Hi Tobias, On Thu, Jul 09, 2020 at 10:47:54PM +0200, Tobias Waldekranz wrote: > Hi netdev, > > TL;DR: Is something like https://github.com/wkz/mdio-tools a good > idea? > > The kernel does not, as far as I know, have a low-level debug > interface to MDIO devices. I.e. something equivalent to i2c-dev or > spi-dev for example. The closest thing I've found are the > SIOCG/SMIIREG ioctls, but they have several drawbacks: > > 1. "Write register" is not always exactly that. The kernel will try to > be extra helpful and do things behind the scenes if it detects a > write to the reset bit of a PHY for example. > > 2. Only one op per syscall. This means that is impossible to implement > many operations in a safe manner. Something as simple as a > read/mask/write cycle can race against an in-kernel driver. > > 3. Addressing is awkward since you don't address the MDIO bus > directly, rather "the MDIO bus to which this netdev's PHY is > connected". This makes it hard to talk to devices on buses to which > no PHYs are connected, the typical case being Ethernet switches. > > The kernel side of mdio-tools, mdio-netlink, tries to address these > problems by adding a GENL interface with which a user can interact > with an MDIO bus directly. > > The user sends a program that the mdio-netlink module executes, > possibly emitting data back to the user. I.e. it implements a very > simple VM. Read/mask/write operations could be implemented by > dedicated commands, but when you start looking at more advanced things > like reading out the VLAN database of a switch you need to state and > branching. > > FAQ: > > - A VM just for MDIO, that seems ridiculous, no? > > It does. But if you want to support the complex kinds of operations > that I'm looking for, without the kernel module having to be aware > of every kind of MDIO device in the world, I haven't found an easier > way. > > - Why not use BPF? > > That could absolutely be one way forward, but the GENL approach was > easy to build out-of-tree to prove the idea. Its not obvious how it > would work though as BPF programs typically run async on some event > (probe hit, packet received etc.) whereas this is a single execution > on behalf of a user. So to what would the program be attached? The > output path is also not straight forward, but it could be done with > perf events i suppose. > > My question is thus; do you think mdio-netlink, or something like it, > is a candidate for mainline? > > Thank you
I will let the PHY library maintainers comment about implementation design choices made by mdio-netlink. However, I want to add a big "+1" from my side for identifying the correct issues in the existing PHY ioctls and doing something about it. I think the mainline kernel needs this. Please be aware that, if your mdio-netlink module, or something equivalent to it, lands in mainline, QEMU/KVM is going to be one of its users (for virtualizing an MDIO bus). So this is going to be more than just for debugging. And, while we're at it: context switches from a VM to a host are expensive. And the PHY library polls around 5 MDIO registers per PHY every second. It would be nice if your mdio-netlink module had some sort of concept of "poll offload": just do the polling in the kernel side and notify the user space only of a change. Thanks, -Vladimir
