[dpdk-dev] [PATCH v16 1/3] build: disable/enable drivers in Arm builds

[Honnappa Nagarahalli]

<snip>
> >
> > > > > > >
> > > > > > > On Tue, Mar 09, 2021 at 08:58:39AM +0000, Juraj Linkeš wrote:
> > > > > > > > Honnappa, Thomas, Bruce, Jerin, you've comments in the past.
> > > > > > > > Do you have
> > > > > > > any further input?
> > > > > > > >
> > > > > > > > I think we just need to agree on the allowlist/blocklist
> > > > > > > > mechanism. The current
> > > > > > > commit allows specifying either an allowlist or a blocklist,
> > > > > > > but not
> > > both.
> > > > > > > However, it would possible to implement specifying both -
> > > > > > > first we'll allow what's in allowlist and then we'll remove
> > > > > > > from that set what's
> > > > > in blocklist.
> > > > > > > Thoughts?
> > > > > > > >
> > > > > > >
> > > > > > > If we have both, I think limiting to only one is by far the sanest
> option.
> > > > +1
> > > >
> > > > > > > I'm not fully convinced by the need to have both, since the
> > > > > > > blocklist allows wildcarding and exception cases. For
> > > > > > > example "net/[!i]*" will blocklist all net drivers except
> > > > > > > those starting with an "i". Admittedly, for usability
> > > > > > > purposes having an allowlist might
> > > > > work better.
> > > > > > >
> > > > > >
> > > > > > If we only want to build a handful of drivers then the list
> > > > > > could be very long
> > > > > (which was the original reason behind having an allowlist), such as
> here:
> > > > > > https://gerrit.fd.io/r/gitweb?p=vpp.git;a=blob;f=build/externa
> > > > > > l/
> > > > > > pa
> > > > > > ck ag
> > > > > >
> es/dpdk.mk;h=c35ac84c27b19411a0cfdf9a3524fdf36024762c;hb=HEAD
> > > > > >
> > > > > > An allowlist could also help with maintenance - users won't
> > > > > > need to add
> > > > > new drivers to their blocklists (if that's what users need, like
> > > > > in the case of VPP).
> > > > >
> > > > > +1 for allowlist.
> > > > May be I am missing something here. By creating an allowlist, does
> > > > it mean drivers are disabled (from compilation) by default? For a
> > > > server platform, where almost all the drivers can be compiled,
> > > > does the allowlist
> > > contain all the drivers?
> > > >
> > >
> > > If no allowlist is specified, then everythin will be built - nothing
> > > will be filtered.
> > That's confusing.
> > If a platform like bluefield has an allow list, a new PMD that gets
> > added will not be compiled for that platform unless someone explicitly adds
> it to the allow list.
> > Is my understanding correct?
> 
> Yes.
With this it becomes very easy to skip compiling on a platform.

> 
> > Whereas if the bluefield platform had a block list, then the new PMD
> > would be compiled for bluefield platform.
> >
> 
> Again, yes.
> 
> Supporting both would give us the option to choose between the two
> behaviors.
> 
> > >
> > > > If we assume by default everything should compile on Arm platform,
> > > > but allow for few exceptions (where things are really painful to
> > > > fix, for
> > > > ex: compiler needs to be fixed), having a blocklist should be
> shorter/better?
> > > >
> > >
> > > The blocklist is, I think, agreed upon by everyone. The question is
> > > whether we want to support the allowlist alongside it and there seem
> > > to be enough reasons to do that.
> > Sorry, may be this is answered already, but, what additional benefit
> > does allowlist provide over the blocklist?
> >
> 
> VPP could use it:
> https://gerrit.fd.io/r/gitweb?p=vpp.git;a=blob;f=build/external/packages/dpdk
> .mk;h=c35ac84c27b19411a0cfdf9a3524fdf36024762c;hb=HEAD
> 
> They're disabling almost everything so an allowlist would fit there. And they
> won't need to update the list when a new driver is added (which they won't
> need).
This is different from how we started this discussion. The current discussion was for DPDK internal use. But the one you are referencing above is for users of DPDK. I am fine for providing the allow list for the users of DPDK. But for DPDK internal, I think block list is enough.

> 
> I think it was Jerin who suggested the allowlist. I don't know of an Arm
> usecase for it, but maybe he has an example.
> 
> > >
> > > > By having an allowlist, we will end up with a large part of the
> > > > code that might not compile on Arm platforms.
> > > >
> > > > >
> > > > > >
> > > > > > > One final thought, if we add a driver allowlist for cross
> > > > > > > files, should we also add one as a top-level meson option
> > > > > > > also for
> > > consistency?
> > > > > > >
> > > > > >
> > > > > > This definitely makese sense. I was thinking about this and
> > > > > > wasn't sure
> > > > > whether I should put it into this commit or a separate one. The
> > > > > commit evolved a bit and now that it's just an implementation of
> > > > > an allow/blocklist it makes sense to include a meson option in
> > > > > it I think
> > > > > - I'll put it into the next version.
> > > > > >
> > > > > > > /Bruce
> > > > > >