Built so a car can run a bigger driving model. Interesting to the rest of us because you can finally read the firmware on the bridge chip.
George Hotz’s comma.ai shipped something on Wednesday that the external-graphics crowd has been asking for since Thunderbolt 3 made eGPUs a thing: a dock whose firmware you can actually read. The comma.ai eGPU dock is called chestnut, it is a PCIe Gen4 x4 to USB4 bridge, and the C source driving its controller is sitting in the open on GitHub under the tinygrad organisation.
There are two versions. Tiny chestnut is $249 and it is the dock alone, bring your own card. Chestnut “ready to drive” is $799 and arrives with an AMD Radeon RX 9060 8GB plus the cabling and mounting hardware. Both carry a 30-day money-back trial and a one-year hardware warranty, per comma’s announcement post.
The reason it exists has nothing to do with PC gaming. Comma has spent a decade squeezing openpilot’s driving models into roughly a 10 W on-device power budget, and that ceiling finally became the bottleneck. Chestnut raises it to about 100 W by hanging a real desktop GPU off a comma four. The company reckons the combination lands in the same compute class as Tesla’s HW4.
That headroom is going somewhere specific. Comma is launching a 1B-parameter driving model alongside it in openpilot 0.11.2, which by its own numbers has 30x the parameters and 100x the FLOPs of the latest on-device model, and is the first of what it calls chestnut-class models. The company also says its fleet now logs close to a million driving minutes a day, with 15% of those drives pulled into the training cluster.
Why a dock for a car matters to PC builders
Strip away the automotive framing and you have a USB4 dock that comma explicitly says works with an ordinary computer. That is the part worth paying attention to. eGPU enclosures have always been closed boxes: you buy the chassis, you trust whatever the vendor flashed onto the bridge silicon, and when it misbehaves at 40 Gbps your only recourse is a firmware blob and a forum thread.
Chestnut breaks that open. The repository holds a C firmware for the ASM2464PD, the ASMedia USB4/Thunderbolt-to-NVMe bridge controller that turns up in a lot of USB4 NVMe enclosures. The README is refreshingly blunt about what works and how quickly: a control message that reads and writes PCIe TLPs manages 3.6 MB/s writing and 1.8 MB/s reading, which is slow, but it can touch anywhere on the PCIe BAR. A separate DMA path into the chip’s 512 KB of SRAM hits around 700 MB/s over 10 Gbps USB3.
Those are not throughput figures for gaming. They are the debug and control plane, and that is exactly the layer nobody has ever been able to poke at on a retail enclosure.
Two caveats worth keeping in view. Gen4 x4 over USB4 is still a narrow pipe next to the x16 slot an RX 9060 was designed for, so this is not a stealth route to full desktop performance. And an open firmware repo is not the same thing as a supported product: comma is selling this to people who want to run bigger driving models, not to Linux users after a better Razer Core. Phoronix notes it is also compatible with tinygrad workloads, which is unsurprising given who wrote the firmware.
Still, at $249 for a dock you can audit and reflash, it is a more interesting object than the price tag suggests. Mounting your graphics card in the passenger footwell remains optional.