Subject: Re: [PATCH net v3 1/3] net: phylink: add phylink_expects_phy() method

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 29 Mar 2023 10:48:09 +0100

On Wed, Mar 29, 2023 at 09:34:05AM +0000, Sit, Michael Wei Hong wrote:
> 
> 
> > -----Original Message-----
> > From: Russell King <[email-masked]>
> > Sent: Wednesday, March 29, 2023 5:01 PM
> > To: Jakub Kicinski <[email-masked]>
> > Cc: Sit, Michael Wei Hong <[email-masked]>;
> > Giuseppe Cavallaro <[email-masked]>; Alexandre Torgue
> > <[email-masked]>; Jose Abreu
> > <[email-masked]>; David S . Miller <[email-masked]>;
> > Eric Dumazet <[email-masked]>; Paolo Abeni
> > <[email-masked]>; Maxime Coquelin
> > <[email-masked]>; Ong, Boon Leong
> > <[email-masked]>; [email-masked]; linux-
> > [email-masked]; linux-arm-
> > [email-masked]; [email-masked]; Looi,
> > Hong Aun <[email-masked]>; Voon, Weifeng
> > <[email-masked]>; Lai, Peter Jun Ann
> > <[email-masked]>
> > Subject: Re: [PATCH net v3 1/3] net: phylink: add
> > phylink_expects_phy() method
> > 
> > On Tue, Mar 28, 2023 at 06:57:20PM -0700, Jakub Kicinski wrote:
> > > On Fri, 24 Mar 2023 16:16:54 +0800 Michael Sit Wei Hong wrote:
> > > > Provide phylink_expects_phy() to allow MAC drivers to check if it
> > is
> > > > expecting a PHY to attach to. Since fixed-linked setups do not
> > need
> > > > to attach to a PHY.
> > > >
> > > > Provides a boolean value as to if the MAC should expect a PHY.
> > > > returns true if a PHY is expected.
> > > >
> > > > Signed-off-by: Michael Sit Wei Hong
> > <[email-masked]>
> > >
> > > Russell, looks good?
> > 
> > Not really, given that phylink_attach_phy() will refuse to attach a
> > PHY
> > when:
> > 
> >         if (WARN_ON(pl->cfg_link_an_mode == MLO_AN_FIXED ||
> >                     (pl->cfg_link_an_mode == MLO_AN_INBAND &&
> >                      phy_interface_mode_is_8023z(interface) && !pl-
> > >sfp_bus)))
> >                 return -EINVAL;
> > 
> > So, if we introduce a helper named "phylink_expects_phy" that
> > returns true when cfg_link_an_mode == MLO_AN_INBAND and the
> > interface mode is e.g. 1000base-X, but then someone tries to attach
> > a PHY, the kernel spits out a warning, backtrace, and a return value
> > of -EINVAL, things are going to look really rather stupid.
> > 
> Should we check for these 3 conditions as well then?
> (pl->cfg_link_an_mode == MLO_AN_INBAND &&
> phy_interface_mode_is_8023z(interface) && !pl->sfp_bus)
> to determine if phylink expects a phy.

If there's a sfp bus, then we don't expect a PHY from the MAC driver
(as there can only be one PHY attached), and as phylink_expects_phy()
is for the MAC driver to use, we should be returning false if
pl->sfp_bus != NULL.

	pl->cfg_link_an_mode == MLO_AN_FIXED ||
	(pl->cfg_link_an_mode == MLO_AN_INBAND &&
	 phy_interface_mode_is_8023z(interface))

Is true when we're in fixed-link mode, or if we're in in-band mode
and using 1000base-X or 25000base-X. These are the conditions that
we don't expect the MAC driver to give us a PHY.

To put that in positive logic, we expect a PHY from the MAC when
we're in PHY mode, or when we're in in-band mode and using SGMII,
QSGMII, USXGMII, RGMII, etc.

The reason for the extra "&& !pl->sfp_bus" in phylink_attach_phy()
is to allow SFPs to connect to the MAC using inband mode with
1000base-X and 2500base-X interface modes. These are not for the
MAC-side of things though.

-- 
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