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Diffstat (limited to 'Documentation/networking/mctp.rst')
| -rw-r--r-- | Documentation/networking/mctp.rst | 48 |
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diff --git a/Documentation/networking/mctp.rst b/Documentation/networking/mctp.rst index 46f74bffce0f..c628cb5406d2 100644 --- a/Documentation/networking/mctp.rst +++ b/Documentation/networking/mctp.rst @@ -212,6 +212,54 @@ remote address is already known, or the message does not require a reply. Like the send calls, sockets will only receive responses to requests they have sent (TO=1) and may only respond (TO=0) to requests they have received. +``ioctl(SIOCMCTPALLOCTAG)`` and ``ioctl(SIOCMCTPDROPTAG)`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +These tags give applications more control over MCTP message tags, by allocating +(and dropping) tag values explicitly, rather than the kernel automatically +allocating a per-message tag at ``sendmsg()`` time. + +In general, you will only need to use these ioctls if your MCTP protocol does +not fit the usual request/response model. For example, if you need to persist +tags across multiple requests, or a request may generate more than one response. +In these cases, the ioctls allow you to decouple the tag allocation (and +release) from individual message send and receive operations. + +Both ioctls are passed a pointer to a ``struct mctp_ioc_tag_ctl``: + +.. code-block:: C + + struct mctp_ioc_tag_ctl { + mctp_eid_t peer_addr; + __u8 tag; + __u16 flags; + }; + +``SIOCMCTPALLOCTAG`` allocates a tag for a specific peer, which an application +can use in future ``sendmsg()`` calls. The application populates the +``peer_addr`` member with the remote EID. Other fields must be zero. + +On return, the ``tag`` member will be populated with the allocated tag value. +The allocated tag will have the following tag bits set: + + - ``MCTP_TAG_OWNER``: it only makes sense to allocate tags if you're the tag + owner + + - ``MCTP_TAG_PREALLOC``: to indicate to ``sendmsg()`` that this is a + preallocated tag. + + - ... and the actual tag value, within the least-significant three bits + (``MCTP_TAG_MASK``). Note that zero is a valid tag value. + +The tag value should be used as-is for the ``smctp_tag`` member of ``struct +sockaddr_mctp``. + +``SIOCMCTPDROPTAG`` releases a tag that has been previously allocated by a +``SIOCMCTPALLOCTAG`` ioctl. The ``peer_addr`` must be the same as used for the +allocation, and the ``tag`` value must match exactly the tag returned from the +allocation (including the ``MCTP_TAG_OWNER`` and ``MCTP_TAG_PREALLOC`` bits). +The ``flags`` field must be zero. + Kernel internals ================ |