netdev - Re: MDIO Debug Interface

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Date:   Thu, 9 Jul 2020 15:36:19 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Tobias Waldekranz <tobias@...dekranz.com>, netdev@...r.kernel.org
Cc:     andrew@...n.ch, hkallweit1@...il.com,
        Vladimir Oltean <olteanv@...il.com>
Subject: Re: MDIO Debug Interface



On 7/9/2020 1:47 PM, 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?

Certainly, the current interface clearly has deficiencies and since
mdio_device instances were introduced, we should have an interface to
debug those from user-space ala i2c-dev or spidev.

Can you post the kernel code for review? Would you entertain having mdio
as an user-space command being part of ethtool for instance (just to
ease the distribution)?
-- 
Florian