[Nouveau] [Mesa-dev] RFC: Fixing nv30 fbo attachments

[Marek Olšák]

On Tue, May 20, 2014 at 11:04 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Tue, May 20, 2014 at 4:51 PM, Marek Olšák <maraeo at gmail.com> wrote:
>> On Tue, May 20, 2014 at 9:58 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>> Hello,
>>>
>>> I attempted doing this a while back, before I really understood what
>>> was going on. I got some advice that went totally over my head, and I
>>> dropped the issue. I think I'm much better-prepared to tackle the
>>> issue this time around.
>>>
>>> To recap, nv30 (and nv40) hw can't handle certain color/depth format
>>> combinations. Specifically, a 16-bit color format must be paired with
>>> a 16-bit depth format, and a 32-bit color format must be paired with a
>>> 32-bit depth format (well, Z24S8). This HW also can't handle different
>>> per-attachment sizes, and also the "linearity" of all the attachments
>>> must be the same. (Whether a surface is linear or not is _generally_
>>> dictated by its w/h, but not always -- POT textures can sometimes end
>>> up allocated linearly by e.g. the ddx, or something else.) The
>>> different sizes are handled by not exposing ARB_fbo. However the rest
>>> of the cases remain.
>>>
>>> Now that I kinda understand how things are structured, I was thinking
>>> of doing the following:
>>>
>>> When rendering (i.e. draw_vbo & co) and the fbo has changed (and has
>>> one of the problems above), I would instead put in a temporary texture
>>> as the depth attachment. Before actually drawing, I would blit from
>>> the real target texture into the temporary texture, and then when
>>> rendering is done, blit back from the temp texture back into the
>>> target. This deals with the target texture getting modified between
>>> draws with various blits/mapping/whatever.
>>>
>>> This means that you'll only get 16 bits of depth even if you ask for
>>> 24 with a 16-bit color format, but the alternative seems too
>>> complex/costly.
>>>
>>> So there are a few questions from this approach:
>>>
>>> 1. Where do I get the temporary texture from? (And more importantly --
>>> when... what happens if allocation fails?)
>>>
>>> 2. Having to blit the depth texture back and forth on every draw seems
>>> _really_ wasteful... anything I can do about that?
>>
>> You can do that in set_framebuffer_state. When binding, blit to a
>> depth buffer which matches the colorbuffer format. When unbinding,
>> blit back.
>
> set_framebuffer_state doesn't allow an error to be returned. Should I
> just print a warning and move on?

Yes, because you have no other choice.

>
> I guess I'm still not 100% on all the terminology -- what do you mean
> exactly by bind/unbind? Do you mean the transfer_map/unmap stuff? So
> basically I would blit once on set_framebuffer_state, and then blit
> back and forth on resource map/unmap, and only ever render to the
> "temporary" buffer without worrying about blitting wihle rendering?

"bind" is when a buffer is set, "unbind" is when the buffer is unset.
set_framebuffer_state unbinds the current framebuffer and binds a new
one.

transfer_map/unmap must use the resource which contains the latest
data, there is no doubt about that.

If there are no transfers, you only have to do the blits between
32-bit and 16-bit in set_framebuffer_state, because that's the only
place where the framebuffer is changed.

Marek