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 <sheng@yasker.org>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
diff --git a/Documentation/target/tcmu-design.txt b/Documentation/target/tcmu-design.txt
index bef81e4..4cebc1e 100644
--- a/Documentation/target/tcmu-design.txt
+++ b/Documentation/target/tcmu-design.txt
@@ -117,7 +117,9 @@
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
@@ -162,6 +164,13 @@
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
is contiguous within the command ring.)