[Mesa-dev] [RFC] st/mesa: Add both front and back buffers for double-buffer visuals

[Boyan Ding]

2017-01-18 18:23 GMT+08:00 Józef Kucia <joseph.kucia at gmail.com>:
> On Wed, Jan 18, 2017 at 2:25 AM, Boyan Ding <boyan.j.ding at gmail.com> wrote:
>> I don't think I find that. The only place in state tracker where
>> _mesa_add_renderbuffer is called is st_framebuffer_create and only one
>> of the front and back buffers is allocated based on the default
>> behavior. When update_framebuffer_state() in state tracker is called
>> after draw buffer changes to front, state tracker will find it refers
>> to a null renderbuffer and remove it afterwards.
>
> Buffers are allocated on demand in st_DrawBuffers() and
> st_ReadBuffer(). The issue is though a bit different. A single GL
> context is used with two windows (drawables).
> glDrawBuffer/glDrawBuffers() is called when the first window is
> current, then after glXMakeCurrent() with the second window a new
> st_framebuffer is created but GL_FRONT renderbuffer is never allocated
> for the newly created st_framebuffer.

Yeah, you're right. I overlooked the on demand allocation, which turns
out to be the key to the problem. After calling _mesa_drawbuffers in
main, renderbuffers will be allocated on demand in state tracker with
DrawBuffers driver hook. But there is one path in main where
DrawBuffers hook isn't invoked after calling _mesa_drawbuffers --
update_framebuffer in main/framebuffer.c. Adding it fixes the problem.

Before sending out the patch, I'm just wonder if putting
_mesa_drawbuffers and DrawBuffers hook invocation into a function will
be a better style. Because there is a function
_mesa_update_draw_buffers, which is _mesa_drawbuffers without
DrawBuffers hook used when switching contexts. _mesa_drawbuffers and
DrawBuffers hook invocation can also fit into a function used to
reconfigure draw buffers for window system framebuffers.

Regards,
Boyan Ding