[ovs-dev] RFC: the meaning of LINUX_DATAPATH and HAVE_IF_DL

[Ben Pfaff]

On Thu, Jan 23, 2014 at 11:51:26PM +0100, Luigi Rizzo wrote:
> following the port of the OVS kernel code to FreeBSD (most of it
> done by Daniele Di Proietto), the varia that control feature
> inclusion may benefit from some rediscussion.
> 
> At the moment ovs uses LINUX_DATAPATH for different purposes:
> 
> 1. indicate that an in-kernel datapath is available
> 2. indicate the availability of certain linux features
>    (rtnetlink, traffic control, vlandev, stats collection)
>    that interact with the in-kernel datapath.
> 
> #1 and #2 are now decoupled (and #2 could be split even further at
> some point), so in places where we want to check for #2 we added
> an extra test on HAVE_IF_DL, which at the moment basically discriminates
> between Linux and *BSD.
> Case #2 occurs in openvswitch/lib/automake.mk, openvswitch/lib/vlandev.c
> and openvswitch/vswitchd/system-stats.c whereas there are over 100
> instances of LINUX_DATAPATH
> 
> Before submitting a patch we'd like to know which of the following
> sounds more suitable:
> 
> a) use " #if defined(LINUX_DATAPATH) && !defined(HAVE_IF_DL)"
>    to test for case 2.
> 
> b) define some other indentifiers for specific features to be used in
>    case 2 (say HAVE_RTNETLINK ...), same as it is done for HAVE_IF_DL
> 
> We have (a) ready, but for readability maybe (b) would be preferable.
> 
> And as an orthogonal issue, it may make sense to rename LINUX_DATAPATH
> into KERNEL_DATAPATH (though it would be just for aesthetic reasons,
> and i am not sure if it is worthwhile considering the number
> of files and places affected)

After review, I think we can just change them all from LINUX_DATAPATH
to just __linux__.  I guess there are a few where you'll want to
either change them to __linux__ || __FreeBSD__ when the port is
available, or to something like KERNEL_DATAPATH if you prefer, but it
seems like a clean way forward.

(How did you find over 100 instances of LINUX_DATAPATH?  I only see
about 30, total.)

I sent out a 2-patch series:
        http://openvswitch.org/pipermail/dev/2014-January/036211.html
        http://openvswitch.org/pipermail/dev/2014-January/036212.html