Device capture not working in metal backend

[mikialex]

Description

The device capture API (device.start_capture or device.stop_capture) not working in the metal backend.

Repro steps

Place any pair of device.start_capture and device.stop_capture between any GPU work doesn't have any effect.

Expected vs observed behavior

As the current capture implementation in metal's backend, the metal's GPU capture should be triggered. Maybe the report should be dumped after stop_capture.

Platform

os: macos 14.5, hardware: m2 pro

[mikialex]

According to https://developer.apple.com/documentation/xcode/capturing-a-metal-workload-programmatically. I did such modifications in metal's capture implementation and the GPU trace is correctly dumped at the given path.

diff --git a/wgpu-hal/src/metal/device.rs b/wgpu-hal/src/metal/device.rs
index efafc98e1..fa8c3d553 100644
--- a/wgpu-hal/src/metal/device.rs
+++ b/wgpu-hal/src/metal/device.rs
@@ -1369,6 +1369,14 @@ impl crate::Device for super::Device {
         shared_capture_manager.set_default_capture_scope(&default_capture_scope);
         shared_capture_manager.start_capture_with_scope(&default_capture_scope);
         default_capture_scope.begin_scope();
+
+        let capture_desc = metal::CaptureDescriptor::new();
+        capture_desc.set_capture_device(&device);
+        capture_desc.set_destination(metal::MTLCaptureDestination::GpuTraceDocument);
+        capture_desc.set_output_url("/Users/myusername/myproject/testtrace.gputrace");
+
+        shared_capture_manager.start_capture(&capture_desc).unwrap();
+

The destination mush be GpuTraceDocument and the path must be end with the gputrace extension name, or capture will not work.I'm not familiar with the metal so this is just a simple test for reference.

[cwfitzgerald]

Are you running inside xcode? The assumption with the existing code is that it was running under xcode.

[mikialex]

no, and I assume this api not required running under xcode. and indeed it should not required running under xcode. I'm using this api to dump metal trace when debug unit test code like this https://github.com/mikialex/rendiation/blob/master/shader/task-graph/src/test.rs.

[cwfitzgerald]

Interesting, I didn't know this could work outside of xcode, and have always run inside xcode (even unit tests) that I wanted to capture.

We probably need some scheme for communicating a path the user wants to save this under, particularly if they want to save multiple paths.