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

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Message-ID: <DM6PR12MB3865B2FBA17BABBC747190D8D8070@DM6PR12MB3865.namprd12.prod.outlook.com>
Date:   Mon, 12 Oct 2020 15:33:45 +0000
From:   Danielle Ratson <danieller@...dia.com>
To:     Jakub Kicinski <kuba@...nel.org>, Ido Schimmel <idosch@...sch.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Jiri Pirko <jiri@...dia.com>,
        "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>, mlxsw <mlxsw@...dia.com>,
        Ido Schimmel <idosch@...dia.com>,
        "johannes@...solutions.net" <johannes@...solutions.net>
Subject: RE: [PATCH net-next 1/6] ethtool: Extend link modes settings uAPI
 with lanes



> -----Original Message-----
> From: Jakub Kicinski <kuba@...nel.org>
> Sent: Monday, October 12, 2020 1:38 AM
> To: Ido Schimmel <idosch@...sch.org>
> Cc: netdev@...r.kernel.org; davem@...emloft.net; Jiri Pirko
> <jiri@...dia.com>; Danielle Ratson <danieller@...dia.com>;
> andrew@...n.ch; f.fainelli@...il.com; mkubecek@...e.cz; mlxsw
> <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?

The use is to enable the user to decide how to achieve a certain speed. 
For example, if he wants to get 100G and the port has 4 lanes, the speed can be achieved it using both 2 lanes of 50G and 4 lanes of 25G, as a port with 4 lanes width can work in 2 lanes mode with double speed each.
So, by specifying "lanes 2" he will achieve 100G using 2 lanes of 50G.
 
> 
> > +/* 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?

I have aligned it to the speed.

> 
> >  	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.

I didn't do !lanes... maybe you mean !req_lanes which is different parameter that specifies if the lanes parameter was passed from user space.

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