我在使用 cdc_acm 驱动程序在 USB 端口上发送意外字符时遇到问题。更令人困惑的是，代码在 Ubuntu 12.04（3.2 内核）上运行良好，但在 Centos 6（3.6 内核）上失败（这个问题的主题）。
USB 设备是 Bluegiga BLED112 蓝牙智能适配器。其嵌入式微控制器将在其 USB 接口上出现意外输入时随时重置。
测试代码打开端口，写入 4 个字节（一条 hello 消息）并期望读取响应。读取永远不会完成，因为意外字符会导致设备重置，从而导致集线器丢弃设备并重新枚举。
为了排除故障，我做了以下操作：
- 下载 cdc_acm 驱动程序的源代码。添加了一堆 printk 调试消息和 stack_dumps 以跟踪发生的情况。
- 我对“库存”cdc_acm 进行了 rmmod，并对我的仪表模块进行了 insmod。所有设备枚举都有效，附加了正确的驱动程序等。
- 由于代码适用于 Ubuntu 12.04/Linux 3.2，因此我获取了 3.2 cdc_acm 代码并在 CentOS 6 / Linux 3.6 平台上编译了该模块。使用 3.2 模块代替 3.6 模块没有什么区别。我恢复到 3.6 模块。
- 使用 usbmon 打开调试文件系统并观察 USB 流量。我可以看到 USB 接口上发送了额外的字符。
- 为了观察发生了什么，在 cdc_acm 模块中的 printk 之上，我合并了 usb mon (cat /sys/kernel/debug/usb/usbmon/3u | logger) 的输出和测试应用程序的输出 ( scan_example /dev/ttyACM0 | logger -s) 所以我有一个时间相关的调试跟踪流。
- USB 端点上发送的虚假字符为 x5E x40 x5E x40 x5E x40 x5E x40 x41（ASCII 中为 ^@^@^@^@A），看起来像是某种探测或试图引起调制解调器的注意这些字符在应用程序的 write() 导致 4 个 hello 字节发送到端点之后立即发送。
由于 cdc_acm 设备应该是调制解调器，因此我尝试通过将其添加到 cdc_acm.c 中的 usb_device_id acm_ids[] 来关闭调制解调器控制
/* bluegiga BLED112*/ { USB_DEVICE(0x2458, 0x0001), .driver_info = NOT_A_MODEM, },重新编译、insmod'd 和 syslog 显示这已被识别（怪异为 8），但功能没有变化。
NetowrkManager 和调制解调器管理器都没有运行，但我仍然怀疑某处正在运行某种调制解调器控制功能，我只是不知道在哪里。
这是带注释的调试日志（MDV 为我添加到 cdc_acm 的那些 printk 添加前缀）
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_bulk
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_done
这是应用程序发送的 4 个字节 00 00 00 01
Feb 13 18:14:32 localhost cpcenter: df046a80 3672670191 C Bi:3:006:4 0 4 = 00000001
Feb 13 18:14:32 localhost cpcenter: 1360797272.669690 write: data2: len=0 contains:
...这些附加字符意外出现 5e 40 5e 40 5e 40...
