Ethereal Wake

With Fast Ethernet, Ethernet introduced something new. Previously, the Physical Signaling layer was integrated into the MAC and connected to the actual media through a Media Attachment Unit. While the user could switch between twisted pair (10Base-T), thinnet (10Base2), thicknet (10Base5), or even fiber (10Base-F) simply by changing MAUs, switching to a different encoding (e.g. Fast Ethernet) would require a completely new interface.

Instead of requiring new networking equipment to manage the PCS of each individual protocol, the MAC communicates with the PHY with a Media Independent Interface (MII). Now, free of protocol-specific encodings, different line rates and protocols could be selected by simply switching between different PHYs, leaving the MAC unaffected. While interchangeable PHYs is now the domain of high-end networking (e.g. SFP modules), the MII interface and its derivatives are the primary mechanism for connecting integrated MACs (and FPGAs) to commodity Ethernet transceivers.

Much of my recent professional development has focused on Ethernet, making it a convenient target for technical essays. Unlike much of the Internet, this series of essays will focus on practical implementation of Ethernet from an FPGA or ASIC perspective. This means the necessary waveforms and encodings to generate ethernet packets when directly connected to a PHY or medium.

The essays will be making extensive references to IEEE 802.3-2022 and every effort will be made to specify the exact clauses for further research by the reader. I will not be using the amendment names (e.g. 802.3z for Gigabit Ethernet) because they are not not useful for finding content within the actual standard and any given clause may have been modified by multiple amendments.

For many industrial embedded projects, embedding debugging support (e.g. JTAG) into the design can be immensely useful. This can be for the convenience of integrated debugging or specialized needs, such as galvanic isolation. The FTDI family of High-Speed USB-Serial transceivers are commonly used for this purpose. FTDI’s Multi-Protocol Synchronous Serial Engine (MPSSE) can offload the JTAG state machine and achieve clock speeds as high as 30 MHz. Software like OpenOCD provides a solid basis for manipulating microcontrollers but the situation is very different when it comes to FPGAs.

When writing a freestanding application, it’s generally necessary for the firmware engineer to handle runtime initialization. Even when a library like newlib includes a rudimentary implementation of crt0.o, initialization is a very application-specific process owing to the need to initialize hardware, memory, and other loading tasks.

In this essay, we examine the current and historical implementation of executable initialization, finishing with a minimal implementation usable with firmware applications.

Note: Most firmware applications need to address the initialization of .data and .bss from nonvolatile memory. That is not addressed in this essay.

While self-hosting Git isn’t that hard (all you need is a shell accessible through SSH), some tools make it easier. One of them is Gitea, a nice, self-contained Go binary that provides you a GitHub clone without all the complexity and dependency hell of something like GitLab. One of the things it does not provide is an integrated continuous integration/delivery (CI/CD) platform. Instead, it implements the same basic patterns at GitHub allowing for pairing with a range of third-party services, cloud or hosted.

Bazel is one of the least terrible build systems out there. It can handle large codebases, mixed languages, and cross-platform builds like a champ. Unfortunately, it suffers from rather poor documentation with an enterprise Java codebase that is a nightmare to decipher.

One of the features I’ve been trying to make use of are linkstamps. The idea behind linkstamps is to embed information such as the Git commit identifier into the resulting binary, providing direct traceability for deployed binaries. Unfortunately, regarding this feature, Bazel suffers from the common documentation anti-pattern where they describe what an option is, not what it does.

When using FreeBSD, the most common method for virtualization and process isolation are jails. Introduced with FreeBSD 4.0 in March of 2000, they predate the closest Linux equivalent, cgroups (and, by extension, Docker), by nearly a decade.

A core part of any virtualization technology is its interaction with the networking infrastructure. In this regard, I’ve found much of the available documentation lacking, often deferring to third party tools which are no longer maintained. As such, I’ve had to scrape multiple sources and reverse engineer system programs to figure out how it’s put together.

In the modern world, people tend to use massive frameworks to accomplish simple tasks. Nothing quite like swatting a fly with a nuclear missile when you load up 100 megs of runtime just to execute ping and post the result to a database. But sometimes, when you’re writing a utility, you want it to be quick and lightweight. And if you’re going through the effort, you might as well see about making it portable.

Inline functions are a notable feature of both C and C++. By exposing the source file to the implementation of a function, they allow a variety of optimization techniques that wouldn’t be possible if it had to call out to a subroutine in a different file (at least without link-time optimization).

However, despite the common syntax, the C and C++ languages implement them in very different ways. While C++ takes a “user friendly” approach and automatically manages the manipulation of multiple implementations, C requires a more manual approach. As a result, inline functions are less common in C and mixed language code, generally using the nuclear option of declaring them static.

On my home server, I use FreeBSD. While FreeBSD beat Linux to the containers by nearly a decade (comparing jails to cgroups), I have to acknowledge that cgroups are the superior design. Whereas jails are a bunch of hacks piled on top of chroot, cgroups are a much cleaner abstraction of the kernel’s namespaces. But even beyond the elegance of the design, software like Docker makes it much easier to run your tools in containers, even if the offloading of sysadmin responsibilities it encourages triggers my OCD.