backports: backport define of SIOCGHWTSTAMP

Ben added ioctl support for the unprivileged SIOCGHWTSTAMP
which is designed to just get the configuration, unlike
SIOCSHWTSTAMP which requires you to set it and only after
it returns the configuration.

Since we are carrying over a small addition to a UAPI header
without carrying it over completely (we use copy-list to
carry over full UAPI headers) its worth making a design note
about this.

We carry UAPI headers for backports to enable compilation
of kernel / driver code to compile without any changes. If
it so happens that a feature is backported it can be added
here but notice that if full subsystems are backported you
should just include the respective full header onto the
copy-list file so that its copied intact. The strategy on this
patch can be used to either backport a specific feature or
to just avoid having to do ifdef changes to compile kernel
or driver carried over by backports.

Userspace is *not expected* to copy over backports headers
to compile userspace programs, userspace programs can
and should consider carrying over a respective copy-list
of the latest UAPI kernel headers they need in their
upstream sources, the kernel the user uses, whether with
backports or not should be able to return -EOPNOTSUPP if
the feature is not available and let it through if its
supported and meats the expected form.

In this particular case if userspace tries to send the
SIOCGHWTSTAMP they'd end up with -ENOTTY.

mcgrof@ergon ~/linux (git::master)$ git describe --contains fd468c74
v3.14-rc1~94^2~622^2~12

commit fd468c74bd4d6949736810a80d6ca05eb20fba84
Author: Ben Hutchings <bhutchings@solarflare.com>
Date:   Thu Nov 14 01:19:29 2013 +0000

    net_tstamp: Add SIOCGHWTSTAMP ioctl to match SIOCSHWTSTAMP

    SIOCSHWTSTAMP returns the real configuration to the application
    using it, but there is currently no way for any other
    application to find out the configuration non-destructively.
    Add a new ioctl for this, making it unprivileged.

    Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
1 file changed