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某通信模块设备,通过USB提供RDNIS和ECM网卡功能。在实际应用中发现,USB RNDIS网卡模式下,当使用AT指令以不同的CID拨号的时候,在Windows主机上能正常拨号成功,但在Linux主机上却会发生拨号失败的情况。作为对比,在同样的测试环境和测试方法下,USB ECM网卡则没有这种异常。
测试流程概括如下:
- 设备侧已经配置为USB RNDIS模式
- 主机侧通过AT指令以CID1拨号成功
- 测试网络功能,主机和设备侧可以ping通
- 主机侧通过AT指令断开CID1拨号
- 主机侧通过AT指令以CID2拨号失败,主机和设备侧不能ping通
以上是问题背景。
USB ECM是USB-IF定义的CDC类规范下的一个子类规范,全称Ethernet Networking Control Model;RNDIS是微软为即插即用的以太网设备制定的一种规范。实现这两种协议的USB设备,通过USB线接入主机后,会在主机侧和设备侧各生成一张网卡。两侧的网卡处在同一个网段,进行网络通信,数据承载通路是USB。下图是从微软官网摘抄的RNDIS框架图:
https://docs.microsoft/en-us/windows-hardware/drivers/network/overview-of-remote-ndis--rndis-
经调查,Linux主机上RNDIS拨号测试失败,主要原因在于:当第一次拨号成功后,断开拨号时,Linux主机上的USB网卡IP地址并没有消失,后续以不同CID拨号后,Linux也没有发起DHCP请求包,DHCP过程失败,IP地址未更新。此时,如果将USB拔插一下就可恢复正常。
通过AT指令通知设备端断开拨号时,设备侧会有down USB网卡的动作,down USB网卡的过程中,设备侧RNDIS会上报rndis disconnect消息来通知主机侧。主机侧可以根据这个消息做相应处理。
在Ubuntu主机和Windows主机上测试断开拨号操作时,在设备端抓取的kernel log片段如下。可以看到,无论是在Windows主机上还是Ubuntu主机上,设备端确实在断开拨号时上报了rndis disconnect消息。
Ubuntu主机环境,设备端log:
root@udx710-module:~#
[ 324.516525] c0 configfs-gadget gadget: rndis_close
[ 324.521239] c0 rndis_set_param_medium: 0 0
[ 324.525296] c0 rndis_signal_disconnect
[ 324.529023] c0 rndis_indicate_status_msg: status 1073807372
Windows主机环境,设备端log:
root@udx710-module:~#
[ 191.340507] c1 configfs-gadget gadget: rndis_close
[ 191.345223] c1 rndis_set_param_medium: 0 0
[ 191.349285] c1 rndis_signal_disconnect
[ 191.353008] c1 rndis_indicate_status_msg: status 1073807372
[ 191.364621] c1 configfs-gadget gadget: rndis reqa1.01 v0000 i0000 l4096
设备端动作
设备端关键代码如下:
rndis_close -> rndis_signal_disconnect -> rndis_indicate_status_msg
drivers/usb/gadget/function/rndis.c
int rndis_signal_disconnect(struct rndis_params *params)
{
params->media_state = RNDIS_MEDIA_STATE_DISCONNECTED;
return rndis_indicate_status_msg(params, RNDIS_STATUS_MEDIA_DISCONNECT);
}
/*
* Device to Host Comunication
*/
static int rndis_indicate_status_msg(struct rndis_params *params, u32 status)
{
rndis_indicate_status_msg_type *resp;
rndis_resp_t *r;
if (params->state == RNDIS_UNINITIALIZED)
return -ENOTSUPP;
r = rndis_add_response(params, sizeof(rndis_indicate_status_msg_type));
if (!r)
return -ENOMEM;
resp = (rndis_indicate_status_msg_type *)r->buf;
resp->MessageType = cpu_to_le32(RNDIS_MSG_INDICATE);
resp->MessageLength = cpu_to_le32(20);
resp->Status = cpu_to_le32(status);
resp->StatusBufferLength = cpu_to_le32(0);
resp->StatusBufferOffset = cpu_to_le32(0);
params->resp_avail(params->v);
return 0;
}
drivers/usb/gadget/function/f_rndis.c
static void rndis_response_available(void *_rndis)
{
struct f_rndis *rndis = _rndis;
struct usb_request *req = rndis->notify_req;
struct usb_composite_dev *cdev = rndis->port.func.config->cdev;
__le32 *data = req->buf;
int status;
if (atomic_inc_return(&rndis->notify_count) != 1)
return;
/* Send RNDIS RESPONSE_AVAILABLE notification; a
* USB_CDC_NOTIFY_RESPONSE_AVAILABLE "should" work too
*
* This is the only notification defined by RNDIS.
