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

[Neil Horman <nhorman@tuxdriver.com>]

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>

> > > > > As a side-effect, this patch also fixes an issue with building on BSD using
> > > > > meson, due to compat lib no longer needing to be listed as a dependency.
> > > > > 
> > > > Can you elaborate here a bit please?  listing a lib as a dependency seems like a
> > > > fundamental function of a build system, was there a bug with meson in this
> > > > capacity?
> > > > 
> > > 
> > > It was a bug in DPDK. There was already a dependency on the compat
> > > library from libeal from linux, but not from BSD, so when a further
> > > dependency was added globally, the BSD build broke, but the linux one
> > > didn't.
> > > 
> > Do you have a link to the breakage details?  I'd like to look at it to see if
> > there is a way around this without enjoining us from adding compat C files in
> > the future.
> > 
> 
> Don't have a link handy, but the basics, as I understand them are as below:
> 
> Commit a8499f65a1d1 ("log: add missing experimental tag"), added the
> experimental tag to a log function. To get this tag definition rte_compat.h
> header was included in the rte_log.h file in the "common" EAL folder. In
> build tests on linux, this was no problem since linuxapp/eal/meson.build had the line:
> 	eal_inc += include_directories('include', '../../../librte_compat')
> However, the BSD eal/meson.build file did not have this line, giving a
> compiler error.
> 
> /Bruce
> 
> PS: Now that I also look at the files more closely, with this move to put
> compat.h into EAL, we can probably remove the special case processing in
> lib/meson.build for libraries with an empty source array.
> 
>