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Re: Pipeline objects open questions

From: Dzmitry Malyshau <dmalyshau@mozilla.com>
Date: Wed, 13 Sep 2017 11:12:53 -0400
Message-ID: <CAHnMvnLcBGp9B9Z9Y+FNbYifkqf6KqN_+iYT2vHXf3L0xQQPeQ@mail.gmail.com>
To: Ben Constable <bencon@microsoft.com>
Cc: "Myles C. Maxfield" <mmaxfield@apple.com>, Corentin Wallez <cwallez@google.com>, public-gpu <public-gpu@w3.org>
> Thanks for the info!
I'll double check my test and provide a RenderDoc capture with system info.

I confirmed again that 0xFFFFFFFF works for 16bit indices, provided Ben
with all the captures, test case description, and relevant info. Looking
forward to see if it's a driver bug or intended behavior.

-Dzmitry

On Tue, Sep 12, 2017 at 10:01 PM, Dzmitry Malyshau <dmalyshau@mozilla.com>
wrote:

> Ben,
>
> >The test you mention tests for the case you are asking about (using
> 0xFFFFFFFF with 16 bit indices) and it says that it should not work. Do you
> mind telling me what your HW / Driver / OS combo is? I am curious how you
> are seeing this case work.
>
> Thanks for the info!
> I'll double check my test and provide a RenderDoc capture with system info.
>
> > Your feedback about the level of documentation combined with the
> availability of a detailed spec and the HLK source is taken. I will
> investigate how we can open up the appropriate things here to make it
> easier for people to inspect things.
>
> Please excuse my sarcastic comment. This is not the best venue to complain
> about DX12 documentation.
>
> > On this particular point, the design is that the cut value has to match
> the format to work correctly. I believe that having the index buffer format
> be part of the pipeline state can solve this problem without having to do a
> lot of state tracking and will work on all APIs.
>
> This is a little bit unfortunate, given that neither API requires the
> format of the index buffer, and we just happen to depend on it because of
> the strip cut value. For instance, it may introduce more headache for
> future developers trying to port their applications on GPUWeb, since they
> may not know ahead of time what sort of index buffer is used, or even use
> different index formats with the same PSO.
>
> Regards,
> Dzmitry
>
>
> On Tue, Sep 12, 2017 at 7:11 PM, Ben Constable <bencon@microsoft.com>
> wrote:
>
>> The test you mention tests for the case you are asking about (using
>> 0xFFFFFFFF with 16 bit indices) and it says that it should not work. Do you
>> mind telling me what your HW / Driver / OS combo is? I am curious how you
>> are seeing this case work.
>>
>>
>>
>> Your feedback about the level of documentation combined with the
>> availability of a detailed spec and the HLK source is taken. I will
>> investigate how we can open up the appropriate things here to make it
>> easier for people to inspect things.
>>
>>
>>
>> On this particular point, the design is that the cut value has to match
>> the format to work correctly. I believe that having the index buffer format
>> be part of the pipeline state can solve this problem without having to do a
>> lot of state tracking and will work on all APIs.
>>
>>
>>
>> *From:* Dzmitry Malyshau [mailto:dmalyshau@mozilla.com]
>> *Sent:* Monday, September 11, 2017 6:31 PM
>> *To:* Myles C. Maxfield <mmaxfield@apple.com>
>> *Cc:* Corentin Wallez <cwallez@google.com>; public-gpu <public-gpu@w3.org
>> >
>> *Subject:* Re: Pipeline objects open questions
>>
>>
>>
>> Ben,
>>
>>
>>
>> > With regard to your test with strip cut index, I think it is dangerous
>> to rely on a non-specced behavior like that.
>>
>>
>>
>> If only, you know, we had a "specced" behavior defined for D3D12. The
>> MSDN pages are rather scarce on details (https://msdn.microsoft.com/en
>> -us/library/windows/desktop/dn986732(v=vs.85).aspx
>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmsdn.microsoft.com%2Fen-us%2Flibrary%2Fwindows%2Fdesktop%2Fdn986732(v%3Dvs.85).aspx&data=02%7C01%7Cbencon%40microsoft.com%7Cd51c41d5b52e432e4b3f08d4f97dfbdc%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636407766875906595&sdata=d9OLmKvlttYvStgOc6YS%2FyG8D%2FltT2656ERRCMY6Ru4%3D&reserved=0>).
>> If I read exactly what it says, then my test case should not have worked.
>>
>> My understanding is that the best specification of D3D12 is the HLK test
>> suite. Could you check the source of the corresponding test (
>> https://msdn.microsoft.com/en-us/library/windows/hardware/
>> dn942192(v=vs.85).aspx
>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmsdn.microsoft.com%2Fen-us%2Flibrary%2Fwindows%2Fhardware%2Fdn942192(v%3Dvs.85).aspx&data=02%7C01%7Cbencon%40microsoft.com%7Cd51c41d5b52e432e4b3f08d4f97dfbdc%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636407766875906595&sdata=IRkuq4taUm9ZcEp%2BNgf7QTDBHY3Excw01N1D3PyvmYE%3D&reserved=0>)
