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<section id="management-component-transport-protocol-mctp">
<h1>Management Component Transport Protocol (MCTP)<a class="headerlink" href="#management-component-transport-protocol-mctp" title="Link to this heading">¶</a></h1>
<p>net/mctp/ contains protocol support for MCTP, as defined by DMTF standard
DSP0236. Physical interface drivers (“bindings” in the specification) are
provided in drivers/net/mctp/.</p>
<p>The core code provides a socket-based interface to send and receive MCTP
messages, through an AF_MCTP, SOCK_DGRAM socket.</p>
<section id="structure-interfaces-networks">
<h2>Structure: interfaces & networks<a class="headerlink" href="#structure-interfaces-networks" title="Link to this heading">¶</a></h2>
<p>The kernel models the local MCTP topology through two items: interfaces and
networks.</p>
<p>An interface (or “link”) is an instance of an MCTP physical transport binding
(as defined by DSP0236, section 3.2.47), likely connected to a specific hardware
device. This is represented as a <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">netdevice</span></code>.</p>
<p>A network defines a unique address space for MCTP endpoints by endpoint-ID
(described by DSP0236, section 3.2.31). A network has a user-visible identifier
to allow references from userspace. Route definitions are specific to one
network.</p>
<p>Interfaces are associated with one network. A network may be associated with one
or more interfaces.</p>
<p>If multiple networks are present, each may contain endpoint IDs (EIDs) that are
also present on other networks.</p>
</section>
<section id="sockets-api">
<h2>Sockets API<a class="headerlink" href="#sockets-api" title="Link to this heading">¶</a></h2>
<section id="protocol-definitions">
<h3>Protocol definitions<a class="headerlink" href="#protocol-definitions" title="Link to this heading">¶</a></h3>
<p>MCTP uses <code class="docutils literal notranslate"><span class="pre">AF_MCTP</span></code> / <code class="docutils literal notranslate"><span class="pre">PF_MCTP</span></code> for the address- and protocol- families.
Since MCTP is message-based, only <code class="docutils literal notranslate"><span class="pre">SOCK_DGRAM</span></code> sockets are supported.</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="n">sd</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">socket</span><span class="p">(</span><span class="n">AF_MCTP</span><span class="p">,</span><span class="w"> </span><span class="n">SOCK_DGRAM</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
</pre></div>
</div>
<p>The only (current) value for the <code class="docutils literal notranslate"><span class="pre">protocol</span></code> argument is 0.</p>
<p>As with all socket address families, source and destination addresses are
specified with a <code class="docutils literal notranslate"><span class="pre">sockaddr</span></code> type, with a single-byte endpoint address:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">typedef</span><span class="w"> </span><span class="n">__u8</span><span class="w"> </span><span class="n">mctp_eid_t</span><span class="p">;</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">mctp_addr</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">mctp_eid_t</span><span class="w"> </span><span class="n">s_addr</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">__kernel_sa_family_t</span><span class="w"> </span><span class="n">smctp_family</span><span class="p">;</span>
<span class="w"> </span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">smctp_network</span><span class="p">;</span>
<span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">mctp_addr</span><span class="w"> </span><span class="n">smctp_addr</span><span class="p">;</span>
<span class="w"> </span><span class="n">__u8</span><span class="w"> </span><span class="n">smctp_type</span><span class="p">;</span>
<span class="w"> </span><span class="n">__u8</span><span class="w"> </span><span class="n">smctp_tag</span><span class="p">;</span>
<span class="p">};</span>
<span class="cp">#define MCTP_NET_ANY 0x0</span>
<span class="cp">#define MCTP_ADDR_ANY 0xff</span>
</pre></div>
</div>
</section>
<section id="syscall-behaviour">
<h3>Syscall behaviour<a class="headerlink" href="#syscall-behaviour" title="Link to this heading">¶</a></h3>
<p>The following sections describe the MCTP-specific behaviours of the standard
socket system calls. These behaviours have been chosen to map closely to the
existing sockets APIs.</p>
<section id="bind-set-local-socket-address">
<h4><code class="docutils literal notranslate"><span class="pre">bind()</span></code> : set local socket address<a class="headerlink" href="#bind-set-local-socket-address" title="Link to this heading">¶</a></h4>
<p>Sockets that receive incoming request packets will bind to a local address,
using the <code class="docutils literal notranslate"><span class="pre">bind()</span></code> syscall.</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="n">addr</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_family</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">AF_MCTP</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_network</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_NET_ANY</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_addr</span><span class="p">.</span><span class="n">s_addr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_ADDR_ANY</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_TYPE_PLDM</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_tag</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_TAG_OWNER</span><span class="p">;</span>
<span class="kt">int</span><span class="w"> </span><span class="n">rc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">bind</span><span class="p">(</span><span class="n">sd</span><span class="p">,</span><span class="w"> </span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="o">&</span><span class="n">addr</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">addr</span><span class="p">));</span>
</pre></div>
</div>
<p>This establishes the local address of the socket. Incoming MCTP messages that
match the network, address, and message type will be received by this socket.
