The ioctl USBTMC488_IOCTL_READ_STB either returns a cached status byte (STB) sent by the device due to a service request (SRQ) condition or the STB obtained from a query to the device with a READ_STATUS_BYTE control message. When the query is interrupted by an SRQ message on the interrupt pipe, the ioctl still returns the requested STB while the STB of the out-of-band SRQ message is cached for the next call of this ioctl. However the cached SRQ STB represents a state that was previous to the last returned STB. Furthermore the cached SRQ STB can be stale and not reflect the current state of the device. This set of patches separates out the behaviour into 3 ioctls: [PATCH 1] USBTMC488_IOCTL_READ_STB always reads the STB from the device and if the associated file descriptor has the srq_asserted bit set it ors in the RQS bit to the returned STB and clears the srq_asserted bit conformant to subclass USB488 devices. [PATCH 2] USBTMC_IOCTL_GET_STB reads the status byte (STB) from the device and returns the STB unmodified to the application. The srq_asserted bit is not taken into account and not changed. [PATCH 3] USBTMC_IOCTL_GET_SRQ_STB only returns the status byte (STB) that was originally sent by the device due to a service request (SRQ) condition. This ioctl checks the srq_asserted bit of the associated file descriptor. If set, the srq_asserted bit is reset and the cached STB with original SRQ information is returned. Otherwise the ioctl returns the error code ENOMSG. The latter 2 ioctls are useful to support non USBTMC-488 compliant devices. Time sensitive applications can read the cached STB without incurring the cost of an urb transaction over the bus. [PATCH 4] Increase the API version number Dave Penkler (4): USB: usbtmc: Fix reading stale status byte USB: usbtmc: Add USBTMC_IOCTL_GET_STB USB: usbtmc: Add separate USBTMC_IOCTL_GET_SRQ_STB USB: usbtmc: Bump USBTMC_API_VERSION value drivers/usb/class/usbtmc.c | 85 ++++++++++++++++++++++++++---------- include/uapi/linux/usb/tmc.h | 3 ++ 2 files changed, 66 insertions(+), 22 deletions(-) -- 2.29.2
