Re: [PATCH v3 12/12] ui/console: call gfx_switch() even if the current scanout is GL

[Akihiko Odaki <akihiko.odaki@gmail.com>]

On 2022/03/09 19:07, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Mar 9, 2022 at 2:01 PM Akihiko Odaki <akihiko.odaki@gmail.com 
> <mailto:akihiko.odaki@gmail.com>> wrote:
> 
>     On 2022/03/09 18:53, Marc-André Lureau wrote:
>      > Hi
>      >
>      > On Wed, Mar 9, 2022 at 1:32 PM Akihiko Odaki
>     <akihiko.odaki@gmail.com <mailto:akihiko.odaki@gmail.com>
>      > <mailto:akihiko.odaki@gmail.com
>     <mailto:akihiko.odaki@gmail.com>>> wrote:
>      >
>      >     On 2022/03/09 18:26, Gerd Hoffmann wrote:
>      >      >    Hi,
>      >      >
>      >      >> dpy_gfx_switch and dpy_gfx_update need to be called to
>     finish the
>      >      >> initialization or switching of the non-OpenGL display.
>     However,
>      >     the proposed
>      >      >> patch only calls dpy_gfx_switch.
>      >      >>
>      >      >> vnc actually does not need dpy_gfx_update because the vnc
>      >     implementation of
>      >      >> dpy_gfx_switch implicitly does the work for
>     dpy_gfx_update, but
>      >     the model of
>      >      >> ui/console expects the two of dpy_gfx_switch and
>     dpy_gfx_update
>      >     is separated
>      >      >> and only calling dpy_gfx_switch violates the model.
>      >     dpy_gfx_update used to
>      >      >> be called even in such a case before and it is a regression.
>      >      >
>      >      > Well, no, the ->dpy_gfx_switch() callback is supposed to do
>      >     everything
>      >      > needed to bring the new surface to the screen.  vnc isn't
>     alone here,
>      >      > gtk for example does the same (see gd_switch()).
>      >      >
>      >
>      >
>      > If dpy_gfx_switch() implies a full dpy_gfx_update(), then we
>     would need
>      > another callback to just set the new surface. This would avoid
>      > intermediary and useless switches to 2d/surface when the scanout
>     is GL.
>      >
>      > For consistency, we should also declare that gl_scanout_texture and
>      > gl_scanout_dmabuf imply full update as well.
>      >
>      >      > Yes, typically this is roughly the same an explicit
>      >     dpy_gfx_update call
>      >      > would do.  So this could be changed if it helps making the
>     opengl
>      >     code
>      >      > paths less confusing, but that should be a separate patch
>     series and
>      >      > separate discussion.
>      >      >
>      >      > take care,
>      >      >    Gerd
>      >      >
>      >
>      >     Then ui/cocoa is probably wrong. I don't think it does the
>     update when
>      >     dpy_gfx_switch is called.
>      >
>      >     Please tell me if you think dpy_gfx_switch shouldn't do the
>     implicit
>      >     update in the future. I'll write a patch to do the update in
>     cocoa's
>      >     dpy_gfx_switch implementation otherwise.
>      >
>      >
>      > Can we ack this series first and iterate on top? It solves a
>     number of
>      > issues already and is a better starting point.
>      >
>      > thanks
>      >
>      > --
>      > Marc-André Lureau
> 
>     The call of dpy_gfx_update in displaychangelistener_display_console
>     should be removed. It would simplify the patch.
> 
>     Also it is still not shown that the series is a better alternative to:
>     https://patchew.org/QEMU/20220213024222.3548-1-akihiko.odaki@gmail.com/
>     <https://patchew.org/QEMU/20220213024222.3548-1-akihiko.odaki@gmail.com/>
> 
>     The series "ui/dbus: Share one listener for a console" has
>     significantly
>     less code than this series and therefore needs some reasoning for that.
> 
> 
> At this point, your change is much larger than the proposed fixes.

My change does not touch the common code except reverting and minimizes 
the risk of regression. It also results in the less code when applied to 
the tree.

> 
> I already discussed the rationale for the current design. To summarize:
> - dispatching DCL in the common code allows for greater reuse if an 
> alternative to dbus emerges, and should help making the code more dynamic
> - the GL context split also is a separation of concerns and should help 
> for alternatives to EGL
> - dbus code only handles dbus specifics

Let me summarize my counterargument:
- The suggested reuse case is not emerged yet.
- The GL context split is not aligned with the reality where the display 
knows the graphics accelerator where the window resides and the context 
should be created. The alternative to EGL can be introduced in a similar 
manner with ui/egl-context.c and ui/egl-helpers.c. If several context 
providers need to be supported, the selection should be passed as a 
parameter, just as the current code does for EGL rendernode.
- implementing the dispatching would allow dbus to share more things 
like e.g. textures converted from DisplaySurface and GunixFDList for 
DMA-BUF. They are not present in all displays and some are completely 
specific to dbus.

> 
> My understanding of your proposal is that you would rather see all this 
> done within the dbus code. I disagree for the reasons above. I may be 
> proven wrong, but so far, this works as expected minor the left-over and 
> regressions you pointed out that should be fixed. Going back to a 
> different design should be done in a next release if sufficiently motivated.

Reverting the dbus change is the safest option if it does not settle.

Regards,
Akuhiko Odaki

 > --
 > Marc-André Lureau