wgsl interpolate(flat) always results in value of 0

[rukai]

Description
wgsl interpolate(flat) always results in value of 0, I think, at least the repro is very concise
Issue does not occur with the equivalent glsl

Repro steps
rukai@5c29302

Expected vs observed behavior
I expect blue output but get red

Platform
wgpu master, linux, vulkan

[rukai]

I discovered in the wgsl spec that flat cannot be used on u32, as integer types are always flat.
In that case we should make the following changes to naga:

• Error when flat is used on an int.
• Fix the repro provided, naga master seems to be outputting spirv with flat regardless of whether interpolate(flat) is used, so possibly the 0 value is caused by something else.

[kvark]

Does renderdoc capture show anything interesting?

[rukai]

renderdoc is showing the correct value is output by the vertex shader:

But I dont think there is a way to inspect the values received by the fragment shader.

[kvark]

You can debug the fragment shaders.

[rukai]

there was a history and debug button but they were greyed out, I think its not supported yet or something, its strange because the docs indicate it should be supported

[kvark]

I see the same thing, unfortunately :(
What GPU are you testing this on?

[kvark]

This looks like a driver bug. Running your sample code on Windows/AMD shows the correct result.

[kvark]

@rukai are you on NVidia proprietary driver by any chance? What's the GPU?

[kvark]

I submitted this issue to NVidia Vulkan support and already got a weak confirmation that this may be fixed soon 🚀
Unfortunately, no reference number. I'll update once I know more.

[rukai]

Yes the card is Nvidia GTX 960 and I am using the Nvidia proprietary driver.
I had incorrectly assumed it wasnt a driver bug considering it worked with glsl.
Thanks for reporting the issue to Nvidia!

[kvark]

More specifically, the driver is confused about the presence of multiple location=1 varyings, and isn't able to ignore the ones that need to be ignored (based on the entry point interface). Generally people don't make SPIR-V with multiple entry points, so that driver path was undertested.

[kvark]

@rukai could you try on 470.62.02 drivers please? The issue is supposedly fixed there. https://developer.nvidia.com/vulkan-driver

[rukai]

I am on the latest driver used by arch https://archlinux.org/packages/extra/x86_64/nvidia-utils/ and its on 470.63.01-01
The issue is not fixed.

[kvark]

@rukai looks like this driver is from 2021.8.10, it's an older driver (with somehow newer minor version?).
The one we need only came out yesterday - https://www.gamingonlinux.com/2021/09/nvidia-vulkan-beta-driver-version-4706202-is-out-now

The newest stable versions of the main NVIDIA driver for Linux are at NVIDIA 470.63.01 released on July 19, 2021 from their "Production Branch" series or 465.31 released on April 29, 2020 from their "New Feature" series. Confused? It's a lot of numbers to remember but both Production and New Feature series are fine for normal use.

[rukai]

Ok I'll test it when that driver makes its way onto arch.

[rukai]

Can now confirm this has been fixed! 🎉