Feb 13 18:14:32 localhost cpcenter: df046a80 3672670232 S Bi:3:006:4 -115 128 <
Feb 13 18:14:32 localhost cpcenter: f3cc5740 3672670297 S Bo:3:006:4 -115 1 = 5e
Feb 13 18:14:32 localhost cpcenter: df2e1300 3672670332 S Bo:3:006:4 -115 1 = 40
Feb 13 18:14:32 localhost cpcenter: f3cc5740 3672670347 C Bo:3:006:4 0 1 >
Feb 13 18:14:32 localhost cpcenter: f3cc5740 3672670392 S Bo:3:006:4 -115 1 = 5e
Feb 13 18:14:32 localhost cpcenter: df2e1180 3672670426 S Bo:3:006:4 -115 1 = 40
Feb 13 18:14:32 localhost cpcenter: df2e1c00 3672670461 S Bo:3:006:4 -115 1 = 5e
Feb 13 18:14:32 localhost cpcenter: df2e1840 3672670496 S Bo:3:006:4 -115 1 = 40
Feb 13 18:14:32 localhost cpcenter: df2e1300 3672670591 C Bo:3:006:4 0 1 >
此时我们会自发断开连接。
Feb 13 18:14:32 localhost kernel: usb 3-1: USB disconnect, device number 6
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_bulk
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_done
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm read_bulk_callback
Feb 13 18:14:32 localhost kernel: MDV 1 acm_read_bulk_callback - urb 1, len 0
Feb 13 18:14:32 localhost kernel: MDV 3 acm_read_bulk_callback - non-zero urb status: -71
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_bulk
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_done
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm read_bulk_callback
Feb 13 18:14:32 localhost kernel: MDV 1 acm_read_bulk_callback - urb 1, len 0
Feb 13 18:14:32 localhost kernel: MDV 3 acm_read_bulk_callback - non-zero urb status: -71
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_bulk
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_done
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm read_bulk_callback
Feb 13 18:14:32 localhost kernel: MDV 1 acm_read_bulk_callback - urb 2, len 0
Feb 13 18:14:32 localhost cpcenter: df2e1d80 3672670629 S Bo:3:006:4 -115 1 = 5e
Feb 13 18:14:32 localhost kernel: MDV 3 acm_read_bulk_callback - non-zero urb status: -71
Feb 13 18:14:32 localhost cpcenter: df2e1300 3672670677 S Bo:3:006:4 -115 1 = 41
Feb 13 18:14:32 localhost cpcenter: f3cc5740 3672670802 C Bo:3:006:4 0 1 >
Feb 13 18:14:32 localhost cpcenter: df2e1180 3672671019 C Bo:3:006:4 0 1 >
Feb 13 18:14:32 localhost cpcenter: df2e1c00 3672671237 C Bo:3:006:4 0 1 >
Feb 13 18:14:32 localhost cpcenter: dfbf8c00 3672673193 C Ii:3:001:1 0:2048 1 = 02
Feb 13 18:14:32 localhost cpcenter: dfbf8c00 3672673207 S Ii:3:001:1 -115:2048 4 <
Feb 13 18:14:32 localhost cpcenter: f3c26c00 3672673221 S Ci:3:001:0 s a3 00 0000 0001 0004 4 <
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_disconnect
Feb 13 18:14:32 localhost kernel: Pid: 29, comm: khubd Tainted: G O 3.5.3-1.el6.elrepo.i686 #1
断开连接时的堆栈跟踪
Feb 13 18:14:32 localhost kernel: Call Trace:
Feb 13 18:14:32 localhost kernel: [<f82dabc5>] acm_disconnect+0x35/0x1f0 [cdc_acm]
Feb 13 18:14:32 localhost kernel: [<c13835db>] usb_unbind_interface+0x4b/0x180
Feb 13 18:14:32 localhost cpcenter: f3c26c00 3672673239 C Ci:3:001:0 0 4 = 00010100
Feb 13 18:14:32 localhost kernel: [<c1318bfb>] __device_release_driver+0x5b/0xb0
