Hi On Mon, Mar 7, 2022 at 4:49 PM Akihiko Odaki <akihiko.od...@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. Imho, it's acceptable in the current proposed form, since there is no apparent bug (unless I miss something). However, I can try to formulate a FIXME comment and add some documentation around the gfx callback to improve the situation. thanks
