Most Wayland compositors do their rendering on the GPU, and many Wayland clients do their rendering on the GPU as well. With the shared memory approach, sending buffers from the client to the compositor in such cases is very inefficient, as the client has to read their data from the GPU to the CPU, then the compositor has to read it from the CPU back to the GPU to be rendered.
The Linux DRM (Direct Rendering Manager) interface (which is also implemented on some BSDs) provides a means for us to export handles to GPU resources. Mesa, the predominant implementation of userspace Linux graphics drivers, implements a protocol that allows EGL users to transfer handles to their GPU buffers from the client to the compositor for rendering, without ever copying data to the GPU.
The internals of how this protocol works are out of scope for this book and would be more appropriate for resources which focus on Mesa or Linux DRM in particular. However, we can provide a short summary of its use.
eglGetPlatformDisplayEXTin concert with
EGL_PLATFORM_WAYLAND_KHRto create an EGL display.
- Configure the display normally, choosing a config appropriate to your
wl_egl_window_createto create a
wl_egl_windowfor a given
eglCreatePlatformWindowSurfaceEXTto create an
- Proceed using EGL normally, e.g.
eglMakeCurrentto make current the EGL context for your surface and
eglSwapBuffersto send an up-to-date buffer to the compositor and commit the surface.
Should you need to change the size of the
wl_egl_window later, use
Some Wayland programmers who don't use libwayland complain that this approach
ties Mesa and libwayland tightly together, which is true. However, untangling
them is not impossible - it just requires a lot of work for you in the form of
linux-dmabuf yourself. Consult the Wayland extension XML for
details on the protocol, and Mesa's implementation at
src/egl/drivers/dri2/platform_wayland.c (at the time of writing). Good luck
Unfortunately, the details for the compositor are both complicated and
out-of-scope for this book. I can point you in the right direction, however:
the wlroots implementation (found at
types/wlr_linux_dmabuf_v1.c at the time
of writing) is straightforward and should set you on the right path.