On Wed, Jan 11, 2017 at 10:31 AM, Erik Faye-Lund <kusmab...@gmail.com> wrote:
> On Wed, Jan 11, 2017 at 7:22 PM, Marek Olšák <mar...@gmail.com> wrote: > > On Wed, Jan 11, 2017 at 7:09 PM, Jason Ekstrand <ja...@jlekstrand.net> > wrote: > >> On Wed, Jan 11, 2017 at 9:32 AM, Samuel Pitoiset < > samuel.pitoi...@gmail.com> > >> wrote: > >>> > >>> > >>> > >>> On 01/11/2017 05:32 PM, Marek Olšák wrote: > >>>> > >>>> On Wed, Jan 11, 2017 at 4:33 PM, Erik Faye-Lund <kusmab...@gmail.com> > >>>> wrote: > >>>>> > >>>>> On Wed, Jan 11, 2017 at 4:14 PM, Nicolai Hähnle <nhaeh...@gmail.com> > >>>>> wrote: > >>>>>> > >>>>>> On 11.01.2017 13:17, Marek Olšák wrote: > >>>>>>> > >>>>>>> > >>>>>>> On Tue, Jan 10, 2017 at 6:48 PM, Jason Ekstrand < > ja...@jlekstrand.net> > >>>>>>> wrote: > >>>>>>>> > >>>>>>>> > >>>>>>>> I'll be honest, I'm not a fan... Given that D3D10 has one defined > >>>>>>>> behavior, > >>>>>>>> D3D9 has another, and GL doesn't specify, I don't really think we > >>>>>>>> should > >>>>>>>> be > >>>>>>>> making a global change to all drivers to do the D3D9 behavior > just to > >>>>>>>> fix > >>>>>>>> one app. Sure, other apps probably have the same bug, but are we > >>>>>>>> going > >>>>>>>> to > >>>>>>>> have apps that expect the D3D10 behavior that we've now explicitly > >>>>>>>> made > >>>>>>>> not > >>>>>>>> work? > >>>>>>>> > >>>>>>>> If we're going to hack around an app bug, I would really rather > see > >>>>>>>> it > >>>>>>>> behind a driconf option rather than a global change to driver > >>>>>>>> behavior. > >>>>>>>> Even better, it'd be cool if we could see the app get fixed. > (Yes, I > >>>>>>>> know > >>>>>>>> that's not likely). > >>>>>>> > >>>>>>> > >>>>>>> > >>>>>>> I think we are not in a position to refuse this workaround, or put > >>>>>>> more precisely, to have a different behavior from everybody else. > By > >>>>>>> "we", I mean i965, radeonsi, svga. All closed drivers use abs. Many > >>>>>>> Mesa drivers also use abs internally (r300, r600, nv30, nv50/nvc0). > >>>>>>> This is not really a workaround for a specific application, even > >>>>>>> though it's strongly motivated by that. It's a fix to align the few > >>>>>>> remaining drivers with all others. > >>>>>>> > >>>>>>> We talked with the publisher about this a very long time ago. > While I > >>>>>>> don't remember the details (Nicolai?), I think they refused to fix > it > >>>>>>> because radeonsi appeared to be the only driver not doing abs. > >>>>>> > >>>>>> > >>>>>> > >>>>>> If I remember correctly, it wasn't so much a refusal as a lack of > >>>>>> follow-through. They even had an option in their framework to add > the > >>>>>> abs(...) when translating shaders, but somehow didn't turn it on > >>>>>> unconditionally for some reason... > >>>>> > >>>>> > >>>>> VP even says so here: > >>>>> https://github.com/virtual-programming/specops-linux/issues/20 > >>>>> > >>>>> They recommend against patching mesa to do abs, though. > >>>> > >>>> > >>>> We should still patch Mesa to align the behavior with closed drivers > >>>> and gallium drivers like r600g and nouveau. In other words, it's too > >>>> late to tell us not to patch Mesa, because r600g and nouveau have been > >>>> "patched" since the beginning. > >>>> > >>>> We only need to decide whether we should do it in the GLSL compiler or > >>>> radeonsi, i.e. whether we should exclude i965 and svga. > >>> > >>> > >>> I do agree with that. > >> > >> > >> I tend to disagree but I've come to the conclusion that I won't stand > in the > >> way either. If both of the other desktop vendors do it and we've > already > >> decided that no implementation we care about will have its performance > >> impacted, it seems like a valid spec-compliant thing to do. I would > prefer > >> it to be behind a driconf option, but if it's unconditional, oh well. > My > >> disagreement is mostly philosophical. > >> > >> Over the last two years of working on Vulkan, I've been fighting broken > >> tests and apps left and right. Vulkan has a huge amount of area where, > if > >> an app does something wrong, they get undefined behavior which is up to > and > >> including program termination. And basically all apps are broken in > some > >> way. Fortunately, the validation layers are finally starting to catch > up to > >> the point where I'm noticing very few bugs that the validation layers > don't > >> catch and things are getting into a better state. However, I've had > more > >> discussions than I can count with people where I have to explain to them > >> that "No, the app is broken. It needs to be fixed. It's not my job to > make > >> it work." Once you start allowing brokenness, you can never stop > allowing > >> it and you paint yourself into a corner. Suddenly, you go to make a > change, > >> and your design decisions are not guided by the spec, they're guided by > the > >> spec *and* all of the broken apps that you have to keep working on your > >> driver because you let something through. > >> > >> In the world of GLES and OpenGL conformance, we fight the same fight. > When > >> people ask me how conformance is coming, I frequently answer with, > "We've > >> got a bunch of people fixing <insert test suite name here> so that our > >> driver passes". It's not that mesa is particularly touchy, it's that a > good > >> chunk of the rest of the industry just hacks around everything inside > their > >> driver and doesn't bother to fix the tests. Sometimes the driver that > >> passes the conformance suite isn't even the one they ship. If we're > going > >> to have a spec and hardware vendors (or the FOSS community) are going to > >> implement it and apps are going to write to it, then we all need to > agree on > >> what it means and play by the rules. If an app doesn't play by the > rules > >> and does something with undefined behavior, then it's a broken app. If > we > >> say, "No, it's ok, you don't have to fix it. We'll just hack around it" > >> we're enablers for their broken behavior and the broken behavior > continues. > >> In this particular case, we're dealing with a broken app. The only real > >> issue is that all of the drivers that point out the issues were not > drivers > >> they tested on. > >> > >> Another reason why I'm not a huge fan is that there is some momentum in > the > >> industry to make GLSL better defined with respect to NaN. I don't know > that > >> anything will ever come of it (because it may break apps) but if > something > >> does, we may find ourselves having to make SQRT and RSQ NaN-correct in > the > >> future and, hey look, it'll break apps. > >> > >> Ok, rant over. Push it if you want. You can even put my nakked-by on > it if > >> you'd like. :-) > > > > I agree with you completely, and I find it unfortunate too that we > > have to add the workaround to GLSL or radeonsi to align its behavior > > with closed drivers. > > Just for reference, I just tested what NVIDIA does on Windows, and > they *don't* seem to do inversesqrt(abs(x)) on my HW/driver. > What about sqrt()? Do they do abs for one and not the other? Because that would be crazy but also possible.
_______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
