[Mesa-dev] [PATCH 1/2] mesa: Set the correct ctx->NewState bitfield for rasterizer discard.

[Marek Olšák]

On Thu, Dec 15, 2011 at 12:18 AM, Paul Berry <stereotype441 at gmail.com> wrote:
> On 14 December 2011 15:00, Marek Olšák <maraeo at gmail.com> wrote:
>>
>> On Wed, Dec 14, 2011 at 11:25 PM, Paul Berry <stereotype441 at gmail.com>
>> wrote:
>> > On 14 December 2011 13:42, Marek Olšák <maraeo at gmail.com> wrote:
>> >>
>> >> I think RASTERIZER_DISCARD has nothing to do with transform feedback.
>> >> I think it's part of the same spec because it's not useful without it.
>> >> As I understand it, _NEW_TRANSFORM_FEEDBACK is dirty when transform
>> >> feedback buffer bindings are changed or just enabled/disabled. On the
>> >> other hand, RASTERIZER_DISCARD enables or disables the rasterizer, so
>> >> it should fall into the same category as face culling for example. (I
>> >> even implemented it using face culling on r600)
>> >>
>> >> Also there would be no way to know whether _NEW_TRANSFORM_FEEDBACK
>> >> changes just TFB buffer bindings, or just RASTERIZER_DISCARD, or both.
>> >>
>> >> Marek
>> >
>> >
>> > I see where you are coming from--I could have implemented rasterizer
>> > discard
>> > on i965 gen6 in the same way.
>> >
>> > However, I think there are three compelling reasons to consider
>> > rasterizer
>> > discard to be related to transform feedback:
>> >
>> > (1) from a user perspective, it really only makes sense to use
>> > rasterizer
>> > discard when transform feedback is active.  Thus, it's highly likely
>> > that
>> > when the rasterizer discard state is changed, transform feedback
>> > settings
>> > will be changed too.
>> >
>> > (2) rasterizer discard functionality is described by the same set of
>> > extensions that enable transform feedback (e.g. EXT_transform_feedback),
>> > so
>> > presumably the inventors of these two features thought they were closely
>> > related.
>> >
>> > (3) the enable bit that Mesa uses to keep track of the state of
>> > rasterizer
>> > discard is in gl_context::TransformFeedback, not gl_context::Transform.
>> >
>> > Item (3) is the most important to me.  One of the scarier things about
>> > i965
>> > driver development is that we have to manually keep track of which
>> > hardware
>> > configuration commands correspond to which dirty bits.  If we miss a
>> > dependency, that causes a subtle bug in which a state change might not
>> > cause
>> > the hardware state to be updated properly.  These kinds of bugs are not
>> > well
>> > exposed by Piglit tests, because most Piglit tests make a large number
>> > of
>> > state changes followed by a draw operation, so a missing dependency
>> > might
>> > easily go undetected.  The thing that allows me to sleep at night is
>> > that
>> > there is a nice one-to-one correspondence between almost all of the
>> > dirty
>> > bits and corresponding substructures of gl_context.  For example, the
>> > _NEW_MODELVIEW dirty bit corresponds to gl_context::ModelView,
>> > _NEW_PROJECTION corresponds to gl_context::Projection, and so on.  That
>> > means any time I am worried that I'm not handling dirty bits correctly,
>> > I
>> > can visually inspect the code and make sure that the dirty bits I'm
>> > consulting match up with which elements of gl_context I'm accessing.  If
>> > we
>> > leave the code as is, then there's a big undocumented exception to that
>> > one-to-one correspondence, wherein
>> > gl_context::TransformFeedback.RasterDiscard is covered by
>> > _NEW_TRANSFORM,
>> > not _NEW_TRANSFORM_FEEDBACK, as one would logically guess based on where
>> > it's located within gl_context.  I'm not confident that I (or other
>> > developers) will remember this exception once we're no longer in the
>> > thick
>> > of implementing transform feedback and rasterizer discard.
>> >
>> > It seems to me that we have three possible approaches to choose from
>> > here:
>> >
>> > (a) Go ahead with this patch, and modify r600 code to recompute the face
>> > culling mode when the _NEW_TRANSFORM_FEEDBACK dirty bit is set.
>> >
>> > (b) Don't apply this patch, and instead move RasterDiscard from
>> > gl_context::TransformFeedback to gl_context::Transform, so that we
>> > restore
>> > the one-to-one correspondence between dirty bits and substructures of
>> > gl_context.
>> >
>> > (c) Do nothing, and rely on programmers to remember that RasterDiscard
>> > is an
>> > exception to the usual correspondence between dirty bits and
>> > substructures
>> > of gl_context.
>> >
>> > I'm really not comfortable with (c) because of the risk of future bugs.
>> > I
>> > suppose I could be talked into (b) if there's popular support for it,
>> > but
>> > it's not my favourite, because as I said earlier, I think there are
>> > actually
>> > a lot of good reasons to think of rasterizer discard as related to
>> > transform
>> > feedback.  My preference is to do (a).
>>
>> (d) Rework the _NEW_* flags such that they roughly match hardware
>> state groups, not OpenGL state groups. Direct3D 11 and Gallium are two
>> examples of how it could be done.
>>
>> I am for (b) or (d). I would have nothing against (a) if TFB buffer
>> bindings were not covered by the same flag. It's mainly about the
>> overhead of state changes, although I admitted there are r600-related
>> reasons too. Also, Gallium will have rasterizer_discard in the rasterizer
>> state (once the patches hit master) - that can be changed though.
>>
>> Marek
>
>
> I would be happy to review patches to do (d) if someone wants to take that
> on.  Sadly, I do not have time to work on it myself right now, since I am
> under deadline pressure to finish OpenGL 3.0 support.
>
> As for your concerns about the overhead of state changes caused by putting
> TFB buffer bindings under the same flag as rasterizer discard, would those
> concerns be addressed by removing the FLUSH_VERTICES(ctx,
> _NEW_TRANSFORM_FEEDBACK) call from bind_buffer_range()?  As you pointed out

Fixing bind_buffer_range wouldn't make the assumed overhead of
changing RASTERIZER_DISCARD just go away. I'd like RASTERIZER_DISCARD
to be kept out of the _NEW_TRANSFORM_FEEDBACK flag as long as Gallium
has it in the rasterizer state. Even _NEW_RASTERIZER_DISCARD would do
the job. i965 could create transform feedback state from
_NEW_TRANSFORM_FEEDBACK|_NEW_RASTERIZER_DISCARD, while Gallium could
use _NEW_TRANSFORM_FEEDBACK for buffer bindings only and
_NEW_RASTERIZER_DISCARD for the rasterizer state.

It's the same issue as with _NEW_TEXTURE, which mixes a lot of
mutually unrelated states.

Marek