Re: [PATCH 1/2][next] UAPI: ethtool: Use __struct_group() in struct ethtool_link_settings

[Gustavo A. R. Silva]

On 21/10/24 14:11, Andrew Lunn wrote:
> >   struct ethtool_link_settings {
> > -	__u32	cmd;
> > -	__u32	speed;
> > -	__u8	duplex;
> > -	__u8	port;
> > -	__u8	phy_address;
> > -	__u8	autoneg;
> > -	__u8	mdio_support;
> > -	__u8	eth_tp_mdix;
> > -	__u8	eth_tp_mdix_ctrl;
> > -	__s8	link_mode_masks_nwords;
> > -	__u8	transceiver;
> > -	__u8	master_slave_cfg;
> > -	__u8	master_slave_state;
> > -	__u8	rate_matching;
> > -	__u32	reserved[7];
> > +	/* New members MUST be added within the __struct_group() macro below. */
> > +	__struct_group(ethtool_link_settings_hdr, hdr, /* no attrs */,
> > +		__u32	cmd;
> > +		__u32	speed;
> > +		__u8	duplex;
> > +		__u8	port;
> > +		__u8	phy_address;
> > +		__u8	autoneg;
> > +		__u8	mdio_support;
> > +		__u8	eth_tp_mdix;
> > +		__u8	eth_tp_mdix_ctrl;
> > +		__s8	link_mode_masks_nwords;
> > +		__u8	transceiver;
> > +		__u8	master_slave_cfg;
> > +		__u8	master_slave_state;
> > +		__u8	rate_matching;
> > +		__u32	reserved[7];
> > +	);
> >   	__u32	link_mode_masks[];
>
> Dumb C question. What are the padding rules for a union, compared to
> base types? Do we know for sure the compiler is not going pad this
> structure differently because of the union?

We've been using the struct_group() family of helpers in Linux for years,
and we haven't seen any issues with padding an alignment. So, it seems
to do its job just fine. :)

Thanks
--
Gustavo

> It is however nicely constructed. The 12 __u8 making 3 32bit words, so
> we have a total of 12 32bit words, or 6 64bit words, before the
> link_mode_masks[], so i don't think padding is technically an issue,
> but it would be nice to know the C standard guarantees this.
>
> 	Andrew