MDIO-NETLINK(9)

💅 mdio-netlink — the netlink hookup to MDIO buses (section 9 of the kernel manual, bestie)

okay so picture this: you're in the kernel manual, section 9, and the page is literally called mdio-netlink. that's the whole vibe. let's get into it 💖

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✨ SYNOPSIS ✨

#include <linux/mdio-netlink.h>

that's it. that's the include. slay 🎀

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🦋 DESCRIPTION 🦋

okay so mdio-netlink gives you a direct interface to talk to devices sitting on MDIO buses. like, no middleman, no cap.

but here's the iconic part — the way it stays race-free while running a bunch of reads/writes as one atomic transaction is that your input is a program in byte code form that gets executed inside a little virtual machine living inside mdio-netlink. yes, a whole VM. it's giving main-character energy and honestly it's similar in spirit to how "channel programs" work on some mainframes. iykyk 🧚

the VM is lowkey minimal — no stack or heap memory, just:

• 8 general purpose registers, each 16-bit
• an implicit program counter (she's just there, doing her thing)

and it serves up this small set of instructions:

• READ — read from an MDIO/XMDIO device to a register 💫
• WRITE — write a register or an immediate value to an MDIO/XMDIO device
• EMIT — emit the contents of a register or an immediate value as an output of the program
• AND — store the bitwise AND of two operands to a register
• OR — store the bitwise OR of two operands to a register
• ADD — store the sum of two operands to a register
• JEQ — add an immediate value to the program counter if two operands are equal
• JNE — add an immediate value to the program counter if two operands are not equal

that's the whole opcode menu. short, sweet, obsessed 🌸

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👑 HISTORY 👑

so why does this exist?? because it improves on the traditional MDIO interface that us userspace programs get in Linux, in a few important ways. let me count them 💅

• MDIO buses are directly addressable. previous solutions relied on at least one Ethernet PHY on the bus being attached to a net device — which is typically not the case when the device is an Ethernet switch, for example. so yeah, that old assumption was not it 🙈
• Complex operations can be performed atomically. the old ioctl(2) interface only supported a single read or write of a single register. as a result, things like read/mask/write operations and accesses to paged PHYs could not be performed safely. huge L, honestly. now they can. periodt 🔥

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🌈 SEE ALSO 🌈

• mdio(8) — go say hi 🫶

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🍓 AUTHORS 🍓

• Tobias Waldekranz <tobias@waldekranz.com> — icon, legend, the moment ⭐

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and that's the page, bestie!! Linux, dated September 17, 2021 — mwah, catch you in the next manpage 💜🎀