[Mesa-dev] [Mesa3d-dev] [PATCH 0/6] [RFC] Formalization of the Gallium shader semantics linkage model
Christoph Bumiller
e0425955 at student.tuwien.ac.at
Fri Dec 17 09:26:51 PST 2010
On 17.12.2010 17:54, Marek Olšák wrote:
> On Fri, Dec 17, 2010 at 4:32 PM, Brian Paul <brianp at vmware.com
> <mailto:brianp at vmware.com>> wrote:
>
> Christoph,
>
> I don't see a patch for the st/mesa program translation code to check
> that we don't exceed the limit. Were you going to take care of
> that too?
>
I didn't plan to for now, at least nothing beyond making the state
tracker return an error if possible, and removing/modifying a certain
comment mentioned below.
> I guess we're assuming that the max number of generic inputs == max
> number of generic outputs. I think that's OK until a counter case
> appears.
>
>
> The way I understand it is that the max number of generic outputs is
> equal to the max number of generic inputs in the next shader stage
> (the same logic applies to some other shader caps too). I guess we
> need to use get_param to determine which shader stages are supported
> by the driver to know which one is next. The name
The problem is that (apart from the linked GL program case) you cannot
know which stage is next until validation time.
You have the same problem with the existing
PIPE_SHADER_CAP_MAX_INPUTS/OUTPUTS - nv50's vertex shaders can output
more variables to geometry shaders than they can to vertex shaders.
Maybe MAX_GENERIC_INDEX should be a non-shader specific cap - for nvc0
the value is the same everywhere, and for hardware that only has VP and
FP as well.
> *PIPE_SHADER_CAP_MAX_GENERIC_INPUT_INDEX* would be less ambiguous
> (still not perfect though).
>
I thought about using something even more verbose, like
PIPE_SHADER_CAP_MAX_GENERIC_INPUT_SEMANTIC_INDEX.
> However I don't believe in usefulness of this new cap, at least not
> without some serious state tracker work. I don't consider failing to
> translate a shader if some CAP is too low particularly useful.
>
The use of the cap is to prevent state tracker writers from thinking
they're free to use GENERIC[0,96,8911] or whatever random numbers they
like and rely on pipe drivers to ensure at all costs that linkage will
be correct.
In mesa/st I see the comment
/* Actually, let's try and zero-base this just for
* readability of the generated TGSI.
*/
So I guess someone thought it would be ok to start at some unspecified
high index. Such random behaviour makes it really hard to get
ARB_separate_shader_objects features (which galliums assumed pipe
drivers would be able to do anyway from the start) sanely.
---
So, maybe we can do without this cap. Maybe it would be better to just
mandate that the GENERIC index be less than
PIPE_SHADER_CAP_MAX_INPUTS/OUTPUTS after all.
Christoph
> (posting to mesa-dev as well)
>
> Marek
>
>
> -Brian
>
>
> On 12/17/2010 05:28 AM, Keith Whitwell wrote:
> > Christoph,
> >
> > This looks good. Thanks for bringing this back to life.
> >
> > Keith
> >
> > On Thu, 2010-12-16 at 07:47 -0800, Christoph Bumiller wrote:
> >> On 12/14/2010 12:36 PM, Keith Whitwell wrote:
> >>> On Mon, 2010-12-13 at 12:01 -0800, Christoph Bumiller wrote:
> >>>> I want to warm this up again adding nvc0 and
> >>>> GL_ARB_separate_shader_objects to the picture.
> >>>>
> >>>> The latter extends GL_EXT_separate_shader_objects to support user
> >>>> defined varyings and guarantees well defined behaviour only if
> >>>> - varyings are declared inside the
> gl_PerVertex/gl_PerFragment block the
> >>>> blocks match exactly in name, type, qualification, and (most
> >>>> significantly) declaration order.