*/
data[0] = cpu_to_le32(1);
data[1] = cpu_to_le32(0);
status = usb_ep_queue(rndis->notify, req, GFP_ATOMIC);
if (status) {
atomic_dec(&rndis->notify_count);
DBG(cdev, "notify/0 --> %d
", status);
}
}
rndis_indicate_status_msg 会做两件事:
1)把RNDIS_MSG_INDICATE message放入response queue,message中带了status(RNDIS_STATUS_MEDIA_DISCONNECT)。
2)调用 resp_avail 发 RESPONSE_AVAILABLE notification(最终调用到rndis_response_available @ f_rndis.c),通过interrupt IN端点发出去,这个数据中没有包含rndis连接状态信息,只是单纯的上报RESPONSE_AVAILABLE (0x00000001)。
主机端动作
1)按微软的RNDIS规范,Host收到RESPONSE_AVAILABLE后,接下来需要往control端点发 GET_ENCAPSULATED_RESPONSE request 才能读取设备端的 response,得到rndis连接状态。
Upon receiving the RESPONSE_AVAILABLE notification, the host reads the control message from the Control endpoint using a GET_ENCAPSULATED_RESPONSE transfer, defined in the following table.
https://docs.microsoft/en-us/windows-hardware/drivers/network/control-channel-characteristics
BmRequestType bRequest wValue wIndex 0xA1 0x01 0x0000
2)再看rndis host驱动:
rndis_status @ rndis_host.c 函数没有做实际动作。
drivers/net/usb/rndis_host.c
void rndis_status(struct usbnet *dev, struct urb *urb)
{
netdev_dbg(dev->net, "rndis status urb, len %d stat %d
",
urb->actual_length, urb->status);
// FIXME for keepalives, respond immediately (asynchronously)
// if not an RNDIS status, do like cdc_status(dev,urb) does
}
static const struct driver_info rndis_info = {
.description = "RNDIS device",
.flags = FLAG_ETHER | FLAG_POINTTOPOINT | FLAG_FRAMING_RN | FLAG_NO_SETINT,
.bind = rndis_bind,
.unbind = rndis_unbind,
.status = rndis_status,
.rx_fixup = rndis_rx_fixup,
.tx_fixup = rndis_tx_fixup,
};
也就是说,Linux主机RNDIS驱动没有严格按照RNDIS协议流程去读取RNDIS_STATUS_MEDIA_DISCONNECT消息,导致它无法获知设备端RNDIS网卡断开的状态,进而无法正确作出网络状态改变的相关处理。
而且作者也在代码注释中表明了态度:强烈建议不要使用RNDIS,而应使用CDC以太网(ECM,NCM,EEM等)这类非专有(non-proprietary)的替代方案。USB CDC规范是USB-IF制定的,RNDIS是微软制定的。
/*
* RNDIS is NDIS remoted over USB. It's a MSFT variant of CDC ACM ... of
* course ACM was intended for modems, not Ethernet links! USB's standard
* for Ethernet links is "CDC Ethernet", which is significantly simpler.
*
* NOTE that Microsoft's "RNDIS 1.0" specification is incomplete. Issues
* include:
* - Power management in particular relies on information that's scattered
* through other documentation, and which is incomplete or incorrect even
* there.
* - There are various undocumented protocol requirements, such as the
* need to send unused garbage in control-OUT messages.
* - In some cases, MS-Windows will emit undocumented requests; this
* matters more to peripheral implementations than host ones.
*
* Moreover there's a no-open-specs variant of RNDIS called "ActiveSync".
*
* For these reasons and others, ** USE OF RNDIS IS STRONGLY DISCOURAGED ** in
* favor of such non-proprietary alternatives as CDC Ethernet or the newer (and
* currently rare) "Ethernet Emulation Model" (EEM).
*/
最后顺便看看为什么ECM没有问题。
ECM设备侧,ecm_close -> ecm_notify -> ecm_do_notify -> 通过interrupt IN端点发出去,这个数据中直接带了ecm连接状态,无需Host专门再发另外的request读取这个状态。
static void ecm_do_notify(struct f_ecm *ecm)
{
struct usb_request *req = ecm->notify_req;
struct usb_cdc_notification *event;
...
event = req->buf;
switch (ecm->notify_state) {
...
case ECM_NOTIFY_CONNECT:
event->bNotificationType = USB_CDC_NOTIFY_NETWORK_CONNECTION;
if (ecm->is_open)
event->wValue = cpu_to_le16(1);
else
event->wValue = cpu_to_le16(0);
event->wLength = 0;
req->length = sizeof *event;
DBG(cdev, "notify connect %s
",
ecm->is_open ? "true" : "false");
ecm->notify_state = ECM_NOTIFY_SPEED;
break;
...
}
ECM主机侧,usbnet_cdc_status @ cdc_ether.c 函数中有处理ecm connection消息并调用usbnet_link_change。
void usbnet_cdc_status(struct usbnet *dev, struct urb *urb)
{
struct usb_cdc_notification *event;
...
event = urb->transfer_buffer;
switch (event->bNotificationType) {
case USB_CDC_NOTIFY_NETWORK_CONNECTION:
netif_dbg(dev, timer, dev->net, "CDC: carrier %s
",
event->wValue ? "on" : "off");
usbnet_link_change(dev, !!event->wValue, 0);
break;
...
}
}
static const struct driver_info cdc_info = {
.description = "CDC Ethernet Device",
.flags = FLAG_ETHER | FLAG_POINTTOPOINT,
.bind = usbnet_cdc_bind,
.unbind = usbnet_cdc_unbind,
.status = usbnet_cdc_status,
.set_rx_mode = usbnet_cdc_update_filter,
.manage_power = usbnet_manage_power,
};
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