Negotiating PoE+ Power in the Pre‑Boot Environment

Related problem is single-board computers that relies on USB-PD for power. USB-PD sources requires the sink to do power delivery negotiation within 5 seconds, or it will cut its power or do funny things. Because USB-PD negotiation is handled in Linux, by the time Linux boots it will be too late, and power supply will cut the power, so it will be stuck in a boot loop: <a href="https:&#x2F;&#x2F;www.spinics.net&#x2F;lists&#x2F;linux-usb&#x2F;msg239175.html" rel="nofollow">https:&#x2F;&#x2F;www.spinics.net&#x2F;lists&#x2F;linux-usb&#x2F;msg239175.html</a><p>They way they&#x27;re trying to solve it is very similar to this article, by doing the USB-PD negotiation during U-boot bootloader stage:<p>- <a href="https:&#x2F;&#x2F;gitlab.collabora.com&#x2F;hardware-enablement&#x2F;rockchip-3588&#x2F;linux&#x2F;-&#x2F;issues&#x2F;8" rel="nofollow">https:&#x2F;&#x2F;gitlab.collabora.com&#x2F;hardware-enablement&#x2F;rockchip-35...</a><p>- <a href="https:&#x2F;&#x2F;lore.kernel.org&#x2F;u-boot&#x2F;20241015152719.88678-1-sebastian.reichel@collabora.com&#x2F;" rel="nofollow">https:&#x2F;&#x2F;lore.kernel.org&#x2F;u-boot&#x2F;20241015152719.88678-1-sebast...</a>

&gt; <i>PoE Standards Overview (IEEE 802.3)</i><p>For the record, 802.3bt was released in 2022:<p>* <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Power_over_Ethernet" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Power_over_Ethernet</a><p>It allows for up to 71W at the far end of the connection.

<i>running Intel Atom processors[...]these were full-fledged x86 computers that required more power than what the standard PoE (802.3af) could deliver</i><p>Those must&#x27;ve been the server Atoms or the later models that aren&#x27;t actually all that low-power, as the ones I&#x27;m familiar with are well under 10W.

Wouldn&#x27;t it be funny to probe peripherals to decide if extra power is demanded or not, then request it all inside UEFI

Interesting. If it were me, i would try to boot the OS at lower CPU clock and maybe I can get away with it. That approach would be less than ideal than author&#x27;s.

Solving PoE+ power negotiation before the OS boots is next-level. This is good and a clever workaround.

&gt; Back in 2015, I was working on a project to build PoE-powered embedded x86 computers and digital signage systems.<p>&gt; Our device required about 23W when fully operational, which pushed us into 802.3at (PoE+) territory<p>The problem the author solved is quite interesting. But I can’t help but think how wasteful it is to load up a full copy of windows just to serve up dumb advertisements.<p>The attack surface of a fully copy of windows 10 is an attackers wet dream.<p>Hope most of these installations are put out to pasture and replaced with very low power solutions.

&gt;the switch is configured to require LLDP for Data Link Layer Classification for devices requiring more than 15.4W<p>This really feels like a switch configuration problem. A compliant PoE PD circuit indicates its power class and shouldn&#x27;t need to bootstrap power delivery. If the PD is compliant and components selected correctly then the PSE is either non-compliant or configured incorrectly.

I know USB PD has trigger chips that will request the power levels that require active negotiation for you; are there not equivalents for PoE?

&gt; I dug deeper and came across the concept of UEFI applications.<p>TIL.

This is awesome. I probably would have used passive poe, which is the defacto workaround everyone seems to use. Good to see someone actually tackle the issue instead of working around it.