Re: [dpdk-dev] [PATCH v3] compat: merge compat library into EAL

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Feb 08, 2019 at 11:55:09AM -0500, Neil Horman wrote:
> On Fri, Feb 08, 2019 at 04:18:38PM +0000, Bruce Richardson wrote:
> > On Fri, Feb 08, 2019 at 10:37:40AM -0500, Neil Horman wrote:
> > > On Thu, Feb 07, 2019 at 03:03:28PM +0000, Bruce Richardson wrote:
> > > > On Thu, Feb 07, 2019 at 09:34:26AM -0500, Neil Horman wrote:
> > > > > On Wed, Feb 06, 2019 at 02:17:45PM +0000, Bruce Richardson wrote:
> > > > > > On Wed, Feb 06, 2019 at 07:22:54AM -0500, Neil Horman wrote:
> > > > > > > On Wed, Feb 06, 2019 at 11:01:30AM +0000, Bruce Richardson wrote:
> > > > > > > > Since compat library is only a single header, we can easily move it into
> > > > > > > > the EAL common headers instead of tracking it separately. The downside of
> > > > > > > > this is that it becomes a little more difficult to have any libs that are
> > > > > > > > built before EAL depend on it. Thankfully, this is not a major problem as
> > > > > > > > the only library which uses rte_compat.h and is built before EAL (kvargs)
> > > > > > > > already has the path to the compat.h header file explicitly called out as
> > > > > > > > an include path.
> > > > > > > > 
> > > > > > > > However, to ensure that we don't hit problems later with this, we can add
> > > > > > > > EAL common headers folder to the global include list in the meson build
> > > > > > > > which means that all common headers can be safely used by all libraries, no
> > > > > > > > matter what their build order.
> > > > > > > > 
> > > > > > > This assumes that the compat lib will always just be a header though, no?  Will
> > > > > > > this work in the event that someone wants to add some compatibility code that
> > > > > > > requires its own C compilation unit?
> > > > > > > 
> > > > > > 
> > > > > > No, it probably won't work, you'll hit an issue with any libraries that
> > > > > > don't depend on EAL and need that functionality. The question is whether
> > > > > > this is likely to be an issue in the future for us. I'd say the possiblity
> > > > > > is fairly remote, but I'm open to input on it.
> > > > > > 
> > > > > Im afraid I don't have any more visibility on that than anyone else.  The fact
> > > > > that it hasn't been needed yet is likely a good sign, but I am concerned at the
> > > > > notion that this change enjoins us from having that flexibility.
> > > > > 
> > > > Yes. However, in general is it not the case that compatibility code belongs
> > > > in the actual library wanting to provide the compatibility? That is what
> > > > has been done up till now. If we do need compatibility code placed more
> > > > centrally, I think EAL is as good a place for it as any - the only library
> > > > which doesn't depend on EAL now is kvargs, so our risk area is pretty low,
> > > > I think.
> > > > 
> > > > Also, if we do need a compat libraries with .c files in it, there is no
> > > > reason we can't undo this change. It would be no more user visible than
> > > > adding a .c file to the existing structure, given that in both cases an
> > > > extra .so file will appear in the build output.
> > > > 
> > > If the consensus is that compat code can all live in the EAL library, then I'm
> > > ok with it, even if its C code.  The only thing I don't want is for our plan to
> > > be, in the event we need C code, to immediately undo this change.  That just
> > > doesn't make sense to me.
> > > 
> > > So, if you're ok with compat C code in eal, then
> > > Acked-by: Neil Horman <nhorman@tuxdriver.com>
> > > 
> > Can you clarify what you would see as the compat C code that would be
> > needed - perhaps an example from another project? From the little function
> > versioning I've done in DPDK, I would have thought what was in the headers
> > was enough for all cases we might encounter.
> > 
> I can't, hence my ACK.  I was really just concerned that we were making a change
> that enjoined us from being able to add C compilation units should we need them,
> but if we can add them directly to the EAL libraries, I'm satisfied with that.
> 
> Neil
> 
Ok, thanks, Neil.

+Thomas, in case you have any concerns or thoughts here.