[PATCH v2 0/5] nouveau/gsp: Clean up IS_ERR vs IS_ERR_OR_NULL usage

[Hongling Zeng <zenghongling@kylinos.cn>]

This series addresses feedback from Danilo Krummrich and Timur Tabi
regarding the use of IS_ERR() vs IS_ERR_OR_NULL() in the nouveau
GSP RPC code.

The key insight is that we should align error checking with the actual
return value contracts of each function:
- Functions that never return NULL should use IS_ERR()
- Functions that can return NULL should use IS_ERR_OR_NULL()

This version adds documentation to clarify the return value contracts,
making it easier for maintainers to validate future changes.

Return Value Analysis

After thorough code analysis, the RPC functions are categorized as:

Never return NULL (use IS_ERR):**
- r535_gsp_msgq_peek()
- r535_gsp_msgq_recv_one_elem()
- r535_gsp_rpc_get()

CAN return NULL (use IS_ERR_OR_NULL):**
- r535_gsp_msgq_recv() - returns NULL when RPC length is invalid
- r535_gsp_msg_recv() - returns NULL when queue drained
- r535_gsp_rpc_handle_reply() - returns NULL for NOWAIT/NOSEQ policies
- r535_gsp_rpc_send() - can return NULL via handle_reply
- r535_gsp_rpc_push() - can return NULL via handle_reply

Changes in v2

- Added kernel-doc comments documenting return value contracts for all
  RPC functions
- Cleaned up incorrect IS_ERR_OR_NULL() usage for functions that never
  return NULL
- Kept IS_ERR_OR_NULL() in places where NULL is actually possible
- Added Fixes tags to all patches

Testing

This fixes the NULL pointer dereference oops that occurs during
nouveau initialization on some systems.

Patches

Hongling Zeng (5):
  nouveau/gsp/rpc: Document RPC function return value contracts
  nouveau/gsp/rpc: Cleanup incorrect IS_ERR_OR_NULL in rpc.c
  nouveau/gsp/rm/alloc: Cleanup IS_ERR_OR_NULL usage
  nouveau/gsp/rm/bar: Cleanup IS_ERR_OR_NULL usage
  nouveau/gsp: Cleanup IS_ERR_OR_NULL in nvkm_gsp_rpc_rd()

 drivers/gpu/drm/nouveau/include/nvkm/subdev/gsp.h   | 2 +-
 .../gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/alloc.c | 2 +-
 .../gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/bar.c  | 2 +-
 .../gpu/drm/nouveau/nvkm/subdev/gsp/rm/r535/rpc.c  | 81 ++++++++++++++++++-
 4 files changed, 81 insertions(+), 6 deletions(-)