[Mesa-dev] RFC: Fixing nv30 fbo attachments

[Marek Olšák]

ARB_buffer_storage doesn't have anything to do with framebuffers.

Marek

On Wed, May 21, 2014 at 2:27 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> 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