[Documentation]: hipHostAllocWriteCombined apparently ignored

[ldrumm]

https://rocm.docs.amd.com/projects/HIP/en/docs-develop/reference/hip_runtime_api/global_defines_enums_structs_files/global_enum_and_defines.html#c.hipHostMallocWriteCombined suggests that the hipHostMallocWriteCombined flag

Allocates the memory as write-combined. On some system configurations, write-combined allocation may be transferred faster across the PCI Express bus, however, could have low read efficiency by most CPUs. It’s a good option for data transfer from host to device via mapped pinned memory.

However, it looks like it's not ever used by the rocm runtime. Its value is stored for later query with hipPointerGetAttributes, but I see no other uses of this value.

I'd like to see a clarification of this in the documentation. If I'm correct and it is indeed unused by the runtime, perhaps we could have some wording along the lines of "for CUDA compatibility; ignored"

[ppanchad-amd]

Hi @ldrumm. Internal ticket has been created to fix documentation. Thanks!

[harkgill-amd]

Hi @ldrumm, the following changes have been made to the documentation ROCm/HIP@82d37bb. The updated documentation, including the new note, can be found at hipHostMallocWriteCombine API. Please let me know if you have any further questions, thanks!

[ldrumm]

As I mentioned above, the flag is ignored by the rocm runtime, so while the added sentence in ROCm/HIP@82d37bb is a helpful note on provenance, it's still not clear that this doesn't actually do anything in HIP.