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Read time: 3 minute(s)
< / 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，
< 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:
loopback_bind()：
< 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)
{
< em class = "hl-comment" > /* (1) 从 gadget 中分配 2 个 bulk endpoint */< / em >
< em class = "hl-comment" > /* allocate endpoints */< / em >
loop-> in_ep = usb_ep_autoconfig(cdev-> gadget, & fs_loop_source_desc);
loop-> out_ep = usb_ep_autoconfig(cdev-> gadget, & fs_loop_sink_desc);
}
loopback_set_alt() → enable_loopback() → alloc_requests():
< strong class = "hl-keyword" > static< / strong > < strong class = "hl-keyword" > int< / strong > alloc_requests(< strong class = "hl-keyword" > struct< / strong > usb_composite_dev *cdev,
< strong class = "hl-keyword" > struct< / strong > f_loopback *loop)
{
< strong class = "hl-keyword" > for< / strong > (i = < span class = "hl-number" > 0< / span > ; i < loop-> qlen & & result == < span class = "hl-number" > 0< / span > ; i++) {
result = -ENOMEM;
in_req = usb_ep_alloc_request(loop-> in_ep, GFP_ATOMIC);
< strong class = "hl-keyword" > if< / strong > (!in_req)
< strong class = "hl-keyword" > goto< / strong > fail;
out_req = lb_alloc_ep_req(loop-> out_ep, loop-> buflen);
< strong class = "hl-keyword" > if< / strong > (!out_req)
< strong class = "hl-keyword" > goto< / strong > fail_in;
in_req-> complete = loopback_complete;
out_req-> complete = loopback_complete;
in_req-> buf = out_req-> buf;
< em class = "hl-comment" > /* length will be set in complete routine */< / em >
in_req-> context = out_req;
out_req-> context = in_req;
< em class = "hl-comment" > /* (2) 先启动 OUT endpoint */< / em >
result = usb_ep_queue(loop-> out_ep, out_req, GFP_ATOMIC);
< strong class = "hl-keyword" > if< / strong > (result) {
ERROR(cdev, < span class = "hl-string" > "%s queue req --> %d\n"< / span > ,
loop-> out_ep-> name, result);
< strong class = "hl-keyword" > goto< / strong > fail_out;
}
}
}
< 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)
{
< strong class = "hl-keyword" > struct< / strong > f_loopback *loop = ep-> driver_data;
< strong class = "hl-keyword" > struct< / strong > usb_composite_dev *cdev = loop-> function.config-> cdev;
< strong class = "hl-keyword" > int< / strong > status = req-> status;
< strong class = "hl-keyword" > switch< / strong > (status) {
< strong class = "hl-keyword" > case< / strong > < span class = "hl-number" > 0< / span > : < em class = "hl-comment" > /* normal completion? */< / em >
< strong class = "hl-keyword" > if< / strong > (ep == loop-> out_ep) {
< em class = "hl-comment" > /*
* We received some data from the host so let's
* queue it so host can read the from our in ep
*/< / em >
< strong class = "hl-keyword" > struct< / strong > usb_request *in_req = req-> context;
in_req-> zero = (req-> actual < req-> length);
in_req-> length = req-> actual;
ep = loop-> in_ep;
req = in_req;
} < strong class = "hl-keyword" > else< / strong > {
< em class = "hl-comment" > /*
* We have just looped back a bunch of data
* to host. Now let's wait for some more data.
*/< / em >
req = req-> context;
ep = loop-> out_ep;
}
< em class = "hl-comment" > /* (3) 环回的关键：
OUT endpoint 接收到的数据 转发到 IN endpoint
IN endpoint 数据发送完成后 req 重新挂载到 OUT endpoint
*/< / em >
< em class = "hl-comment" > /* queue the buffer back to host or for next bunch of data */< / em >
status = usb_ep_queue(ep, req, GFP_ATOMIC);
}
< / 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 > /
bulk_buflen qlen< / pre > < / div > < / div > < / article > < / main > < / div >
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