🔶 Chestnut：搭载开源固件的 eGPU 扩展坞
英文标题：🔶 Chestnut – eGPU dock with open-source firmware
comma.ai 推出名为 Chestnut 的 PCIe Gen4 x4 至 USB4 eGPU 扩展坞，其桥接芯片固件以 C 源码形式在 GitHub 开源。产品提供 249 美元的仅扩展坞版本，以及包含 AMD Radeon RX 9060 8GB 的 799 美元“ready to drive”版本。Chestnut 的主要用途是让 comma four 连接桌面 GPU，将自动驾驶模型的可用功耗从约 10W 提升至约 100W，并支持参数量达 10 亿的新驾驶模型。对开发者而言，其意义还在于可审计、修改和重新刷写 ASM2464PD 桥接控制器固件，为长期封闭的 USB4/eGPU 调试与控制层提供了开放入口。
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中文
它的造出来是为了让汽车运行更大的驾驶模型。对其他人来说，它有趣之处在于：你终于可以阅读桥接芯片上的固件了。
George Hotz 的 comma.ai 于周三发布了一款产品，这是自 Thunderbolt 3 让 eGPU 成为现实以来外接显卡用户一直期待的东西：一款你确实可以阅读其固件的扩展坞。comma.ai 的 eGPU 扩展坞名为 chestnut，它是一款 PCIe Gen4 x4 至 USB4 桥接设备，驱动其控制器的 C 语言源码已在 tinygrad 组织的 GitHub 上公开。
产品有两个版本。Tiny chestnut 售价 249 美元，仅提供扩展坞，显卡需自行准备。Chestnut“ready to drive”版本售价 799 美元，随附一张 AMD Radeon RX 9060 8GB 显卡，以及线缆和安装硬件。根据 comma 的公告文章，两个版本均提供 30 天无理由退款试用和一年硬件保修。
它诞生的原因与 PC 游戏无关。Comma 花了十年时间，试图将 openpilot 的驾驶模型压缩进约 10W 的车端功耗预算内，而这一上限终于成为瓶颈。Chestnut 通过为 comma four 外接一块真正的桌面 GPU，将功耗上限提高到约 100W。该公司认为，这一组合的计算能力可达到与 Tesla HW4 相近的级别。
这些额外的性能空间有明确用途。Comma 将在 openpilot 0.11.2 中同时发布一款参数量为 10 亿的驾驶模型；按该公司自己的数据，这款模型的参数量是最新车端模型的 30 倍，FLOPs 是其 100 倍，也是该公司所称“chestnut 级”模型的首款产品。该公司还表示，其车队现在每天记录接近 100 万分钟的驾驶数据，其中 15% 的行程会被纳入训练集群。
为什么一款汽车扩展坞对 PC 装机用户很重要
抛开汽车应用背景来看，这是一款 comma 明确表示可与普通计算机配合使用的 USB4 扩展坞。这正是值得关注的部分。eGPU 机箱一直都是封闭盒子：你购买机箱、信任厂商写入桥接芯片的任何固件；当它在 40 Gbps 下表现异常时，你唯一的解决办法就是一个固件二进制包和论坛帖子。
Chestnut 打破了这种封闭状态。其 代码仓库 包含 ASM2464PD 的 C 语言固件；ASM2464PD 是 ASMedia 的 USB4/Thunderbolt 至 NVMe 桥接控制器，在许多 USB4 NVMe 硬盘盒中都能见到。README 对已实现功能及其速度描述得直截了当：一条可读取和写入 PCIe TLP 的控制消息，写入速度可达 3.6 MB/s、读取速度可达 1.8 MB/s，虽然很慢，但它可以访问 PCIe BAR 中的任何位置。另一条进入该芯片 512 KB SRAM 的 DMA 路径，在 10 Gbps USB3 条件下可达到约 700 MB/s。
这些并不是游戏吞吐量数据。它们属于调试和控制平面，而这恰恰是过去人们从未能够在零售扩展坞上触及的层级。
有两点注意事项需要牢记。通过 USB4 传输的 Gen4 x4，相对于 RX 9060 原本设计使用的 x16 插槽而言仍是一条狭窄通道，因此这并不是一条获得完整桌面性能的隐秘捷径。此外，开源固件仓库并不等同于一款获得完善支持的产品：comma 将其销售给希望运行更大驾驶模型的人，而不是希望得到更好 Razer Core 的 Linux 用户。Phoronix 指出，它也兼容 tinygrad 工作负载；考虑到是谁编写了固件，这并不令人意外。
尽管如此，作为一款售价 249 美元、可供审计和重新刷写的扩展坞，它比其价格标签所暗示的更有意思。至于把显卡安装在副驾驶脚部空间内，仍然是可选项。
English / 原文
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.