Comma.ai's Chestnut eGPU Dock: Open-Source Firmware for $249

Comma.ai, the company behind openpilot, has shipped an eGPU dock that breaks from the closed-box norm. The Chestnut dock is a PCIe Gen4 x4 to USB4 bridge, and its controller firmware is open source. This is a first for the eGPU market, where vendors typically ship binary blobs and offer little transparency.

Two versions are available: the bare dock for $249, and a "ready to drive" bundle for $799 that includes an AMD Radeon RX 9060 8GB, cabling, and mounting hardware. Both come with a 30-day money-back guarantee and a one-year warranty.

The dock's purpose is not gaming. Comma has spent a decade fitting driving models into a ~10 W power budget, and that ceiling has become a bottleneck. Chestnut raises it to ~100 W by connecting a desktop GPU to a comma four device. The company claims this lands in the same compute class as Tesla's HW4.

Alongside the dock, comma is releasing a 1B-parameter driving model in openpilot 0.11.2, which it says has 30x the parameters and 100x the FLOPs of the latest on-device model. The fleet now logs nearly a million driving minutes per day, with 15% of those drives used for training.

Why this matters beyond cars

Strip away the automotive angle, and you have a USB4 dock that works with any computer. eGPU enclosures have always been closed: you trust whatever firmware the vendor flashed, and when things break at 40 Gbps, you're stuck with a blob and a forum thread.

Chestnut changes that. The firmware repository contains C source for the ASM2464PD, an ASMedia USB4/Thunderbolt-to-NVMe bridge controller used in many USB4 NVMe enclosures. The README is candid about performance: a control message path for reading/writing PCIe TLPs manages 3.6 MB/s writes and 1.8 MB/s reads—slow, but it can access any PCIe BAR. A separate DMA path into the chip's 512 KB SRAM hits ~700 MB/s over 10 Gbps USB3.

These aren't gaming throughput numbers; they're the debug and control plane, a layer that has been inaccessible on retail enclosures.

Caveats

Gen4 x4 over USB4 is still a narrow pipe compared to the x16 slot an RX 9060 expects, so this isn't a path to full desktop performance. Also, an open firmware repo doesn't equal a supported product—comma is selling to people who want bigger driving models, not Linux users seeking a better Razer Core. Phoronix notes it's compatible with tinygrad workloads, which is unsurprising given the firmware's origin.

At $249 for an auditable, reflashable dock, it's a more interesting object than the price suggests. Mounting your GPU in the passenger footwell remains optional.