Feasibility of adding a software renderer

[showier-drastic]

Is it possible to add support for Flutter's built-in software renderer, to run this on dumb framebuffer, on SoCs without GPU? I'm evaluating to run this on RK3506.

Does it make sense to do so? Will such a Pull request be accepted if implemented?

[ardera]

Hey, yeah of course! I even had that in the back of my mind:

flutter-pi/src/egl_gbm_render_surface.c

Lines 575 to 590 in f0b3330

	static int
	egl_gbm_render_surface_present_fbdev(struct surface *s, const struct fl_layer_props *props, struct fbdev_commit_builder *builder) {
	struct egl_gbm_render_surface *egl_surface;
	/// TODO: Implement by mmapping the current front bo, copy it into the fbdev
	/// TODO: Print a warning here if we're not using explicit linear tiling and use glReadPixels instead of gbm_bo_map in that case
	egl_surface = CAST_THIS(s);
	(void) egl_surface;
	(void) props;
	(void) builder;
	UNIMPLEMENTED();
	return 0;
	}

Though I would also look into using lavapipe or llvmpipe as CPU based vulkan/OpenGL implementations, and then use mmap to present on fbdev. Last I heard the Flutter software renderer was more meant for unit tests, and I'm not sure it's that performant: flutter-team-archive/engine#26995 (comment) And I think it's easier to implement.

Might I ask which board this is you want to target, specifically? Or is it a custom one?

[gotnull]

It makes sense, and it does work in practice. I've been driving Flutter's software renderer through the embedder API on an Allwinner H700 board with no DRM/KMS and presenting to /dev/fb0, which is essentially the present step you'd add after that surface hands back a CPU buffer.

On the "more meant for unit tests" worry: the renderer itself was never the limiting factor for real 2D/UI at 720x480. The fiddly parts were around it, not in it. The engine buffer comes out as Skia kN32 (BGRA), so it needs repacking into the framebuffer's bitfield offsets, and a double-buffered panel needs a pan back to the visible buffer each frame or you get nothing.

I went the standalone-embedder route instead of patching the GBM surface, so the comparison isn't apples to apples, but if it's useful to compare the present path the source is here: https://pub.dev/packages/flutter_fbdev