>> to see if the case using 0xFFFFFFFF value with 16-bit buffer is covered?
>>
>> I'm hoping to see this behavior analogous to stencil tests. One can have
>> a reference value of 0x34 matching the stencil buffer value of 0x04 if the
>> corresponding stencil read mask is 0x0F, for example.
>>
>>
>>
>> > Are we wanting to allow multiple index types in the MVP (both 16 and
>> 32)? I realize that there are bandwidth limitations etc driving people’s
>> decisions here, but I am skeptical that this design point is something that
>> gives us lock-in if we just choose 32 bit for the time being.
>>
>>
>>
>> It's a bit important since it affects the pipeline creation API. Surely
>> we can (and will) break the API after MVP, but we better have strong
>> reasons for it.
>>
>>
>>
>> Myles,
>>
>>
>>
>> > Also, I’m not sure how Dzmitry fit 0xFFFFFFFF into a u16? Maybe I’m
>> misunderstanding.
>>
>>
>>
>> My index buffer is 16-bit and has a value of 0xFFFF. I used `
>> *D3D12_INDEX_BUFFER_STRIP_CUT_VALUE_0xFFFFFFFF`* for PSO creation and
>> got it respecting the 16-bit value.
>>
>>
>>
>> > Also similar to above, I’m not sure if alpha-to-coverage is necessary
>> for the MVP.
>>
>>
>>
>> Do you find it controversial? If not, let's have it in the MVP.
>>
>>
>>
>> > It sounds like you’re asking for us to choose between two cases:
>>
>>
>>
>> I see the question caused some confusion. Depth bounds test != depth test.
>>
>> I voted for both to be explicit, potentially by using WebIDL optional
>> dictionary entries/default values semantics.
>>
>>
>>
>> Regards,
>>
>> Dzmitry
>>
>>
>>
>> On Mon, Sep 11, 2017 at 8:30 PM, Myles C. Maxfield <mmaxfield@apple.com>
>> wrote:
>>
>>
>>
>>
>>
>> On Sep 8, 2017, at 12:24 PM, Corentin Wallez <cwallez@google.com> wrote:
>>
>>
>>
>> Hey all,
>>
>>
>>
>> While what goes into pipeline objects is mostly clear (see this doc
>> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fgpuweb%2Fgpuweb%2Fblob%2Fmaster%2Fdesign%2FPipelines.md&data=02%7C01%7Cbencon%40microsoft.com%7Cd51c41d5b52e432e4b3f08d4f97dfbdc%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636407766875906595&sdata=Siu6xnMPzneh5A%2FHjlMbm9I3W%2B83JYMUvRKGD7%2FCewU%3D&reserved=0>),
>> there is still a bunch of open questions:
>>
>>    - How do we take advantage of the pipeline caching present in D3D12
>>    and Vulkan? Do we expose it to the application or is it done magically in
>>    the WebGPU implementation?
>>
>> No comment from us right now. We need more time to discuss internally.
>>
>>
>>    - Should the type of the indices be set in RenderPipelineDescriptor?
>>    If not, how is the D3D12 IBStripCutValue chosen?
>>
>> I’m not sure that triangle strips are necessary in the first place for
>> the MVP.
>>
>>
>>
>> Also, I’m not sure how Dzmitry fit 0xFFFFFFFF into a u16? Maybe I’m
>> misunderstanding.
>>
>>
>>    - Should the vertex attributes somehow be included in the
>>    PipelineLayout so vertex buffers are treated as other resources and changed
>>    in bulk with them?
>>
>> I agree with the comments previously made here.
>>
>>
>>    - Does the sample count of the pipeline state come from the
>>    RenderPass too?
>>
>> I’m not sure what you mean by “come from.” Are you asking whether or not
>> the render pass must include the sample count in addition to the pipeline
>> state including the same sample count? If you are asking that, I don’t see
>> why we should require redundant information. The sample count must at least
>> be present in the pipeline state because that’s where Metal looks when it
>> needs to configure the rasterizer.
>>
>>
>>    - Should enablement of independent attachment blend state be explicit
>>    like in D3D12 or explicit? Should alpha to coverage be part of the
>>    multisample state or the blend state?
>>
>> Similar to above, there’s no reason to require redundant information.
>>
>>
>>
>> Also similar to above, I’m not sure if alpha-to-coverage is necessary for
>> the MVP.
>>
>>
>>    - About Vulkan’s VkPipelineDepthStencilStateCreateInfo::depthBoundTestEnable
>>    and D3D12's D3D12_DEPTH_STENCIL_DESC1::DepthBoundsTestEnable? Should
>>    “depth test enable” be implicit or explicit?
>>
>> I want to be clear about what you’re asking. It sounds like you’re asking
>> for us to choose between two cases:
>>
>>    1. A boolean variable (depthBoundTestEnable), which determines
>>    whether or not the implementation cares about another n-state variable
>>    (less-than, less-than-or-equal-to, equal, etc.)
>>    2. An (n+1)-state variable (never, less-than, less-than-or-equal-to,
>>    equal, etc.)
>>
>>
>>
>> I don’t think it matters very much, but option 2. seems cleaner.
>>
>>
>>
>> What do you all think about these?
>>
>>
>>
>> Corentin
>>
>>
>>
>>
>>
>
>
Received on Wednesday, 13 September 2017 15:13:18 UTC

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