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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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