[RFC PATCH v4 0/3] bsg: add io_uring command support for SCSI passthrough

[Yang Xiuwei <yangxiuwei@kylinos.cn>]

Hi all,

This RFC series adds io_uring command support to the BSG (Block layer
SCSI Generic) driver, enabling asynchronous SCSI passthrough operations
with zero-copy support via fixed buffers.

The implementation follows the io_uring uring_cmd pattern used by other
drivers (e.g., nvme). A new bsg_uring_cmd structure fits within the 80-byte
cmd field of a 128-byte SQE, with 8 bytes reserved for future extensions.
The structure uses protocol-agnostic field names and follows the sg_io_v4
design with separate din_xferp/dout_xferp fields. SCSI status information
is returned in the CQE res2 field using a compact 64-bit encoding.

Currently only BSG_SUB_PROTOCOL_SCSI_CMD is implemented. Scatter/gather
I/O and bidirectional transfers are not yet supported.

The implementation has been tested with a user-space test program covering
basic SCSI commands (INQUIRY, READ CAPACITY, READ, WRITE), zero-copy mode,
and error handling.

The v4 implementation uses a callback function pointer pattern to maintain
independence of the generic BSG layer from specific protocol implementations,
avoiding build-time dependencies on SCSI-specific code.

Previous versions:
- v1: https://lore.kernel.org/all/20260112084606.570887-1-yangxiuwei@kylinos.cn/
- v3: https://lore.kernel.org/r/cover.1768536312.git.yangxiuwei@kylinos.cn

Changes since v1:
-----------------
- Protocol-agnostic field names (request/request_len instead of cdb_addr/cdb_len)
- Removed __packed attribute for better code generation

Why v2 was superseded:
----------------------
v2 unified din_xferp/dout_xferp into a single xfer_addr field, which
diverged from the established sg_io_v4 interface. v3 reverts to separate
din_xferp/dout_xferp fields to maintain consistency with the existing
BSG interface.

Changes since v3:
- Fixed undefined symbol error by using callback function pointer pattern
  instead of direct function calls
- Removed protocol-specific validation from generic layer (moved to
  scsi_bsg_uring_cmd)


Yang Xiuwei (3):
  bsg: add bsg_uring_cmd uapi structure
  bsg: add uring_cmd support to BSG generic layer
  bsg: implement SCSI BSG uring_cmd handler

 block/bsg-lib.c          |   2 +-
 block/bsg.c              |  35 ++++++-
 drivers/scsi/scsi_bsg.c  | 210 ++++++++++++++++++++++++++++++++++++++-
 include/linux/bsg.h      |   6 +-
 include/uapi/linux/bsg.h |  21 ++++
 5 files changed, 270 insertions(+), 4 deletions(-)

-- 
2.25.1