cxl: Add mechanism for delivering AFU driver specific events

This adds an afu_driver_ops structure with fetch_event() and event_delivered() callbacks. An AFU driver such as cxlflash can fill this out and associate it with a context to enable passing custom AFU specific events to userspace. This also adds a new kernel API function cxl_context_pending_events(), that the AFU driver can use to notify the cxl driver that new specific events are ready to be delivered, and wake up anyone waiting on the context wait queue. The current count of AFU driver specific events is stored in the field afu_driver_events of the context structure. The cxl driver checks the afu_driver_events count during poll, select, read, etc. calls to check if an AFU driver specific event is pending, and calls fetch_event() to obtain and deliver that event. This way, the cxl driver takes care of all the usual locking semantics around these calls and handles all the generic cxl events, so that the AFU driver only needs to worry about it's own events. fetch_event() return a struct cxl_event_afu_driver_reserved, allocated by the AFU driver, and filled in with the specific event information and size. Total event size (header + data) should not be greater than CXL_READ_MIN_SIZE (4K). Th cxl driver prepends an appropriate cxl event header, copies the event to userspace, and finally calls event_delivered() to return the status of the operation to the AFU driver. The event is identified by the context and cxl_event_afu_driver_reserved pointers. Since AFU drivers provide their own means for userspace to obtain the AFU file descriptor (i.e. cxlflash uses an ioctl on their scsi file descriptor to obtain the AFU file descriptor) and the generic cxl driver will never use this event, the ABI of the event is up to each individual AFU driver.

PBPhilippe Bergheaud committed 10 years ago