Re: [Qemu-devel] [PATCH v2 1/2 for-2.12?] qapi: Parameter gl of DisplayType now accept an enum

[Gerd Hoffmann]

On Thu, Apr 12, 2018 at 03:11:53PM +0100, Elie Tournier wrote:
> Hello,
> 
> On Tue, Apr 10, 2018 at 03:33:35PM +0200, Gerd Hoffmann wrote:
> > > # @off: Disable OpenGL (default).
> 
> Just to be sure, I have to add @ in front of all parameter, right?
> 
> > > 
> > > > + # 'on'    Use OpenGL, pick context type automatically.
> > > > + #         Would better be named 'auto' but is called 'on' for backward
> > > > + #         compatibility with bool type.
> > > 
> > > See below...
> > 
> > > DisplayOptions was added in 2.12.  This is a backwards-incompatible
> > > change in QMP (you CANNOT change 'bool' to 'DisplayGLMode' across
> > > releases, because the on-the-wire representation differs; pre-patch it
> > > would be "gl":true, post-patch it is "gl":"on").  So if it affects QMP,
> > > and we want it, this patch either HAS to go in 2.12, or we have to have
> > > more finesse (perhaps by using an 'alternate' type in the QAPI) so that
> > > it remains backwards-compatible.
> > > 
> > > /me goes and looks at introspection output...
> > > 
> > > You may be in luck - there is no instance of 'window-close' in the
> > > introspection output, which means 'DisplayType' exists only for ease of
> > > command-line parsing and is not currently used by QMP, so tweaks here
> > > are not affecting the command line.
> > 
> > Yes, right now the struct is only used to store the parsed command line
> > opts, so no effect on QMP.
> > 
> > Plan for the future is to also parse command line options with generic
> > qapi/json code instead of the home-grown parser, but that switch didn't
> > happen yet.
> > 
> > > That said, you can STILL name the enum value something smarter than 'on'
> > > IF you make the change now, for 2.12, given that the QAPI type was only
> > > introduced in 2.12 (you only have to worry about backwards compatibility
> > > if 2.11 already parsed gl=on).
> > 
> > gl=on is older, so that must continue to work.  Making both "on" and
> > "auto" work isn't a problem for our home-grown parser (aka
> > parse_display() in vl.c).  But having quirks like this makes the switch
> > to generic parser code more difficuilt, so I'd prefer to avoid that ...
> 
> Is it possible to upstream this change before 2.12 release?

No need to hurry with this.  QMP doesn't see the structs, so no
compatibility problem here.  gl=on cmd line option was present in 2.11 +
older already, so that must continue to work no matter whenever this
makes 2.12 or not.

cheers,
  Gerd