Better discoverability of allowlist support

[Oblomov]

Is your feature request related to a problem? Please describe

I'm getting back into SYCL after a few years, and testing the Intel OneAPI implementation (from official .deb, version 2025.0) I was surprised to find out that sycl-ls (and my own custom syclinfo) would only show the Intel OpenCL platform, despite me having two other platforms (rusticl and pocl) that properly declare SPIR-V support. I also tried running with SYCL_UR_TRACE at various level, and I could see that the relevant platforms were being probed, but without any information about why they were being discarded.

Ultimately @bashbaug pointed me at the documentation for SYCL_DEVICE_ALLOWLIST (relevant Fediverse thread) and I was able to have them show up by setting it to the empty string.

Describe the solution you would like

The presence of an allowlist (particularly a built-in one) should be better advertised. At the very least, a message on the reason why a platform/device is being discarded should be given in the trace.

(Please let me know if this is the wrong component to report this issue against, so that I may move it to a more appropriate one.)

Describe alternatives you have considered

No response

Additional context

No response

[AlexeySachkov]

Thanks for the report, @Oblomov

I can confirm that closed-source oneAPI version has some built-in setting for SYCL_DEVICE_ALLOWLIST. From what I see that's done by supplying sycl.conf file which provides default configuration and you should be able to see what exactly we are filtering there.

I agree that the documentation on this matter should be improved. This issue will be cloned to an internal tracker to follow-up on that in the oneAPI documentation.

Also, from what I see, intel/llvm version does not have the same default config and from that point of view it is probably better for experimentation with other backends/platforms. Hopefully, one day we will even ship pre-built binaries from intel/llvm so its easier to use.