linux-kernel - Re: [PATCH 1/1] net: phy: marvell: avoid bringing down fibre link when autoneg is bypassed

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <Zv_wv67TGIUz5IZy@shell.armlinux.org.uk>
Date: Fri, 4 Oct 2024 14:42:23 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Andrew Lunn <andrew@...n.ch>
Cc: Qingtao Cao <qingtao.cao.au@...il.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] net: phy: marvell: avoid bringing down fibre link
 when autoneg is bypassed

On Fri, Oct 04, 2024 at 03:26:33PM +0200, Andrew Lunn wrote:
> On Fri, Oct 04, 2024 at 11:35:30AM +1000, Qingtao Cao wrote:
> > Hi Andrew,
> > 
> > Please see my inline replies.
> > 
> > On Fri, Oct 4, 2024 at 12:30 AM Andrew Lunn <andrew@...n.ch> wrote:
> > 
> >     On Thu, Oct 03, 2024 at 12:25:12PM +1000, Qingtao Cao wrote:
> >     > On 88E151x the SGMII autoneg bypass mode defaults to be enabled. When it
> >     is
> >     > activated, the device assumes a link-up status with existing
> >     configuration
> >     > in BMCR, avoid bringing down the fibre link in this case
> >     >
> >     > Test case:
> >     > 1. Two 88E151x connected with SFP, both enable autoneg, link is up with
> >     speed
> >     >    1000M
> >     > 2. Disable autoneg on one device and explicitly set its speed to 1000M
> >     > 3. The fibre link can still up with this change, otherwise not.
> > 
> >     What is actually wrong here?
> > 
> >     If both ends are performing auto-neg, i would expect a link at the
> >     highest speeds both link peers support.
> > 
> >     If one peer is doing autoneg, the other not, i expect link down, this
> >     is not a valid configuration, since one peer is going to fail to
> >     auto-neg.
> > 
> > 
> > Well, technically speaking, thanks to the 88E151X's bypass mode, in such case
> > with one end using autoneg but the other is using 1000M explicitly, the link
> > could still be up, but not with the current code.
> 
> So we can make an invalid configuration work. Question is, should we?
> 
> Are we teaching users they can wrongly configure their system and
> expect it to work? They then think it is actually a valid
> configuration and try the same on some other board with other PHYs,
> and find it does not work?
> 
> Does Marvell document why this bypass mode exists? When it should be
> used? What do they see as its use cases?

The paragraph about it is couched in terms of "if the MAC or the PHY
implements the auto-negotiation function and the other end does not".

That seems to point towards a MAC <-> PHY link rather than across a
media. So I tend to agree with you that we should not be enabling
bypass mode on a media side link.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!