Claude review: gpu: nova-core: fb: make sure to unregister SysmemFlush on boot failure

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Overall Series Review

Subject: gpu: nova-core: fb: make sure to unregister SysmemFlush on boot failure
Author: Eliot Courtney <ecourtney@nvidia.com>
Patches: 3
Reviewed: 2026-04-12T11:22:40.453075

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This is a single-patch fix for a real resource leak bug in `nova-core`. When `Gpu::new` fails after `SysmemFlush::register()` succeeds (e.g., during falcon creation or GSP boot), the sysmem flush page is leaked: it is freed by Rust's automatic drop of `CoherentHandle`, but the GPU hardware register still points to the now-freed DMA address. This could lead to the GPU writing to stale/reused memory.

The approach is sound: add a `Drop` impl that performs best-effort cleanup via revocable BAR access, while preserving the existing infallible manual `unregister()` path for normal teardown. The guard against double-unregister warnings (`registered_dma_handle != 0`) is a necessary and correct consequence of having both manual and automatic cleanup paths.

**Verdict: The patch is correct and should be accepted.** I have one minor observation below but no blocking issues.

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