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

[Andrew Lunn]

>  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?

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