ethtool ioctl ABI: preferred way to expand uapi structure ethtool_eee for additional link modes?

["Marek Behún" <kabel@kernel.org>]

Hello,

the legacy ioctls ETHTOOL_GSET and ETHTOOL_SSET, which pass structure
ethtool_cmd, were superseded by ETHTOOL_GLINKSETTINGS and
ETHTOOL_SLINKSETTINGS.

This was done because the original structure only contains 32-bit words
for supported, advertising and lp_advertising link modes. The new
structure ethtool_link_settings contains member
  s8 link_mode_masks_nwords;
and a flexible array
  __u32 link_mode_masks[];
in order to overcome this issue.

But currently we still have only legacy structure ethtool_eee for EEE
settings:
  struct ethtool_eee {
    __u32 cmd;
    __u32 supported;
    __u32 advertised;
    __u32 lp_advertised;
    __u32 eee_active;
    __u32 eee_enabled;
    __u32 tx_lpi_enabled;
    __u32 tx_lpi_timer;
    __u32 reserved[2];
  };

Thus ethtool is unable to get/set EEE configuration for example for
2500base-T and 5000base-T link modes, which are now available in
several PHY drivers.

We can remedy this by either:

- adding another ioctl for EEE settings, as was done with the GSET /
  SSET

- using the original ioctl, but making the structure flexible (we can
  replace the reserved fields with information that the array is
  flexible), i.e.:

  struct ethtool_eee {
    __u32 cmd;
    __u32 supported;
    __u32 advertised;
    __u32 lp_advertised;
    __u32 eee_active;
    __u32 eee_enabled;
    __u32 tx_lpi_enabled;
    __u32 tx_lpi_timer;
    s8 link_mode_masks_nwords; /* zero if legacy 32-bit link modes */
    __u8 reserved[7];
    __u32 link_mode_masks[];
    /* filled in if link_mode_masks_nwords > 0, with layout:
     * __u32 map_supported[link_mode_masks_nwords];
     * __u32 map_advertised[link_mode_masks_nwords];
     * __u32 map_lp_advertised[link_mode_masks_nwords];
     */
  };

  this way we will be left with another 7 reserved bytes for future (is
  this enough?)

What would you prefer?

Marek