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

[Ilia Mirkin]

Can you not render to a texture buffer? That might not be supported by
nv30 actually.

On Tue, May 20, 2014 at 8:49 PM, Marek Olšák <maraeo at gmail.com> wrote:
> 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