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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-host-ehci_internal_structure.dita">Edit online</a></span><h1 class="- topic/title title topictitle1" id="ariaid-title1">EHCI 内部结构</h1><div class="date inPage">28 May 2024</div><div style="color: gray;">
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Read time: 1 minute(s)
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</div><div class="- topic/body concept/conbody body conbody"><div class="- topic/p p" data-ofbid="d148513e22__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_f3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule.png" alt="image8"/></div><br/></div><p class="- topic/p p" data-ofbid="d148513e26__20250121171610">EHCI 的核心就是把数据传输分成了两类来进行调度：</p><ul class="- topic/ul ul" id="usb_host_ehci_internal_structure__ul_g3x_lpz_21c" data-ofbid="usb_host_ehci_internal_structure__ul_g3x_lpz_21c"><li class="- topic/li li" data-ofbid="d148513e29__20250121171610">
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<p class="- topic/p p" data-ofbid="d148513e31__20250121171610"><code class="+ topic/ph pr-d/codeph ph codeph">Asynchronous Schedule</code>。用来传输对时间延迟要求不高的 Endpoint 数据，包括
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<code class="+ topic/ph pr-d/codeph ph codeph">Control Transfer</code> 和 <code class="+ topic/ph pr-d/codeph ph codeph">Bulk Transfer</code>。</p>
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</li><li class="- topic/li li" data-ofbid="d148513e42__20250121171610">
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<p class="- topic/p p" data-ofbid="d148513e44__20250121171610"><code class="+ topic/ph pr-d/codeph ph codeph">Periodic Schedule</code>。用来传输对时间延迟要求高的 Endpoint 数据，包括
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<code class="+ topic/ph pr-d/codeph ph codeph">Isochronous Transfer</code> 和 <code class="+ topic/ph pr-d/codeph ph codeph">Interrupt
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Transfer</code>。</p>
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</li></ul><section class="- topic/section section" id="usb_host_ehci_internal_structure__section_h3x_lpz_21c" data-ofbid="usb_host_ehci_internal_structure__section_h3x_lpz_21c"><h2 class="- topic/title title sectiontitle">Asynchronous Queue Schedule</h2>
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<div class="- topic/p p" data-ofbid="d148513e60__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_i3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_async.png" alt="image9"/></div><br/></div>
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<p class="- topic/p p" data-ofbid="d148513e65__20250121171610"><code class="+ topic/ph pr-d/codeph ph codeph">Asynchronous Schedule</code> 内部实现非常的简单就只有一级链表，链表中只有 <code class="+ topic/ph pr-d/codeph ph codeph">Queue
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Head</code> 类型的描述符。每个时间片内传输完 Period 数据以后，再尽可能的传输 Asynchronous 数据即可。</p>
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<p class="- topic/p p" data-ofbid="d148513e73__20250121171610">核心的描述符如下：</p>
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<ul class="- topic/ul ul" id="usb_host_ehci_internal_structure__ul_j3x_lpz_21c" data-ofbid="usb_host_ehci_internal_structure__ul_j3x_lpz_21c"><li class="- topic/li li" data-ofbid="d148513e77__20250121171610"><p class="- topic/p p" data-ofbid="d148513e78__20250121171610">Queue Head</p><div class="- topic/p p" data-ofbid="d148513e80__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_k3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_qh.png" alt="image10"/></div><br/></div></li><li class="- topic/li li" data-ofbid="d148513e84__20250121171610"><p class="- topic/p p" data-ofbid="d148513e85__20250121171610">Queue Element Transfer Descriptor (qTD)</p><div class="- topic/p p" data-ofbid="d148513e87__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_m3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_qtd.png" alt="image11"/></div><br/></div></li></ul>
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</section><section class="- topic/section section" id="usb_host_ehci_internal_structure__section_n3x_lpz_21c" data-ofbid="usb_host_ehci_internal_structure__section_n3x_lpz_21c"><h2 class="- topic/title title sectiontitle">Periodic Queue Schedule</h2>
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<div class="- topic/p p" data-ofbid="d148513e97__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_o3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_period.png" alt="image12"/></div><br/></div>
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<p class="- topic/p p" data-ofbid="d148513e102__20250121171610"><code class="+ topic/ph pr-d/codeph ph codeph">Periodic Schedule</code> 内部实现如上图所示，核心是两级链表：</p>
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<ul class="- topic/ul ul" id="usb_host_ehci_internal_structure__ul_p3x_lpz_21c" data-ofbid="usb_host_ehci_internal_structure__ul_p3x_lpz_21c"><li class="- topic/li li" data-ofbid="d148513e108__20250121171610"><p class="- topic/p p" data-ofbid="d148513e109__20250121171610">第一级链表如上图 <code class="+ topic/ph pr-d/codeph ph codeph">绿色</code> 所示。是各种传输结构的实际描述符，主要包含以下几种类型的描述符：</p><div class="- topic/p p" data-ofbid="d148513e114__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_q3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_period_type.png" alt="image13"/></div><br/></div></li><li class="- topic/li li" data-ofbid="d148513e118__20250121171610"><p class="- topic/p p" data-ofbid="d148513e119__20250121171610">第二级链表如上图 <code class="+ topic/ph pr-d/codeph ph codeph">橙色</code> 所示。是一个指针数组，数组中保存的是指向第一级链表的指针。这里每个数组成员代表一个时间分片
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Frame/Micro-Frame 的起始位置，每个时间片会根据指针传输第一级链表中的数据，直到第一级链表的结尾。指针的格式如下：</p><div class="- topic/p p" data-ofbid="d148513e124__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_s3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_period_framelist.png" alt="image14"/></div><br/></div><p class="- topic/p p" data-ofbid="d148513e128__20250121171610">这里的调度思想就是：第一级链表是一个传输数据全集，第二级链表决定了某个时间片里要传输的数据。</p><p class="- topic/p p" data-ofbid="d148513e130__20250121171610">这样合理的安排二级链表的指针，比如间隔
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8 次指向同一位置这部分数据的 interval 就是 8，间隔 4 次指向同一位置这部分数据的 interval 就是 4。 第一级链表也是要根据
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interval 排序的。</p><p class="- topic/p p" data-ofbid="d148513e132__20250121171610"><code class="+ topic/ph pr-d/codeph ph codeph">Periodic Schedule</code> 核心的描述符除了 QH、QTD 还有
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ITD：</p></li><li class="- topic/li li" data-ofbid="d148513e136__20250121171610"><p class="- topic/p p" data-ofbid="d148513e137__20250121171610">Isochronous (High-Speed) Transfer Descriptor (iTD)</p><div class="- topic/p p" data-ofbid="d148513e139__20250121171610"><br/><div class="imagecenter"><img class="- topic/image image imagecenter" id="usb_host_ehci_internal_structure__image_u3x_lpz_21c" src="../../../images/usb/usb_ehci_schedule_itd.png" alt="image15"/></div><br/></div></li></ul>
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<div class=" wh_topic_toc "><div class="wh_topic_label">在本页上</div><ul><li class="section-item"><div class="section-title"><a href="#usb_host_ehci_internal_structure__section_h3x_lpz_21c" data-tocid="usb_host_ehci_internal_structure__section_h3x_lpz_21c">Asynchronous Queue Schedule</a></div></li><li class="section-item"><div class="section-title"><a href="#usb_host_ehci_internal_structure__section_n3x_lpz_21c" data-tocid="usb_host_ehci_internal_structure__section_n3x_lpz_21c">Periodic Queue Schedule</a></div></li></ul></div>
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