Hi, Oliver > -----Original Message----- > From: Oliver Neukum [mailto:oliver@xxxxxxxxxx] > Sent: Saturday, March 30, 2013 10:34 AM > To: Mike Verstegen > Cc: linux-usb@xxxxxxxxxxxxxxx > Subject: Re: cdc_acm device - unexpected characters sent to USB device > > On Saturday 30 March 2013 03:20:15 Mike Verstegen wrote: > > > - Downloaded the source code for the cdc_acm driver. > > - Added a bunch of printk debug messages and stack_dumps > to follow what's going on. > > - I rmmod'd the "stock" cdc_acm and insmod'd my > instrumented module. > > - All the device enumeration works, right driver attached, etc. > > - Since the code works on Ubuntu 12.04/Linux 3.2, I > grabbed the 3.2 cdc_acm code and compiled that module on the > Centos 6 / Linux 3.6 platform. Using that 3.2 module instead > of the 3.6 module did not make a difference. I reverted to > the 3.6 module. > > - Turned on the debug file system with usbmon and watched > the USB traffic. I can see that there are extra characters > being sent on the USB interface. > > - To watch what's going on, on top of the printk's in the > cdc_acm module, I've merged the output of usb mon (cat > /sys/kernel/debug/usb/usbmon/3u | logger) and the output of > the test application (scan_example /dev/ttyACM0 | logger -s) > so I have a single stream of time correlated debug trail. > > - The spurious characters sent on the USB endpoint are > x5E x40 x5E x40 x5E x40 x5E x40 x41 (in ASCII its ^@^@^@^@A ) > which looks like some sort of probing or trying to get the > attention of a modem These characters are sent immediately > after the application's write() causes the 4 hello bytes to > be sent to the end point. > > To clarify, is acm_tty_write() called with the additional > bytes or isn't it? > > Regards > Oliver > > Hi, Oliver Such a simple question you asked -- but it lead to a useful discovery. My logs show that acm_tty_write is called multiple times - The application opens the device and then calls write() to send the 4 byte message to the fd. This results in a call to acm_tty_write - From somewhere else, acm_tty_write is called several times, each with a count=1 and one of the offending characters noted above. To get find out where the unexpected calls to acm_tty_write were coming from, I added a dump_stack at the entry point to acm_tty_write. From those stack frames, I can see that the first acm_tty_write call is from my application, but the second and subsequent calls are from a kworker thread. So at this point, it doesn't look like a cdc_acm driver problem, but instead an unexpected call to acm_tty_write from another process. Having briefly looked the trace output of the kworker queue, it may a challenge to find out what thread in there is making this call. Thanks Mike-- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
