netdev - Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through phy_mii

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Message-ID: <87pmrgjhk4.fsf@waldekranz.com>
Date:   Thu, 04 Nov 2021 12:17:47 +0100
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Grygorii Strashko <grygorii.strashko@...com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org,
        Vignesh Raghavendra <vigneshr@...com>,
        Sean Anderson <sean.anderson@...o.com>
Subject: Re: [RFC PATCH] net: phy/mdio: enable mmd indirect access through
 phy_mii_ioctl()

On Wed, Nov 03, 2021 at 20:36, Andrew Lunn <andrew@...n.ch> wrote:
> On Wed, Nov 03, 2021 at 08:42:07PM +0200, Grygorii Strashko wrote:
>> 
>> 
>> On 03/11/2021 02:27, Andrew Lunn wrote:
>> > > > What i find interesting is that you and the other resent requester are
>> > > > using the same user space tool. If you implement C45 over C22 in that
>> > > > tool, you get your solution, and it will work for older kernels as
>> > > > well. Also, given the diverse implementations of this IOTCL, it
>> > > > probably works for more drivers than just those using phy_mii_ioctl().
>> > > 
>> > > Do you mean change uapi, like
>> > >   add mdio_phy_id_is_c45_over_c22() and
>> > >   flag #define MDIO_PHY_ID_C45_OVER_C22 0x4000?
>> > 
>> > No, i mean user space implements C45 over C22. Make phytool write
>> > MII_MMD_CTRL and MII_MMD_DATA to perform a C45 over C22.
>> 
>> Now I give up - as mentioned there is now way to sync User space vs Kernel
>> MMD transactions and so no way to get trusted results.

Except that there is a way: https://github.com/wkz/mdio-tools

I can see that Sean has already mentioned it in the other branch of the
thread (thanks for the plug :)). I have posted it to netdev before:

https://lore.kernel.org/netdev/C42DZQLTPHM5.2THDSRK84BI3T@wkz-x280/

It allows you to send an entire "MDIO program" to the kernel, where
mdio-netlink will (1) lock the bus, (2) run your program in a small VM,
and (3) unlock the bus.

There are currently two programs in the project:

- `mdio`: A basic register peek/poke program that uses the mdio-netlink
  module in the kernel to do its thing. The source is structured in such
  a way that custom access modes can be easily added. Today there are
  accessors for C22 PHYs, C45 MMDs, Marvell Alaska paged PHYs, Marvell
  LinkStreet switches, and XRS700x switches.

- `mvls`: Specialized read-only debug tool for Marvell LinkStreet
  switches. This does _not_ rely on the mdio-netlink kernel module,
  instead it uses the standard devlink API. Let's you dump the VTU/ATU
  etc.

You could easily add a new addressing mode to `mdio` to do C45-over-C22
accesses. Would that work for you Grygorii?

> Except that it will probably work 99% of the time, which is enough for
> a debug tool.

Why though, why would we not build something that is rock solid? Ever
since ce69e2162f15, the flood gates are open. Any vendor can implement
mdio-netlink on their own, or just download mine. We are only punishing
ourselves at this point.

> phylib is pretty idle most of the time, it just polls
> the PHY once a second to see if the link status has changed. And does
> not poll at all if interrupts are wired up. And you can always do a
> read three times and see if you get the same answer, or do a write
> followed by a read to see if the write actually happened correctly, or
> corrupted some other register.

As a staunch opponent of Vendor SDKs myself, I get where you are coming
from - really.

I guess I just don't have the brain power to operate this kind of Rube
Goldberg machinery and debug my problems at the same time. We have a
mutex right there already, let's use it!

I'll shut up now - I just had to make one final appeal :)