1877114 - Codepen webgpu demo (https://codepen.io/liumingxun/pen/RweXYKq) doesnt render with Nightly

Status: RESOLVED FIXED

(Core :: Graphics: WebGPU, defect)

Description

[Mayank Bansal]

Go to https://codepen.io/liumingxun/pen/RweXYKq

AR: Only the blue background is seen. Chrome renders the colorful triangle too.
ER: Not so

Comment 1

[Erich Gubler [:ErichDonGubler] (he/him)]

Attachment: bug-1877114-translated-to-rust.zip

I suspect that we have a Firefox-specific problem in this issue, rather than a WGPU backend problem. With the attached Rust crate source bug-1877114-translated-to-rust.zip, I have translated the JS usage of the WebGPU API to the Rust API of wgpu. Things seem to work more or less as expected. More investigation to do, I guess!

:nical suggested that the next step be to record a WGPU trace from the Firefox execution and the translated Rust code, seeing if there are any different WGPU API calls being made.

Comment 2

[Erich Gubler [:ErichDonGubler] (he/him)]

Attachment: trace-native.zip

Comment 3

[Erich Gubler [:ErichDonGubler] (he/him)]

Attachment: trace-ff.zip

Comment 4

[Erich Gubler [:ErichDonGubler] (he/him)]

AHA! I noticed in comparing a trace (attached as trace-native.zip) from my native Rust port (bug-1877114-translated-to-rust.zip from before) to a trace of Firefox while running the offending CodePen (generated with WGPU_TRACE=… firefox …, attached as trace-ff.zip) that Firefox is specifying a data range of 0...24 (a byte size) for the 24 floats that it uses, while my native code is (correctly) using the byte length, which would write the index range of 0..96 (viz., 24 32-bit floats = 96 bytes in length).

After diving into Firefox source, I have identified that we have a bug where we incorrectly forward the passed size argument of GPUQueue.writeBuffer (an element count). If you remove the size argument for writeBuffer in the offending CodePen (i.e., change device.queue.writeBuffer(vertexBuffer, 0, vertices, 0, vertices.length) to device.queue.writeBuffer(vertexBuffer, 0, vertices, 0), then things render as expected.

I will write a patch for this soon.

Comment 5

[Mayank Bansal]

This is working now that bug 1879988 is fixed.