Hi
On Mon, Mar 7, 2022 at 4:49 PM Akihiko Odaki <akihiko.odaki@gmail.com> wrote:
(taking notes, mostly for myself)
Could you provide a failing test case or a more concrete suggestion on
what to do instead? I am all ears :)
thanks
Let me describe a more concrete case. Think that egl-headless and vnc
are enabled. The guest devices are serial virtio-gpu-gl. vnc selects
serial at first.
(I am not sure how you would select serial first, but let's assume you did)
vnc uses register_displaychangelistener and console_select to switch
consoles.
displaychangelistener_display_console does the actual switching, and
calls dpy_gfx_switch and dpy_gfx_update if con->scanout.kind is
SCANOUT_SURFACE. It calls dpy_gl_scanout_texture or
dpy_gl_scanout_dmabuf if con->scanout.kind is SCANOUT_TEXTURE or
SCANOUT_DMABUF. It works for a OpenGL display, but it doesn't vnc in
combination with egl-headless.
(hmm, what doesn't work? With this patch, the DisplaySurface is always
switched, no matter what con->scanout.kind is)
virtio-gpu-gl starts scanning out texture. It would happen in the
following sequence:
1. virtio-gpu-gl calls qemu_console_resize. con->scanout.kind will be
SCANOUT_SURFACE.
2. qemu_console_resize calls dpy_gfx_switch, delivering DisplaySurface
to egl-headless. con->scanout.kind will be SCANOUT_TEXTURE.
(qemu_console_resize calls dpy_gfx_replace_surface. con.scanout.kind
is still SCANOUT_SURFACE)
(It calls displaychangelistener_gfx_switch() which will call
egl_gfx_switch() and update the current surface)
3. virtio-gpu-gl calls dpy_gl_scanout_texture. egl-headless starts
rendering the texture to the DisplaySurface.
(now con.scanout.kind becomes SCANOUT_TEXTURE)
In the end, the DisplaySurface will have the image rendered, and
con->scanout.kind will be SCANOUT_TEXTURE.
(so far it works as expected, right?)
Now vnc switches to virtio-gpu-gl.
4. vnc calls console_select
5. displaychangelistener_display_console finds con->scanout.kind is
SCANOUT_TEXTURE so it tries to scanout texture, but vnc is not an OpenGL
display. It essentially becomes no-op and the display would be blank.
This patch will change it to call dpy_gfx_switch but not to call
dpy_gfx_update.
Alright, I think I see what you mean. egl-headless is associated with
a specific con, and thus is not involved during vnc console switching.
However, dpy_gfx_switch on vnc is not a no-op. It updates the surface,
resize the client and mark the area dirty. Because the con.surface is
kept updated by egl-headless, the client gets the updated content.
Iow, it still works.
If we always called dpy_gfx_update() during
displaychangelistener_display_console(), it would mean for the
listener to display the surface content, even when the scanout kind is
set for a texture. Or we need to change the behaviour of
dpy_gfx_update() to depend on the current scanout kind.