Mesa OpenGL core vs. compatibility profile and Qt5

[Anthony Mallet <anthony.mallet%laas.fr@localhost>]

Hi,

I'm seeking for a bit of clarification regarding the so called OpengL
"core" vs. "compatibility" profile, as I have quite zero knowledge on
these topics.

I've got a Qt5 application that needs an OpenGL 3.3+ rendering
context, running on NetBSD amd64/intel -current, mesa from
xsrc-current. Out of the box, the code fails to get the correct OpenGL
context and reports an insufficient "OpenGL version 3.0". According to
glxinfo this is the version from the "compatibility profile" on my
hardware.

If I patch the application like so, adding right after the
QApplication creation these bits:

+  QSurfaceFormat format;
+  format.setProfile(QSurfaceFormat::CoreProfile);
+  format.setVersion(3, 3);
+  QSurfaceFormat::setDefaultFormat(format);

then the application works fine, reporting "OpenGL version 4.5". This
corresponds to the glxinfo "core profile" version). I found this trick
online, I'm sure of the implications of this patch, if any.

My dumb questions are:
- Is it expected (i.e. on purpose, by design choice) that we have
  such a "3.0 compatibility profile"? For instance, I noticed that on
  some other Linux machine glxinfo reports the same "4.6" version for
  both compat/core profiles (and thus the above application works out
  of the box).

- Is my understanding correct that Qt5 requests by default an OpenGL
  compatibility profile?

- All in all, does my patch seems like "a correct fix"? Is there any
  advantage to use a compat profile like what Qt seems to do by default?

Thanks for any clue, or interesting readings on the topic!
Best,
Anthony