Re: What is the purpose of the first phylink_validate() call from phylink_create()?

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

On Thu, Oct 05, 2023 at 01:25:23AM +0300, Vladimir Oltean wrote:
> Hi Russell,
> 
> In phylink_create() we have this code which populates pl->supported with
> a maximal link mode configuration and then makes a best-effort attempt
> to reduce it to what the physical port actually supports:
> 
> diff --git a/drivers/net/phy/phylink.c b/drivers/net/phy/phylink.c
> index 3951e5af8cb5..1e89634ec8ae 100644
> --- a/drivers/net/phy/phylink.c
> +++ b/drivers/net/phy/phylink.c
> @@ -1677,10 +1677,6 @@ struct phylink *phylink_create(struct phylink_config *config,
>  	__set_bit(PHYLINK_DISABLE_STOPPED, &pl->phylink_disable_state);
>  	timer_setup(&pl->link_poll, phylink_fixed_poll, 0);
>  
> -	bitmap_fill(pl->supported, __ETHTOOL_LINK_MODE_MASK_NBITS);
> -	linkmode_copy(pl->link_config.advertising, pl->supported);
> -	phylink_validate(pl, pl->supported, &pl->link_config);
> -
>  	ret = phylink_parse_mode(pl, fwnode);
>  	if (ret < 0) {
>  		kfree(pl);
> 
> However:
> 
> - in MLO_AN_FIXED mode, the later call to phylink_parse_fixedlink() will
>   overwrite this pl->supported and pl->link_config.advertising with
>   another set
> 
> - in MLO_AN_INBAND mode, the later call to phylink_parse_mode() will
>   also overwrite pl->supported and pl->link_config.advertising
> 
> - with a PHY (either in MLO_AN_INBAND or MLO_AN_PHY modes),
>   phylink_bringup_phy() will overwrite pl->supported and
>   pl->link_config.advertising with stuff from the PHY
> 
> Of these 3 cases, phylink_bringup_phy() is the only one which
> potentially does not come immediately after phylink_create().
> So, the effect of the phylink_validate() from phylink_create() will be
> visible only when it's not overwritten, for example when phylink_connect_phy()
> (or one of variants) isn't called at probe time but is delayed until
> ndo_open().
> 
> Since mvneta calls phylink_of_phy_connect() from mvneta_open() and I can
> test that, I'm comparing the "ethtool" output produced before running
> "ip link set dev eth0 up", in 2 cases:
> 
> - With the phylink_validate() from phylink_create() kept in place:
> 
> $ ethtool eth0
> Settings for eth0:
>         Supported ports: [ TP    AUI     MII     FIBRE   BNC     Backplane ]
>         Supported link modes:   10baseT/Half 10baseT/Full
>                                 100baseT/Half 100baseT/Full
>                                 1000baseT/Full
>                                 1000baseKX/Full
>                                 1000baseX/Full
>                                 100baseT1/Full
>                                 1000baseT1/Full
>                                 100baseFX/Half 100baseFX/Full
>                                 10baseT1L/Full
>                                 10baseT1S/Full
>                                 10baseT1S/Half
>                                 10baseT1S_P2MP/Half
>         Supported pause frame use: Symmetric
>         Supports auto-negotiation: Yes
>         Supported FEC modes: Not reported
>         Advertised link modes:  Not reported
>         Advertised pause frame use: No
>         Advertised auto-negotiation: No
>         Advertised FEC modes: Not reported
>         Speed: Unknown!
>         Duplex: Half
>         Auto-negotiation: off
>         Port: MII
>         PHYAD: 0
>         Transceiver: internal
>         Supports Wake-on: d
>         Wake-on: d
>         Link detected: no
> 
> - And with it removed (the diff from the beginning):
> 
> $ ethtool eth0
> Settings for eth0:
>         Supported ports: [  ]
>         Supported link modes:   Not reported
>         Supported pause frame use: No
>         Supports auto-negotiation: No
>         Supported FEC modes: Not reported
>         Advertised link modes:  Not reported
>         Advertised pause frame use: No
>         Advertised auto-negotiation: No
>         Advertised FEC modes: Not reported
>         Speed: Unknown!
>         Duplex: Half
>         Auto-negotiation: off
>         Port: MII
>         PHYAD: 0
>         Transceiver: internal
>         Supports Wake-on: d
>         Wake-on: d
>         Link detected: no

You've found the exact reason for it - so that we report something that
seems at least reasonable to userspace, rather than reporting absolutely
nothing which may cause issues.

The original code in mvneta would've done this:

int mvneta_ethtool_get_settings(struct net_device *dev, struct ethtool_cmd *cmd)
{
        struct mvneta_port *pp = netdev_priv(dev);

        if (!pp->phy_dev)
                return -ENODEV;

        return phy_ethtool_gset(pp->phy_dev, cmd);
}

Thus making the call fail if the device wasn't up - and that may be
an alternative if we're expecting a PHY but we have none.

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