Submitting an asynchronous transfer refused any buffer needing a cache
line stand-in, and that test also refuses every buffer whose length is not
a whole number of cache lines, which an interrupt transfer's rarely is: a
HID keyboard reads eight bytes. Every submission returned -EFAULT before
a descriptor was written, and a class driver resubmitting from its
completion callback never sees a second chance.
The refusal existed because the copy out of a stand-in is done by the
blocked caller, and an asynchronous transfer has none. The work queue
thread handling the completion will do: a buffer given to DRVR_ASYNCH
comes from DRVR_ALLOC, so it is kernel memory reachable from any thread.
Use the same stand-in machinery as every other transfer and finish the DMA
in the completion, just before the callback. A cancelled transfer returns
its stand-in on cancellation.
The callback also moves outside the spinlock. It is class driver code
that queues work and takes its own locks, and it may now free a stand-in.
Whether a completion is synchronous is still decided under the lock, since
a posted waiter may be carrying a new transfer immediately.
The asynchronous setup now records the requested length, as the
synchronous setup does. The byte count handed to the callback is worked
out from it and the residue, and was previously whatever the endpoint held
from an earlier transfer.
Assisted-by: Claude:claude-opus-5
Signed-off-by: Justin Hammond <justin@dynam.ac>
a5431319bb