessons:

  • Above 4G Decoding matters, but enabling it does not guarantee success.
  • OEM workstation GPU support is not the same as generic datacenter GPU support.
  • PCIe slot layout matters.
  • BIOS behavior matters.
  • Power cabling matters.
  • Passive datacenter cards bring server assumptions into non-server cases.

The Dell Precision T7820 was not a crazy idea. It looked good on paper. It had the right kind of chassis, the right PSU class, and the right workstation pedigree. But for my P40 build, it was a dead end.

In the next main postYYY, I’ll cover the build that finally worked: moving to an ASUS B550-E motherboard, Ryzen 5600X, normal ATX power, custom P40 cooling, Ubuntu, NVIDIA driver 580, and the deeply satisfying moment when nvidia-smi finally showed a healthy Tesla P40.