Add and use rtems_blkdev_request_done(). Block device transfer requests
must signal the completion status now with rtems_blkdev_request_done().
The return value of the block device IO control will be ignored for
transfer requests.
The first parameter of rtems_blkdev_request_cb is now the transfer
request structure.
Renamed rtems_blkdev_request::req_done to rtems_blkdev_request::done to
break third party drivers at compile time, otherwise this API change
would result in runtime errors.
Read-ahead requests were previously executed in the context of the
reading task. This blocks the reading task until the complete read
with read-ahead transfer is finished. A read-ahead task is introduced
to off-load the read-ahead transfer. This allows the reading task to
work with the requested block more quickly. The read-ahead is triggered
after two misses of ascending consecutive blocks or a read hit of a
block read by the most-recent read-ahead transfer. The read-ahead
feature is configurable and can be disabled.
