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<div class=" wh_topic_content body "><main role="main"><article class="- topic/topic concept/concept topic concept" role="article" aria-labelledby="ariaid-title1"><span class="edit-link" style="font-size:12px; opacity:0.6; text-align:right; vertical-align:middle"><a target="_blank" href="http://172.16.35.88/tasks/jdssno1uvvbf2mltu9kb9v3if05d5gopuakboe8hlud18rma/edit/F:/aicdita/aicdita-cn/topics/sdk/usb/usb-loopback_function.dita">Edit online</a></span><h1 class="- topic/title title topictitle1" id="ariaid-title1">Loopback Function</h1><div class="date inPage">31 Jan 2024</div><div style="color: gray;">
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Read time: 3 minute(s)
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</div><div class="- topic/body concept/conbody body conbody"><p class="- topic/p p" data-ofbid="d82164e22__20250121171526"><code class="+ topic/ph pr-d/codeph ph codeph">Loopback Function</code> 提供的功能更为简单，它分配了两个 bulk endpoint，所做的就是把
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<code class="+ topic/ph pr-d/codeph ph codeph">out_ep</code> 接收到的数据 转发到 <code class="+ topic/ph pr-d/codeph ph codeph">in_ep</code>。</p><div class="- topic/p p" data-ofbid="d82164e32__20250121171526">主要流程如下：<pre class="+ topic/pre pr-d/codeblock pre codeblock language-c" id="usb_loopback_function__pre_lvf_bkz_21c" data-ofbid="usb_loopback_function__pre_lvf_bkz_21c">drivers\usb\gadget\function\f_loopback.c:
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loopback_bind()：
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<strong class="hl-keyword">static</strong> <strong class="hl-keyword">int</strong> loopback_bind(<strong class="hl-keyword">struct</strong> usb_configuration *c, <strong class="hl-keyword">struct</strong> usb_function *f)
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{
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<em class="hl-comment">/* (1) 从 gadget 中分配 2 个 bulk endpoint */</em>
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<em class="hl-comment">/* allocate endpoints */</em>
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loop->in_ep = usb_ep_autoconfig(cdev->gadget, &fs_loop_source_desc);
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loop->out_ep = usb_ep_autoconfig(cdev->gadget, &fs_loop_sink_desc);
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}
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loopback_set_alt() → enable_loopback() → alloc_requests():
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<strong class="hl-keyword">static</strong> <strong class="hl-keyword">int</strong> alloc_requests(<strong class="hl-keyword">struct</strong> usb_composite_dev *cdev,
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<strong class="hl-keyword">struct</strong> f_loopback *loop)
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{
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<strong class="hl-keyword">for</strong> (i = <span class="hl-number">0</span>; i < loop->qlen && result == <span class="hl-number">0</span>; i++) {
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result = -ENOMEM;
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in_req = usb_ep_alloc_request(loop->in_ep, GFP_ATOMIC);
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<strong class="hl-keyword">if</strong> (!in_req)
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<strong class="hl-keyword">goto</strong> fail;
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out_req = lb_alloc_ep_req(loop->out_ep, loop->buflen);
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<strong class="hl-keyword">if</strong> (!out_req)
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<strong class="hl-keyword">goto</strong> fail_in;
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in_req->complete = loopback_complete;
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out_req->complete = loopback_complete;
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in_req->buf = out_req->buf;
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<em class="hl-comment">/* length will be set in complete routine */</em>
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in_req->context = out_req;
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out_req->context = in_req;
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<em class="hl-comment">/* (2) 先启动 OUT endpoint */</em>
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result = usb_ep_queue(loop->out_ep, out_req, GFP_ATOMIC);
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<strong class="hl-keyword">if</strong> (result) {
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ERROR(cdev, <span class="hl-string">"%s queue req --> %d\n"</span>,
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loop->out_ep->name, result);
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<strong class="hl-keyword">goto</strong> fail_out;
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}
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}
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}
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<strong class="hl-keyword">static</strong> <strong class="hl-keyword">void</strong> loopback_complete(<strong class="hl-keyword">struct</strong> usb_ep *ep, <strong class="hl-keyword">struct</strong> usb_request *req)
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{
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<strong class="hl-keyword">struct</strong> f_loopback *loop = ep->driver_data;
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<strong class="hl-keyword">struct</strong> usb_composite_dev *cdev = loop->function.config->cdev;
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<strong class="hl-keyword">int</strong> status = req->status;
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<strong class="hl-keyword">switch</strong> (status) {
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<strong class="hl-keyword">case</strong> <span class="hl-number">0</span>: <em class="hl-comment">/* normal completion? */</em>
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<strong class="hl-keyword">if</strong> (ep == loop->out_ep) {
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<em class="hl-comment">/*
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* We received some data from the host so let's
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* queue it so host can read the from our in ep
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*/</em>
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<strong class="hl-keyword">struct</strong> usb_request *in_req = req->context;
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in_req->zero = (req->actual < req->length);
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in_req->length = req->actual;
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ep = loop->in_ep;
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req = in_req;
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} <strong class="hl-keyword">else</strong> {
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<em class="hl-comment">/*
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* We have just looped back a bunch of data
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* to host. Now let's wait for some more data.
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*/</em>
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req = req->context;
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ep = loop->out_ep;
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}
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<em class="hl-comment">/* (3) 环回的关键：
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OUT endpoint 接收到的数据 转发到 IN endpoint
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IN endpoint 数据发送完成后 req 重新挂载到 OUT endpoint
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*/</em>
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<em class="hl-comment">/* queue the buffer back to host or for next bunch of data */</em>
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status = usb_ep_queue(ep, req, GFP_ATOMIC);
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}
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</pre></div><div class="- topic/p p" data-ofbid="d82164e36__20250121171526">也支持一些参数调整：<pre class="+ topic/pre pr-d/codeblock pre codeblock language-c" id="usb_loopback_function__codeblock_hsw_hkz_21c" data-ofbid="usb_loopback_function__codeblock_hsw_hkz_21c"># ls functions/Loopback.<span class="hl-number">0</span>/
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bulk_buflen qlen</pre></div></div></article></main></div>
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