[Mesa-dev] RFC: Fixing nv30 fbo attachments

[Ilia Mirkin]

On Tue, May 20, 2014 at 6:20 PM, Marek Olšák <maraeo at gmail.com> wrote:
> 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.

OK, I see. That makes sense, I think. So I just have to be careful
with the transfers to make sure to blit things back first (or maybe I
don't even have to do that, haven't checked the transfer api), similar
for blit. And I'll need to turn off ARB_buffer_storage.

  -ilia