> >>>> - varyings are assigned matching location qualifiers:
> >>>> like: layout(location = 3) in vec4 normal
> >>>> "The number of input locations available to a shader is limited."
> >>>>
> >>>> So, I propose to (loosely) identify GENERIC semantic indices
> with these
> >>>> location qualifiers and let the pipe driver set a limit on
> the allowed
> >>>> maximum (e.g PIPE_SHADER_CAP_MAX_INPUTS, and not demand to at
> least
> >>>> support 219 of them - nvc0 offsers 0x200 bytes for generic
> inputs/outputs).
> >>>
> >>> This sounds fine actually. We kicked this around before& I was
> >>> basically ok with the last iteration of the proposal, but this
> seems ok
> >>> too.
> >>>
> >>> As far as I can tell from a gallium perspective you're really just
> >>> proposing a new pipe cap _MAX_INPUTS (actually
> _MAX_GENERIC_INDEX would
> >>> be clearer), which the state tracker thereafter has to respect?
> >>>
> >>> That would be fine with me.
> >> First attempt at a patch introducing such a cap attached.
> >>
> >>>
> >>>> My motivation is mostly that the hardware routing table for
> shader
> >>>> varyings that was present on nv50 has been removed with nvc0
> (Fermi).
> >>>> And I'm glad, because filling 4 routing tables (since we have
> 5 shader
> >>>> types now) is somewhat annoying. And so applying relocations
> to shaders
> >>>> - it can be done, it's probably not too time consuming, but
> it's just
> >>>> plain *unnecessary* (and thus stupid) for OpenGL.
> >>>>
> >>>> Now about d3d9 ...
> >>>> 1. don't care, I don't see a d3d9 state tracker
> >>>> 2.
> http://msdn.microsoft.com/en-us/library/bb509647%28v=VS.85%29.aspx
> >>>> says "n is an optional integer between 0 and the number of
> resources
> >>>> supported" - what "supported" means here isn't clear to me,
> but, I
> >>>> didn't find any example where someone used something OpenGL
> doesn't have
> >>>> (like COLOR2).
> >>>> 3.
> >>>>
> http://msdn.microsoft.com/en-us/library/bb944006%28v=vs.85%29.aspx#Varying_Shader_Inputs_and_Semantics
> >>>> says "Input semantics are similar to the values in the
> D3DDECLUSAGE."
> >>>> and
> >>>> DECLUSAGE sounds like you're limited to sane values.
> >>>
> >>> I think you're on the right track with (1)... It's fairly
> pointless
> >>> trying to discuss code here which isn't public& I don't think
> people
> >>> need to be worrying about what may or may not be important for
> code they
> >>> can't see.
> >>>
> >>> I know this idea previously got tied up with speculation about
> what a
> >>> DX9 state tracker might or might not require, but in
> retrospect I wish
> >>> I'd been able to steer conversation away from that.
> >>>
> >>> The work on closed components may drive a lot of the feature
> development
> >>> and new interfaces, but there's usually enough flexibility
> that this
> >>> sort of cleanup isn't a big deal.
> >>>
> >>>
> >>> Keith
> >>>
> >>>> Not sure if anyone wants to think about this issue at this
> time (since
> >>>> implementation of ARB_separate_shader_objects is probably far
> in the GL4
> >>>> future), but I'd be happy about any comments.
> >>>>
> >>>> Regards,
> >>>> Christoph
> >>>>
> >>>> On 04/13/2010 12:55 PM, Luca Barbieri wrote:
> >>>>> This patch series is intended to resolve the issue of
> semantic-based shader linkage in Gallium.
> >>>>> It can also be found in the RFC-gallium-semantics branch.
> >>>>>
> >>>>> It does not change the current Gallium design, but rather
> formalizes some limitations to it, and provides infrastructure to
> implement this model more easily in drivers, along with a full
> nv30/nv40 implementation.
> >>>>>
> >>>>> These limitations are added to allow an efficient
> implementation for both hardware lacking special support and
> hardware having support but also special constraints.
