drm: img: implement imported dmabuf re-export

[Icenowy]

This allows re-exporting dmabufs that is exported to PVRSRV, which is needed by Zink renderonly.

The logic here is like drm_gem_prime_handle_to_fd().

Summary by Sourcery

Allow re-exporting dma-bufs that originate from imported dma-buf PMRs in the IMG Rogue DRM driver.

New Features:

• Support re-exporting dma-bufs backed by PMRs of type PMR_TYPE_DMABUF, enabling external dma-buf reuse.

Enhancements:

• Adjust PMR reference handling to distinguish between internally exported and externally imported dma-bufs, avoiding unnecessary unref operations.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Enable re-exporting imported dmabuf-backed PMRs by special-casing PMR_TYPE_DMABUF in PhysmemExportDmaBuf, reusing the existing dma_buf instead of creating a new export, while preserving refcounting and cleanup semantics based on whether the dma_buf is external or internally exported.

Sequence diagram for re-exporting imported dmabuf-backed PMR

sequenceDiagram
  participant Caller
  participant PhysmemExportDmaBuf
  participant PMR
  participant PMRWrapper
  participant dma_buf

  Caller->>PhysmemExportDmaBuf: request export for psPMR
  PhysmemExportDmaBuf->>PMR: PMR_IsExportable(psPMR)
  PMR-->>PhysmemExportDmaBuf: status
  PhysmemExportDmaBuf->>PMRWrapper: PhysmemGetOrCreatePMRWrapper(psPMR)
  PMRWrapper-->>PhysmemExportDmaBuf: psPMRWrapper

  PhysmemExportDmaBuf->>PMR: PMR_GetType(psPMR)
  PMR-->>PhysmemExportDmaBuf: PMR_TYPE_DMABUF
  PhysmemExportDmaBuf->>PMR: PhysmemGetDmaBuf(psPMR)
  PMR-->>PhysmemExportDmaBuf: psDmaBuf
  PhysmemExportDmaBuf->>dma_buf: get_dma_buf(psDmaBuf)
  Note over PhysmemExportDmaBuf,dma_buf: Reuse existing dma_buf and bump refcount

  PhysmemExportDmaBuf->>Caller: return file descriptor for reused dma_buf

  alt later failure while exporting fd
    PhysmemExportDmaBuf->>dma_buf: dma_buf_put(psDmaBuf)
    Note over PhysmemExportDmaBuf,PMR: PMRUnrefPMR is skipped because bIsExternalDmaBuf is true
  end

Loading

File-Level Changes

Change	Details	Files
Allow re-export of imported dmabufs instead of rejecting PMR_TYPE_DMABUF PMRs outright.	Remove the hard failure when attempting to export a PMR of type PMR_TYPE_DMABUF Introduce a bIsExternalDmaBuf flag to distinguish imported dmabufs from internally exported ones When PMR_GetType indicates PMR_TYPE_DMABUF, fetch the existing dma_buf via PhysmemGetDmaBuf and take a reference with get_dma_buf instead of exporting a new dma_buf Skip PMRRefPMR for external dmabuf-backed PMRs and ensure PMRUnrefPMR is only called for non-external PMRs on failure paths	drivers/gpu/drm/img-rogue/physmem_dmabuf.c
Retain the existing export path for non-dmabuf PMRs while adjusting error handling and refcounting flow.	Move PMRRefPMR into the non-PMR_TYPE_DMABUF branch after creating or retrieving the PMR wrapper Keep using DEFINE_DMA_BUF_EXPORT_INFO and dma_buf_export for non-external PMRs Guard the fail_export cleanup to avoid unref on external dmabuf PMRs	drivers/gpu/drm/img-rogue/physmem_dmabuf.c

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