netdev - Re: [PATCH net] Revert "ixgbe: Manual AN-37 for troublesome link partners for X550 SFI"

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Date: Tue, 21 May 2024 17:41:43 +0100
From: Simon Horman <horms@...nel.org>
To: Jacob Keller <jacob.e.keller@...el.com>
Cc: netdev@...r.kernel.org, Jeff Daly <jeffd@...icom-usa.com>,
	kernel.org-fo5k2w@...arbi.fr
Subject: Re: [PATCH net] Revert "ixgbe: Manual AN-37 for troublesome link
 partners for X550 SFI"

On Mon, May 20, 2024 at 05:21:27PM -0700, Jacob Keller wrote:
> This reverts commit 565736048bd5f9888990569993c6b6bfdf6dcb6d.
> 
> According to the commit, it implements a manual AN-37 for some
> "troublesome" Juniper MX5 switches. This appears to be a workaround for a
> particular switch.
> 
> It has been reported that this causes a severe breakage for other switches,
> including a Cisco 3560CX-12PD-S.
> 
> The code appears to be a workaround for a specific switch which fails to
> link in SFI mode. It expects to see AN-37 auto negotiation in order to
> link. The Cisco switch is not expecting AN-37 auto negotiation. When the
> device starts the manual AN-37, the Cisco switch decides that the port is
> confused and stops attempting to link with it. This persists until a power
> cycle. A simple driver unload and reload does not resolve the issue, even
> if loading with a version of the driver which lacks this workaround.
> 
> The authors of the workaround commit have not responded with
> clarifications, and the result of the workaround is complete failure to
> connect with other switches.
> 
> This appears to be a case where the driver can either "correctly" link with
> the Juniper MX5 switch, at the cost of bricking the link with the Cisco
> switch, or it can behave properly for the Cisco switch, but fail to link
> with the Junipir MX5 switch. I do not know enough about the standards
> involved to clearly determine whether either switch is at fault or behaving
> incorrectly. Nor do I know whether there exists some alternative fix which
> corrects behavior with both switches.
> 
> Revert the workaround for the Juniper switch.
> 
> Fixes: 565736048bd5 ("ixgbe: Manual AN-37 for troublesome link partners for X550 SFI")
> Link: https://lore.kernel.org/netdev/cbe874db-9ac9-42b8-afa0-88ea910e1e99@intel.com/T/
> Link: https://forum.proxmox.com/threads/intel-x553-sfp-ixgbe-no-go-on-pve8.135129/#post-612291
> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> Cc: Jeff Daly <jeffd@...icom-usa.com>
> Cc: kernel.org-fo5k2w@...arbi.fr

One of those awkward situations where the only known (in this case to Jacob
and me) resolution to a regression is itself a regression (for a different
setup).

I think that in these kind of situations it's best to go back to how things
were.

Reviewed-by: Simon Horman <horms@...nel.org>