> >>>>>
> >>>>> Note that this does NOT resolve all issues, and there are
> quite a bit left to future refinement.
> >>>>>
> >>>>> In particular, the following issues are still open:
> >>>>> 1. COLOR clamping (and floating point framebuffers)
> >>>>> 2. A linkage table CSO allowing to specify non-identity linkage
> >>>>> 3. BCOLOR/FACE-related issues
> >>>>> 4. Adding a cap to inform the state tracker that more than
> 219 generic indices are provided
> >>>>>
> >>>>> This topic was already very extensively discussed.
> >>>>> See
> http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg10865.html
> for some early inconclusive discussion around an early
> implementation that modified the GLSL linker (which is NOT being
> proposed here)
> >>>>> See
> http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg12016.html
> for some more discussion that seemed to mostly reach a consensus
> over the approach proposed here.
> >>>>> See in particular
> http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg12041.html
> .
> >>>>>
> >>>>> That said, I'm going to try to repeat all information here,
> partially by copy&pasting from earlier messages.
> >>>>> This message should probably be adapted into gallium/docs
> if/when this is accepted.
> >>>>>
> >>>>> Here is the short summary; the long rationale follows after it.
> >>>>>
> >>>>> The proposal here is to add the following limitations to
> Gallium, for the intermediate semantics:
> >>>>> 1. TGSI_SEMANTIC_NORMAL is removed, using a commit by Michal
> Krol that was never merged
> >>>>> 2. Every semantic except GENERIC, COLOR and BCOLOR can only
> be used with semantic index 0
> >>>>> 3. COLOR and BCOLOR can only be used with semantic index 0-1
> (note that this doesn't apply to fragment outputs)
> >>>>> 4. GENERIC can be used with semantic indices 0-218 on any
> driver, if BCOLOR is not used
> >>>>> 5. GENERIC can be used with semantic indices 0-216 on any
> driver, if BCOLOR IS used
> >>>>> 6. GENERIC can be used with semantic indices 0-255 on almost
> all drivers (those that don't need the 0-218 limitation)
> >>>>> 7. Some drivers may also choose to support GENERIC with
> arbitrary indices, but that should generally not happen
> >>>>>
> >>>>> The reason of this, in short, is that this maps directly to
> DirectX 9 SM3, which is the most problematic interface of all.
> >>>>>
> >>>>> The peculiar problem we have here is that we have two
> competing constraints that force us into choosing the exact SM3 value:
> >>>>> 1. The VMware SVGA driver must deal with an SM3 host
> interface and would ideally want to directly feed the Gallium
> semantics to the host
> >>>>> 2. An hypotetical DirectX 9 state tracker needs to support
> SM3 and would ideally want to directly feed the SM3 semantics to
> Gallium
> >>>>>
> >>>>> Note that this is not a reference to the VMware DirectX 9
> state tracker, since its authors haven't provided details about
> its handling of shader semantics.
> >>>>>
> >>>>> SM3 ends up supporting 219 generic indices: 16 indices in 14
> classes, minus POSITION0, PSIZE0, COLOR0, COLOR1 and FOG0 which
> are the only ones that wouldn't be mapped to GENERIC.
> >>>>> However, Gallium drivers that don't benefit from having
> specific contraints (like svga and r600) are supposed to support
> 256 indices, and my nv30/nv40 work does that.
> >>>>>
> >>>>> The expected implementation, if no hardware support exists,
> is to build a list of relocations to apply to either the fragment
> or the vertex shader, and patch one of them at validation time to
> match the other.
> >>>>> Data structures are provided in gallium/auxiliary to ease
> this, and try to minimize the number of times where this needs to
> be performed.
> >>>>>
> >>>>> Let's now proceed to the discussion and detailed rationale,
> mostly constructed by copy&pasting older messages.
> >>>>> ...
> >
> >
> >
> >
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