Docs: describe the async-DMA workspace as it behaves - #2088
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📝 WalkthroughWalkthroughThe documentation defines Worker-level DMA workspace provisioning, zero-address behavior, SDMA quarantine rules, registration support limits, and conditions for removing the ChangesDMA workspace documentation
Estimated code review effort: 1 (Trivial) | ~5 minutes Merge Risk: 🔵 Low · up to This docs-only PR does not change runtime behavior, but two remaining statements could mislead maintainers about which DMA engines are provisioned and when unsupported SDMA requests fail. The change is low risk, but those wording issues should be corrected or explicitly accepted by the owner. Poem
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In `@src/common/platform/include/common/dma_workspace.h`:
- Line 28: Update the provisioning rule documentation near
dma_workspace_supported_mask() to state that the effective engines are the
caller-requested mask intersected with the platform-supported mask, with the
SDMA quarantine applied. Keep the documentation consistent with
DeviceRunnerBase::provision_dma_workspace(required_mask, ...) and
dma_workspace_provision.
- Line 39: Update the SDMA support-matrix wording in the documentation comment
to say requests are rejected during Worker initialization/provisioning,
replacing the inaccurate “at registration” lifecycle stage. Keep the existing
provider and request context unchanged.
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docs/comm-domain.mdsrc/common/platform/include/aicpu/aicpu_device_config.hsrc/common/platform/include/common/dma_workspace.h
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Three in-tree statements contradicted the code. dma_workspace.h claimed the runtime provisions a workspace for each engine the device supports. Provisioning takes both a caller's request and the platform's support into account, which is a different rule and hid why a caller supplies a request at all. State the real rule, and state that engine selection belongs to the kernel, which is what get_dma_workspace(args, kind) already implements: every provisioned address is injected and an unprovisioned kind reads back 0. The same paragraph placed the rejection of an unsupported SDMA request "at registration"; the check runs while the workspace is provisioned, during Worker initialization, and no registration boundary examines it. aicpu_device_config.h read 0 as "no callable has provisioned that engine" and promised that a callable declaring an engine is rejected before launch when the address would be zero. Callables carry no workspace declaration and nothing rejects anything: the only consumer of get_dma_workspace_addr() is the scheduler's unconditional copy into GlobalContext. A reader who greps for the declaration finds nothing, so the comment sends them looking for absent machinery. comm-domain.md called the same thing the "callable-declared workspace mechanism" for the same reason, and described enable_sdma as an ordinary capability opt-in. It is a quarantine for an unfixed CANN defect: the 16 KB workspace is the descriptor table for 48 CP-process STARS streams, so SdmaWorkspaceManager::Init() cannot hand back the address without creating them, and a Worker holding them gets one device-reset attempt instead of three. Record that, and record the exit condition, so the flag can be deleted when either side lifts the constraint rather than being carried forever as config. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Summary
Three in-tree statements about the async-DMA workspace contradicted the code. All three were found while reading the subsystem, not while changing it, so this is docs-only — no behavior change, no code touched.
dma_workspace.hclaimed the runtime provisions a workspace "for each async-DMA engine it supports". It provisions the ones a caller's mask names, which is a different rule and hid why a caller supplies a mask at all. The comment now states the real rule (supported minus quarantined) and that selecting an engine belongs to the kernel — which is whatget_dma_workspace(args, kind)already implements.aicpu_device_config.hread a zero address as "no callable has provisioned that engine" and promised that "a callable that declares the engine is rejected before launch if the platform cannot provide a non-zero address". Neither mechanism exists:grep -rn "dma_workspace" src/common/task_interface/is empty;get_dma_workspace_addr()is the scheduler's unconditional copy intoGlobalContext(scheduler_cold_path.cpp:962,:1303).This is the
doc-consistency.md§3 case: a reader who greps for the declaration finds nothing and goes looking for absent machinery.comm-domain.mdcalled the same thing the "callable-declared workspace mechanism" for the same reason, and describedenable_sdmaas an ordinary capability opt-in. It is a quarantine for an unfixed CANN defect, and that matters because it has an exit condition that the current wording hides:SdmaWorkspaceManager::Init()is indivisible — the 16 KB workspace is the descriptor table for 48 CP-process STARS streams (CopyOpResToDevice()memcpys the per-streamsqId/cqIdtable and points the workspace at it), so there is no way to obtain the address without holding the streams;So defaulting it on is not currently possible, and the flag is not config to be generalized — it should be deleted once CANN bounds the final CP-process stream release, or once pto-isa offers an
Init()that separates the workspace from the streams. The doc now says that.Testing
markdownlint-cli2clean ondocs/comm-domain.mdclang-format --dry-run -Werrorclean on both headerscheck_headers.py/check_retired_names.pypassctest -LE requires_hardware -j4) — comment-only, but the headers are compiled by most targetsNo runtime effect: every changed line is a comment or prose.