Upcoming Qt switch to OpenGL ES on arm64

[Steve Langasek]

Hi Lisandro,

On Fri, Nov 23, 2018 at 11:05:11PM -0300, Lisandro Damián Nicanor Pérez Meyer wrote:
> Andy: explicitly CCing you because I think it answers part of a question you 
> did but in another part of the thread.

> El viernes, 23 de noviembre de 2018 06:58:13 -03 Steve McIntyre escribió:
> > On Fri, Nov 23, 2018 at 03:27:57AM +0300, Dmitry Eremin-Solenikov wrote:
> [snip]
> > >Can you build two packages and allow user to select, which one he wants to
> > >install? Or those packages will be binary incompatible?

> > That's a good question, yes. It'w ahst I was wondering too.

> And that's a perfectly valid question, one we did in 2015, Ubuntu tried out 
> (as Dmitry pointed out) and did not work.

> Why?

> Short story: really *too* complicated and error prone.

> Long story:

> Please first check this image:

> <https://qt-kde-team.pages.debian.net/images/qt5_build_deps.png>

> That's almost all of Qt for 5.10 (we have now new submodules, so I need to 
> update it).

> The Desktop/GLES decision is done at the root of the graph, qtbase. This 
> decision changes the API/ABI of libqt5gui5, one of the libraries provided by 
> qtbase.

> So, as the API/ABI changes then we would need to (probably) ship two set of 
> headers and (for sure) two different libraries, let's say libqt5gui5 for 
> Desktop and libqt5gui5gles for GLES.

> But it doesn't ends there. The whole graph you saw is actually the *entire* 
> Qt. Upstream provides it either as a big fat tarball or as submodules. We took 
> the submodules route because building the whole tarball as one would take 
> literally days in slow arches. And a single mistake could be disastrous.

> Now whatever switch is applied to qtbase it's "inherited" by the rest of the 
> submodules. So if we ship two versions of libqt5gui5 then we would probably 
> need to ship two versions of the libs provided by qtdeclarative, which is 
> affected by this switch.

> This waterfall schema means *multiple* libraries would have to start doing 
> this two-binaries thing, as Ubuntu devs discovered. But remember that Qt is 
> really a set of submodules, so in any later version any submodule could start 
> using this switch for something. So whatever change could mean yet another set 
> of binaries with a transition with multiple rebuilds of the big part of rdeps 
> of Qt... no, we don't want to enter that mess.

Hmm, so I'm not sure this reflects the actual state of the art wrt dual Qt
stacks as it existed in Ubuntu at the time Ubuntu Touch was sunsetted.

Yes, GL vs. GLES impacts the ABI of libqt5gui5; HOWEVER, the set of
reverse-dependencies that are actually impacted by the GL-specific ABI
difference is actually quite small; and by using clever symbols files, the
impact on the dependency tree can be minimized.

If anyone wants to dig into this further, perhaps for proof-of-concept, here
is packaging that could be used as a starting point for the symbols files:

  https://launchpad.net/ubuntu/+source/qtbase-opensource-src-gles/5.7.1+dfsg-2ubuntu4~1

And here is the list of all packages that required dual-stack at least as of
2017, when Ubuntu stopped development on this:

$ wget -O - -q http://old-releases.ubuntu.com/ubuntu/dists/zesty/universe/source/Sources.gz \
    zcat | grep-dctrl -FPackage -r qt.*gles -sPackage
Package: qt3d-opensource-src-gles
Package: qtbase-opensource-src-gles
Package: qtdeclarative-opensource-src-gles
Package: qtlocation-opensource-src-gles
Package: qtmir-gles
Package: qtmultimedia-opensource-src-gles
Package: qtubuntu-gles
$

i.e. 7 source packages total, and 2 of them Ubuntu-Touch-specific (qtmir,
qtubuntu).

Maybe you were already aware of this, but it didn't come across to me in
your mail, sorry.  If you still think it is too much maintenance overhead to
provide a dual stack for these 5 libraries (plus any others that later start
to use GL-dependant ABIs), I think you're absolutely entitled to that view.

-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                   https://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org
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