The reference to ‘incoming’ is important here; a bound socket will only receive
messages with the TO bit set, to indicate an incoming request message, rather
than a response.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">smctp_tag</span></code> value will configure the tags accepted from the remote side of
this socket. Given the above, the only valid value is <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code>, which
will result in remotely “owned” tags being routed to this socket. Since
<code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code> is set, the 3 least-significant bits of <code class="docutils literal notranslate"><span class="pre">smctp_tag</span></code> are not
used; callers must set them to zero.</p>
<p>A <code class="docutils literal notranslate"><span class="pre">smctp_network</span></code> value of <code class="docutils literal notranslate"><span class="pre">MCTP_NET_ANY</span></code> will configure the socket to
receive incoming packets from any locally-connected network. A specific network
value will cause the socket to only receive incoming messages from that network.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">smctp_addr</span></code> field specifies a local address to bind to. A value of
<code class="docutils literal notranslate"><span class="pre">MCTP_ADDR_ANY</span></code> configures the socket to receive messages addressed to any
local destination EID.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">smctp_type</span></code> field specifies which message types to receive. Only the
lower 7 bits of the type is matched on incoming messages (ie., the
most-significant IC bit is not part of the match). This results in the socket
receiving packets with and without a message integrity check footer.</p>
</section>
<section id="sendto-sendmsg-send-transmit-an-mctp-message">
<h4><code class="docutils literal notranslate"><span class="pre">sendto()</span></code>, <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code>, <code class="docutils literal notranslate"><span class="pre">send()</span></code> : transmit an MCTP message<a class="headerlink" href="#sendto-sendmsg-send-transmit-an-mctp-message" title="Link to this heading">¶</a></h4>
<p>An MCTP message is transmitted using one of the <code class="docutils literal notranslate"><span class="pre">sendto()</span></code>, <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code> or
<code class="docutils literal notranslate"><span class="pre">send()</span></code> syscalls. Using <code class="docutils literal notranslate"><span class="pre">sendto()</span></code> as the primary example:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="n">addr</span><span class="p">;</span>
<span class="kt">char</span><span class="w"> </span><span class="n">buf</span><span class="p">[</span><span class="mi">14</span><span class="p">];</span>
<span class="kt">ssize_t</span><span class="w"> </span><span class="n">len</span><span class="p">;</span>
<span class="cm">/* set message destination */</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_family</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">AF_MCTP</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_network</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_addr</span><span class="p">.</span><span class="n">s_addr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">8</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_tag</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_TAG_OWNER</span><span class="p">;</span>
<span class="n">addr</span><span class="p">.</span><span class="n">smctp_type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_TYPE_ECHO</span><span class="p">;</span>
<span class="cm">/* arbitrary message to send, with message-type header */</span>
<span class="n">buf</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MCTP_TYPE_ECHO</span><span class="p">;</span>
<span class="n">memcpy</span><span class="p">(</span><span class="n">buf</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"hello, world!"</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">)</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">sendto</span><span class="p">(</span><span class="n">sd</span><span class="p">,</span><span class="w"> </span><span class="n">buf</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">),</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span>
<span class="w"> </span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="o">&</span><span class="n">addr</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">addr</span><span class="p">));</span>
</pre></div>
</div>
<p>The network and address fields of <code class="docutils literal notranslate"><span class="pre">addr</span></code> define the remote address to send to.
