linux-kernel - Re: [PATCH net] net: sparx5: fix reconfiguration of PCS on link mode change

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Date: Fri, 5 Apr 2024 11:20:47 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Daniel Machon <daniel.machon@...rochip.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Lars Povlsen <lars.povlsen@...rochip.com>,
	Steen Hegelund <Steen.Hegelund@...rochip.com>,
	UNGLinuxDriver@...rochip.com,
	Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: sparx5: fix reconfiguration of PCS on link mode
 change

On Fri, Apr 05, 2024 at 11:53:15AM +0200, Daniel Machon wrote:
> It was observed that the PCS would be misconfigured on link mode change,
> if the negotiated link mode went from no-inband capabilities to in-band
> capabilities. This bug appeared after the neg_mode change of phylink [1],
> but is really due to the wrong config being used when reconfiguring the PCS.

I don't see how the change you point to could have changed the
behaviour. Old code:

	conf.inband = phylink_autoneg_inband(mode);
	conf.autoneg = phylink_test(advertising, Autoneg);

New code:

	conf.inband = neg_mode == PHYLINK_PCS_NEG_INBAND_DISABLED ||
		      neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED;
	conf.autoneg = neg_mode == PHYLINK_PCS_NEG_INBAND_ENABLED;

where, for SGMII or 802.3z modes, neg_mode will be one of
PHYLINK_PCS_NEG_INBAND_DISABLED or PHYLINK_PCS_NEG_INBAND_ENABLED if
phylink_autoneg_inband(mode) is true, or PHYLINK_PCS_NEG_OUTBAND if
not.

It does change conf.autoneg slightly in that this will always be true
for SGMII, but will only be true for Autoneg + 802.3z modes.

As far as your code change goes, it looks correct to me, but I think
it's fixing a bug that goes back long before the commit you have
identified.

However, I think there's another issue here which is more relevant to
the problem you describe in your commit message. If you look at
port_conf_has_changed(), you will notice that it fails to compare
conf.inband, and thus fails to notice any change in the setting of
that configuration item. This will result in sparx5_port_pcs_set()
not even being called if only conf.inband changes state.

Thus, changing from in-band to out-of-band or vice versa won't have
any effect if this is the only change that occurs, and this also
exists prior to my change.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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