On Thu, Dec 15, 2011 at 12:18 AM, Paul Berry <stereotype...@gmail.com> wrote: > On 14 December 2011 15:00, Marek Olšák <mar...@gmail.com> wrote: >> >> On Wed, Dec 14, 2011 at 11:25 PM, Paul Berry <stereotype...@gmail.com> >> wrote: >> > On 14 December 2011 13:42, Marek Olšák <mar...@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 _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
