target/user: Report capability of handling out-of-order completions to userspace

TCMU_MAILBOX_FLAG_CAP_OOOC was introduced, and userspace can check the flag
for out-of-order completion capability support.

Also update the document on how to use the feature.

Signed-off-by: Sheng Yang <[e-mail]>
Reviewed-by: Andy Grover <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

Documentation/target/tcmu-design.txt

···
 when the commands are completed.
 
 version - 1 (userspace should abort if otherwise)
-flags - none yet defined.
+flags:
+- TCMU_MAILBOX_FLAG_CAP_OOOC: indicates out-of-order completion is
+  supported.  See "The Command Ring" for details.
 cmdr_off - The offset of the start of the command ring from the start
 of the memory region, to account for the mailbox size.
 cmdr_size - The size of the command ring. This does *not* need to be a
···
 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.
+
+If TCMU_MAILBOX_FLAG_CAP_OOOC is set for mailbox->flags, kernel is
+capable of handling out-of-order completions. In this case, userspace can
+handle command in different order other than original. Since kernel would
+still process the commands in the same order it appeared in the command
+ring, userspace need to update the cmd->id when completing the
+command(a.k.a steal the original command's entry).
 
 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

drivers/target/target_core_user.c

···
 
 	mb = udev->mb_addr;
 	mb->version = TCMU_MAILBOX_VERSION;
+	mb->flags = TCMU_MAILBOX_FLAG_CAP_OOOC;
 	mb->cmdr_off = CMDR_OFF;
 	mb->cmdr_size = udev->cmdr_size;
 

include/uapi/linux/target_core_user.h

···
 
 #define TCMU_MAILBOX_VERSION 2
 #define ALIGN_SIZE 64 /* Should be enough for most CPUs */
+#define TCMU_MAILBOX_FLAG_CAP_OOOC (1 << 0) /* Out-of-order completions */
 
 struct tcmu_mailbox {
 	__u16 version;