[RFC PATCH v3 0/3] bsg: add io_uring command support for SCSI passthrough

[Yang Xiuwei <yangxiuwei@kylinos.cn>]

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.

Changes since v1:
-----------------
- Protocol-agnostic field names (request/request_len instead of cdb_addr/cdb_len)
- Removed __packed attribute for better code generation
- Reduced reserved space from 16 bytes to 8 bytes to fit within 80-byte SQE

Why v2 was superseded: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.

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.c              |  66 ++++++++++++++
 drivers/scsi/scsi_bsg.c  | 192 +++++++++++++++++++++++++++++++++++++++
 include/linux/bsg.h       |   4 +
 include/uapi/linux/bsg.h  |  24 +++++
 4 files changed, 286 insertions(+)

-- 
2.25.1