If <code class="docutils literal notranslate"><span class="pre">smctp_tag</span></code> has the <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code>, the kernel will ignore any bits set
in <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_VALUE</span></code>, and generate a tag value suitable for the destination
EID. If <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code> is not set, the message will be sent with the tag
value as specified. If a tag value cannot be allocated, the system call will
report an errno of <code class="docutils literal notranslate"><span class="pre">EAGAIN</span></code>.</p>
<p>The application must provide the message type byte as the first byte of the
message buffer passed to <code class="docutils literal notranslate"><span class="pre">sendto()</span></code>. If a message integrity check is to be
included in the transmitted message, it must also be provided in the message
buffer, and the most-significant bit of the message type byte must be 1.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code> system call allows a more compact argument interface, and the
message buffer to be specified as a scatter-gather list. At present no ancillary
message types (used for the <code class="docutils literal notranslate"><span class="pre">msg_control</span></code> data passed to <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code>) are
defined.</p>
<p>Transmitting a message on an unconnected socket with <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code>
specified will cause an allocation of a tag, if no valid tag is already
allocated for that destination. The (destination-eid,tag) tuple acts as an
implicit local socket address, to allow the socket to receive responses to this
outgoing message. If any previous allocation has been performed (to for a
different remote EID), that allocation is lost.</p>
<p>Sockets will only receive responses to requests they have sent (with TO=1) and
may only respond (with TO=0) to requests they have received.</p>
</section>
<section id="recvfrom-recvmsg-recv-receive-an-mctp-message">
<h4><code class="docutils literal notranslate"><span class="pre">recvfrom()</span></code>, <code class="docutils literal notranslate"><span class="pre">recvmsg()</span></code>, <code class="docutils literal notranslate"><span class="pre">recv()</span></code> : receive an MCTP message<a class="headerlink" href="#recvfrom-recvmsg-recv-receive-an-mctp-message" title="Link to this heading">¶</a></h4>
<p>An MCTP message can be received by an application using one of the
<code class="docutils literal notranslate"><span class="pre">recvfrom()</span></code>, <code class="docutils literal notranslate"><span class="pre">recvmsg()</span></code>, or <code class="docutils literal notranslate"><span class="pre">recv()</span></code> system calls. Using <code class="docutils literal notranslate"><span class="pre">recvfrom()</span></code>
as the primary example:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="n">addr</span><span class="p">;</span>
<span class="kt">socklen_t</span><span class="w"> </span><span class="n">addrlen</span><span class="p">;</span>
<span class="kt">char</span><span class="w"> </span><span class="n">buf</span><span class="p">[</span><span class="mi">14</span><span class="p">];</span>
<span class="kt">ssize_t</span><span class="w"> </span><span class="n">len</span><span class="p">;</span>
<span class="n">addrlen</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">addr</span><span class="p">);</span>
<span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">recvfrom</span><span class="p">(</span><span class="n">sd</span><span class="p">,</span><span class="w"> </span><span class="n">buf</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">),</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span>
<span class="w"> </span><span class="p">(</span><span class="k">struct</span><span class="w"> </span><span class="nc">sockaddr_mctp</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="o">&</span><span class="n">addr</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">addrlen</span><span class="p">);</span>
<span class="cm">/* We can expect addr to describe an MCTP address */</span>
<span class="n">assert</span><span class="p">(</span><span class="n">addrlen</span><span class="w"> </span><span class="o">>=</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">buf</span><span class="p">));</span>
<span class="n">assert</span><span class="p">(</span><span class="n">addr</span><span class="p">.</span><span class="n">smctp_family</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">AF_MCTP</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"received %zd bytes from remote EID %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">rc</span><span class="p">,</span><span class="w"> </span><span class="n">addr</span><span class="p">.</span><span class="n">smctp_addr</span><span class="p">);</span>
</pre></div>
</div>
<p>The address argument to <code class="docutils literal notranslate"><span class="pre">recvfrom</span></code> and <code class="docutils literal notranslate"><span class="pre">recvmsg</span></code> is populated with the
remote address of the incoming message, including tag value (this will be needed
in order to reply to the message).</p>
<p>The first byte of the message buffer will contain the message type byte. If an
integrity check follows the message, it will be included in the received buffer.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">recv()</span></code> system call behaves in a similar way, but does not provide a
remote address to the application. Therefore, these are only useful if the
remote address is already known, or the message does not require a reply.</p>
<p>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.</p>
</section>
<section id="ioctl-siocmctpalloctag-and-ioctl-siocmctpdroptag">
<h4><code class="docutils literal notranslate"><span class="pre">ioctl(SIOCMCTPALLOCTAG)</span></code> and <code class="docutils literal notranslate"><span class="pre">ioctl(SIOCMCTPDROPTAG)</span></code><a class="headerlink" href="#ioctl-siocmctpalloctag-and-ioctl-siocmctpdroptag" title="Link to this heading">¶</a></h4>
<p>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 <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code> time.</p>
<p>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.</p>
<p>Both ioctls are passed a pointer to a <code class="docutils literal notranslate"><span class="pre">struct</span> <span class="pre">mctp_ioc_tag_ctl</span></code>:</p>
<div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span><span class="w"> </span><span class="nc">mctp_ioc_tag_ctl</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">mctp_eid_t</span><span class="w"> </span><span class="n">peer_addr</span><span class="p">;</span>
<span class="w"> </span><span class="n">__u8</span><span class="w"> </span><span class="n">tag</span><span class="p">;</span>
<span class="w"> </span><span class="n">__u16</span><span class="w"> </span><span class="n">flags</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">SIOCMCTPALLOCTAG</span></code> allocates a tag for a specific peer, which an application
can use in future <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code> calls. The application populates the
<code class="docutils literal notranslate"><span class="pre">peer_addr</span></code> member with the remote EID. Other fields must be zero.</p>
<p>On return, the <code class="docutils literal notranslate"><span class="pre">tag</span></code> member will be populated with the allocated tag value.
