Re: [net-next PATCH 2/2] net: phylink: Fix issues with link balancing w/ BMC present

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Wed, Apr 16, 2025 at 08:29:00AM -0700, Alexander Duyck wrote:
> From: Alexander Duyck <alexanderduyck@fb.com>
> 
> This change is meant to address the fact that there are link imbalances
> introduced when using phylink on a system with a BMC. Specifically there
> are two issues.
> 
> The first issue is that if we lose link after the first call to
> phylink_start but before it gets to the phylink_resolve we will end up with
> the phylink interface assuming the link was always down and not calling
> phylink_link_down resulting in a stuck interface.

That is intentional.

phylink strictly orders .mac_link_down and .mac_link_up, and starts from
an initial position that the link _will_ be considered to be down. So,
it is intentional that .mac_link_down will _never_ be called after
phylink_start().

> The second issue is that when a BMC is present we are currently forcing the
> link down. This results in us bouncing the link for a fraction of a second
> and that will result in dropped packets for the BMC.

... but you don't explain how that happens.

> The third issue is just an extra "Link Down" message that is seen when
> calling phylink_resume. This is addressed by identifying that the link
> isn't balanced and just not displaying the down message in such a case.

Hmm, this one is an error, but is not as simple as "don't print the
message" as it results in a violation of the rule I mentioned above.
We need phylink_suspend() to record the state of the link at that
point, and avoid calling phylink_link_down() if the link was down
prior to suspend.

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