A UEFI app that sends LLDP-MED pkt at boot to negotiate PoE+ power before the OS

Summary

Back in 2015, I was working on a project to build PoE-powered embedded x86 computers and digital signage systems. These were full Windows 10 Professional machines running Intel Atom processors, designed to simplify deployment by drawing power directly over Ethernet. Our goal was to eliminate the need to run traditional AC power to these devices, which can be costly and impractical in many deployment scenarios. But unlike typical IoT or low-power devices, these were full-fledged x86 computers that required more power than what the standard PoE (802.3af) could deliver, which maxes out at 15.4W at the PSE (Power Sourcing Equipment), such as a PoE network switch or injector. Our device required about 23W when fully operational, which pushed us into 802.3at (PoE+) territory. In most client environments their PoE+ switches provided the power we needed with no problem. But some environments had network switches that would not give us the additional power. PoE Standards Overview (IEEE 802.3) Standard Max Power at PSE Max Power at PD Voltage Range Pairs Used Year 802.3af (PoE) 15.4 W 12.95 W 44–57 V DC 2 pairs 2003 802.3at (PoE+) 30 W 25.5 W 50–57 V DC 2 pairs 2009 The problem was that our embedded systems only supported physical‑layer classification which is limited to signaling power requirements through resistive detection and pulsed current signatures during initial PoE handshaking. Only relying on this method can be problematic if the switch is configured to require LLDP for Data Link Layer Classification for devices requiring more than 15.4W. Which is a problem because at minimum our computers required at least 18W in order to boot into the operating system. So our systems would initially start to boot, but then eventually shut off before it got into Windows. We were stuck in a frustrating Catch-22, we needed to send LLDP packets to get more power, but we couldn’t even boot the OS to send them. So What Do You Do When the OS Can't Help? We did some testing and measured ...