The allocated tag will have the following tag bits set:</p>
<blockquote>
<div><ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code>: it only makes sense to allocate tags if you’re the tag
owner</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">MCTP_TAG_PREALLOC</span></code>: to indicate to <code class="docutils literal notranslate"><span class="pre">sendmsg()</span></code> that this is a
preallocated tag.</p></li>
<li><p>... and the actual tag value, within the least-significant three bits
(<code class="docutils literal notranslate"><span class="pre">MCTP_TAG_MASK</span></code>). Note that zero is a valid tag value.</p></li>
</ul>
</div></blockquote>
<p>The tag value should be used as-is for the <code class="docutils literal notranslate"><span class="pre">smctp_tag</span></code> member of <code class="docutils literal notranslate"><span class="pre">struct</span>
<span class="pre">sockaddr_mctp</span></code>.</p>
<p><code class="docutils literal notranslate"><span class="pre">SIOCMCTPDROPTAG</span></code> releases a tag that has been previously allocated by a
<code class="docutils literal notranslate"><span class="pre">SIOCMCTPALLOCTAG</span></code> ioctl. The <code class="docutils literal notranslate"><span class="pre">peer_addr</span></code> must be the same as used for the
allocation, and the <code class="docutils literal notranslate"><span class="pre">tag</span></code> value must match exactly the tag returned from the
allocation (including the <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_OWNER</span></code> and <code class="docutils literal notranslate"><span class="pre">MCTP_TAG_PREALLOC</span></code> bits).
The <code class="docutils literal notranslate"><span class="pre">flags</span></code> field must be zero.</p>
</section>
</section>
</section>
<section id="kernel-internals">
<h2>Kernel internals<a class="headerlink" href="#kernel-internals" title="Link to this heading">¶</a></h2>
<p>There are a few possible packet flows in the MCTP stack:</p>
<ol class="arabic">
<li><p>local TX to remote endpoint, message <= MTU:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>sendmsg()
-> mctp_local_output()
: route lookup
-> rt->output() (== mctp_route_output)
-> dev_queue_xmit()
</pre></div>
</div>
</li>
<li><p>local TX to remote endpoint, message > MTU:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>sendmsg()
-> mctp_local_output()
-> mctp_do_fragment_route()
: creates packet-sized skbs. For each new skb:
-> rt->output() (== mctp_route_output)
-> dev_queue_xmit()
</pre></div>
</div>
</li>
<li><p>remote TX to local endpoint, single-packet message:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>mctp_pkttype_receive()
: route lookup
-> rt->output() (== mctp_route_input)
: sk_key lookup
-> sock_queue_rcv_skb()
</pre></div>
</div>
</li>
<li><p>remote TX to local endpoint, multiple-packet message:</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span>mctp_pkttype_receive()
: route lookup
-> rt->output() (== mctp_route_input)
: sk_key lookup
: stores skb in struct sk_key->reasm_head
mctp_pkttype_receive()
: route lookup
-> rt->output() (== mctp_route_input)
: sk_key lookup
: finds existing reassembly in sk_key->reasm_head
: appends new fragment
-> sock_queue_rcv_skb()
</pre></div>
</div>
</li>
</ol>
<section id="key-refcounts">
<h3>Key refcounts<a class="headerlink" href="#key-refcounts" title="Link to this heading">¶</a></h3>
<blockquote>
<div><ul class="simple">
<li><p>keys are refed by:</p>
<ul>
<li><p>a skb: during route output, stored in <code class="docutils literal notranslate"><span class="pre">skb->cb</span></code>.</p></li>
<li><p>netns and sock lists.</p></li>
</ul>
</li>
<li><p>keys can be associated with a device, in which case they hold a
reference to the dev (set through <code class="docutils literal notranslate"><span class="pre">key->dev</span></code>, counted through
<code class="docutils literal notranslate"><span class="pre">dev->key_count</span></code>). Multiple keys can reference the device.</p></li>
</ul>
</div></blockquote>
</section>
</section>
</section>
</div>
</div>
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