Feb 13 18:14:32 localhost kernel: [<c1318d05>] device_release_driver+0x25/0x40
Feb 13 18:14:32 localhost kernel: [<c1317f0c>] bus_remove_device+0xcc/0x130
Feb 13 18:14:32 localhost kernel: [<c131612f>] ? device_remove_attrs+0x2f/0x90
Feb 13 18:14:32 localhost kernel: [<c1316275>] device_del+0xe5/0x180
Feb 13 18:14:32 localhost kernel: [<c1380326>] usb_disable_device+0x96/0x240 Feb 13 18:14:32 localhost kernel: [<c1379f91>] usb_disconnect+0x91/0x130
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_bulk
Feb 13 18:14:32 localhost kernel: [<c137a2c0>] hub_port_connect_change+0xb0/0xa60
Feb 13 18:14:32 localhost kernel: [<c1380f4e>] ? usb_control_msg+0xce/0xe0
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm acm_write_done
Feb 13 18:14:32 localhost kernel: [<c137b296>] hub_events+0x536/0x810
Feb 13 18:14:32 localhost cpcenter: f3c26c00 3672673243 S Co:3:001:0 s 23 01 0010 0001 0000 0
Feb 13 18:14:32 localhost cpcenter: f3c26c00 3672673250 C Co:3:001:0 0 0
Feb 13 18:14:32 localhost kernel: [<c1065bdf>] ? finish_wait+0x4f/0x70
Feb 13 18:14:32 localhost kernel: [<c137b5aa>] hub_thread+0x3a/0x1d0
Feb 13 18:14:32 localhost cpcenter: df2e1840 3672673260 C Bo:3:006:4 -71 0
Feb 13 18:14:32 localhost kernel: [<c1065a70>] ? wake_up_bit+0x30/0x30
Feb 13 18:14:32 localhost kernel: [<c137b570>] ? hub_events+0x810/0x810
Feb 13 18:14:32 localhost kernel: [<c106564c>] kthread+0x7c/0x90
Feb 13 18:14:32 localhost cpcenter: f3c16c80 3672673292 C Bi:3:006:4 -71 0
Feb 13 18:14:32 localhost cpcenter: df2e1d80 3672673453 C Bo:3:006:4 -71 0
Feb 13 18:14:32 localhost cpcenter: f3c16d40 3672673553 C Bi:3:006:4 -71 0
Feb 13 18:14:32 localhost kernel: [<c10655d0>] ? kthread_freezable_should_stop+0x60/0x60
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm read_bulk_callback
Feb 13 18:14:32 localhost kernel: [<c14dedbe>] kernel_thread_helper+0x6/0x10
Feb 13 18:14:32 localhost kernel: MDV 1 acm_read_bulk_callback - urb 3, len 0
Feb 13 18:14:32 localhost kernel: MDV:cdc-acm stop_data_traffic
Feb 13 18:14:32 localhost cpcenter: f3d19500 3672674474 C Ii:3:006:2 -108:64 0
Feb 13 18:14:32 localhost kernel: MDV 2 acm_read_bulk_callback - disconnected
Feb 13 18:14:32 localhost cpcenter: df2e1300 3672674636 C Bo:3:006:4 -71 0
Feb 13 18:14:32 localhost cpcenter: f3c16140 3672674753 C Bi:3:006:4 -71 0
答案1
添加更多内核跟踪后，我发现
localhost kernel: [<c12c6757>] process_echoes+0x117/0x2c0
localhost kernel: [<c12c8409>] n_tty_receive_char+0x379/0x770
localhost kernel: [<c121fce4>] ? rb_erase+0xb4/0x120
localhost kernel: [<c12c89f6>] n_tty_receive_buf+0x1f6/0x380
localhost kernel: [<c14d62cb>] ? __schedule+0x39b/0x6d0
它显示 tty 子系统回显和删除字符——这些字符是导致问题的原因。以下代码删除了 tty 线路规则，现在它可以工作了
struct termios usb_termio; // set the USB to raw mode
memset(&usb_termio, 0, sizeof(usb_termio)); // clear the structure
cfmakeraw(&usb_termio);
if (tcsetattr(BLED_fd, TCSANOW, &usb_termio)< 0) {
perror("tcsetattr usb");
return(1);
}
Stack Overflow 的 @Sergey Vlasov 分析了 USB 消息跟踪，并从另一条路径得出了相同的结论。他的解释帮助我更好地理解 usbmon 输出 https://stackoverflow.com/questions/14866899/linux-cdc-acm-device-unexpected-characters-sent-to-device