As the latest work on the RadeonSI driver -- the open-source AMD Linux driver for the Radeon HD 7000 "Southern Islands" GPUs and newer -- Marek Olšák has now provided support in mainline Mesa for layered rendering and the OpenGL AMD_vertex_shader_layer extension.
Via the OpenGL.org registry, "This extension exposes the gl_Layer built-in variable in the vertex shader, allowing rendering to be directed to layered framebuffer attachments with only a vertex and fragment shader present. Combined with features such as instancing, or static vertex attributes and so on, this allows a wide variety of techniques to be implemented without the requirement for a geometry shader to be present."
It was just last month that Intel's Mesa DRI driver began supporting this OpenGL extension. Hooking in the support in the Gallium3D world required changes to the Mesa state tracker, and other Gallium3D code, before Marek Olšák enabled it. Hopefully the other OpenGL 3 capable Gallium3D drivers will begin supporting the layered rendering feature in the near future.
In terms of where the open-source RadeonSI OpenGL performance currently is at, check out last week's 21-way open-source comparison and the other many recent Phoronix articles.