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
