Since 4f3c8d6eddc272b386464524235440a418ed2029 I'm experiencing this STB race condition. TL;DR an old, cached STB value can be used after a new one is reported in reply to a control message. Hacking up the latest driver code to ignore the cached stb value gets around the issue. My USBTMC device has an interrupt endpoint with a 1ms interval. 1) A query is sent to the USBTMC device. 2) An SRQ is reported via the interrupt endpoint with MAV set. 3) Userspace performs a read to get the reply since MAV is set after being notified by poll(). 4) A condition on the USBTMC device triggers an SRQ before the response is read but is not reported until later due to 1ms poll interval. MAV is still set in the reported stb. 5) The USBTMC device submits a reply and clears MAV. 6) Userspace receives the reply and does USBTMC488_IOCTL_READ_STB. [ 2584.683589] 000: usbtmc 1-1.1:1.0: Enter ioctl_read_stb iin_ep_present: 1 7) The SRQ comes in but too late to be used as a cached value by usbtmc488_ioctl_read_stb() so it is stored for later. [ 2584.683673] 000: usbtmc 1-1.1:1.0: int status: 0 len 2 [ 2584.683684] 000: usbtmc 1-1.1:1.0: srq received bTag 81 stb 51 8) A control msg is sent and the interrupt endpoint replies with the STB. MAV is (correctly) not set. [ 2584.684657] 000: usbtmc 1-1.1:1.0: int status: 0 len 2 [ 2584.687642] 000: usbtmc 1-1.1:1.0: stb:0x41 received 0 9) Userspace sends a query in response to the device condition reported in the first STB bit. 10) Userspace does USBTMC488_IOCTL_READ_STB before waiting with poll() and finds MAV is already set (incorrectly) since the cached value from the last SRQ is used even though the bit was more recently reported not set in response to a control message. 11) Userspace then attempts to read based on this false MAV 3and finds no response. Am I making any incorrect assumptions about how this should work? If not what could we do to fix this behavior other than not using a cached stb value? FWIW I'm the author of the userspace code and the USBTMC device firmware. Regards, George McCollister Software Architect NovaTech LLC
