diff options
Diffstat (limited to 'Documentation/target/tcmu-design.txt')
| -rw-r--r-- | Documentation/target/tcmu-design.txt | 43 |
1 files changed, 28 insertions, 15 deletions
diff --git a/Documentation/target/tcmu-design.txt b/Documentation/target/tcmu-design.txt index 5518465..43e94ea 100644 --- a/Documentation/target/tcmu-design.txt +++ b/Documentation/target/tcmu-design.txt @@ -138,27 +138,40 @@ signals the kernel via a 4-byte write(). When cmd_head equals cmd_tail, the ring is empty -- no commands are currently waiting to be processed by userspace. -TCMU commands start with a common header containing "len_op", a 32-bit -value that stores the length, as well as the opcode in the lowest -unused bits. Currently only two opcodes are defined, TCMU_OP_PAD and -TCMU_OP_CMD. When userspace encounters a command with PAD opcode, it -should skip ahead by the bytes in "length". (The kernel inserts PAD -entries to ensure each CMD entry fits contigously into the circular -buffer.) - -When userspace handles a CMD, it finds the SCSI CDB (Command Data -Block) via tcmu_cmd_entry.req.cdb_off. This is an offset from the -start of the overall shared memory region, not the entry. The data -in/out buffers are accessible via tht req.iov[] array. Note that -each iov.iov_base is also an offset from the start of the region. - -TCMU currently does not support BIDI operations. +TCMU commands are 8-byte aligned. They start with a common header +containing "len_op", a 32-bit value that stores the length, as well as +the opcode in the lowest unused bits. It also contains cmd_id and +flags fields for setting by the kernel (kflags) and userspace +(uflags). + +Currently only two opcodes are defined, TCMU_OP_CMD and TCMU_OP_PAD. + +When the opcode is CMD, the entry in the command ring is a struct +tcmu_cmd_entry. Userspace finds the SCSI CDB (Command Data Block) via +tcmu_cmd_entry.req.cdb_off. This is an offset from the start of the +overall shared memory region, not the entry. The data in/out buffers +are accessible via tht req.iov[] array. iov_cnt contains the number of +entries in iov[] needed to describe either the Data-In or Data-Out +buffers. For bidirectional commands, iov_cnt specifies how many iovec +entries cover the Data-Out area, and iov_bidi_count specifies how many +iovec entries immediately after that in iov[] cover the Data-In +area. Just like other fields, iov.iov_base is an offset from the start +of the region. When completing a command, userspace sets rsp.scsi_status, and rsp.sense_buffer if necessary. Userspace then increments mailbox.cmd_tail by entry.hdr.length (mod cmdr_size) and signals the kernel via the UIO method, a 4-byte write to the file descriptor. +When the opcode is PAD, userspace only updates cmd_tail as above -- +it's a no-op. (The kernel inserts PAD entries to ensure each CMD entry +is contiguous within the command ring.) + +More opcodes may be added in the future. If userspace encounters an +opcode it does not handle, it must set UNKNOWN_OP bit (bit 0) in +hdr.uflags, update cmd_tail, and proceed with processing additional +commands, if any. + The Data Area: This is shared-memory space after the command ring. The organization |
