netdev - Re: [PATCH net-next 1/6] ethtool: Extend link modes settings uAPI with lanes

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Message-ID: <20201011153759.1bcb6738@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Sun, 11 Oct 2020 15:37:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jiri@...dia.com,
        danieller@...dia.com, andrew@...n.ch, f.fainelli@...il.com,
        mkubecek@...e.cz, mlxsw@...dia.com,
        Ido Schimmel <idosch@...dia.com>, johannes@...solutions.net
Subject: Re: [PATCH net-next 1/6] ethtool: Extend link modes settings uAPI
 with lanes

On Sat, 10 Oct 2020 18:41:14 +0300 Ido Schimmel wrote:
> From: Danielle Ratson <danieller@...dia.com>
> 
> Currently, when auto negotiation is on, the user can advertise all the
> linkmodes which correspond to a specific speed, but does not have a
> similar selector for the number of lanes. This is significant when a
> specific speed can be achieved using different number of lanes.  For
> example, 2x50 or 4x25.
> 
> Add 'ETHTOOL_A_LINKMODES_LANES' attribute and expand 'struct
> ethtool_link_settings' with lanes field in order to implement a new
> lanes-selector that will enable the user to advertise a specific number
> of lanes as well.
> 
> When auto negotiation is off, lanes parameter can be forced only if the
> driver supports it. Add a capability bit in 'struct ethtool_ops' that
> allows ethtool know if the driver can handle the lanes parameter when
> auto negotiation is off, so if it does not, an error message will be
> returned when trying to set lanes.

What's the use for this in practical terms? Isn't the lane count
basically implied by the module that gets plugged in?

> +/* Lanes, 1, 2, 4 or 8. */
> +#define ETHTOOL_LANES_1			1
> +#define ETHTOOL_LANES_2			2
> +#define ETHTOOL_LANES_4			4
> +#define ETHTOOL_LANES_8			8

Not an extremely useful set of defines, not sure Michal would agree.

> +#define ETHTOOL_LANES_UNKNOWN		0

>  struct link_mode_info {
>  	int				speed;
> +	int				lanes;

why signed?

>  	u8				duplex;
>  };

> @@ -274,16 +277,17 @@ const struct nla_policy ethnl_linkmodes_set_policy[] = {
>  	[ETHTOOL_A_LINKMODES_SPEED]		= { .type = NLA_U32 },
>  	[ETHTOOL_A_LINKMODES_DUPLEX]		= { .type = NLA_U8 },
>  	[ETHTOOL_A_LINKMODES_MASTER_SLAVE_CFG]	= { .type = NLA_U8 },
> +	[ETHTOOL_A_LINKMODES_LANES]		= { .type = NLA_U32 },

NLA_POLICY_VALIDATE_FN(), not sure why the types for this
validation_type are limited.. Johannes, just an abundance of caution?

> +	} else if (!lsettings->autoneg) {
> +		/* If autoneg is off and lanes parameter is not passed from user,
> +		 * set the lanes parameter to UNKNOWN.
> +		 */
> +		ksettings->lanes = ETHTOOL_LANES_UNKNOWN;

you assume UNKNOWN is zero by doing !lanes in auto_linkmodes -
that's inconsistent.

> +	}
> +
>  	ret = ethnl_update_bitset(ksettings->link_modes.advertising,
>  				  __ETHTOOL_LINK_MODE_MASK_NBITS,
>  				  tb[ETHTOOL_A_LINKMODES_OURS], link_mode_names,