block: Adding ROW scheduling algorithm

This patch adds the implementation of a new scheduling algorithm - ROW.
The policy of this algorithm is to prioritize READ requests over WRITE
as much as possible without starving the WRITE requests.
The requests are kept in queues according to their priority. The dispatch
is done in a Round Robin manner with a different slice for each queue.
READ request queues get bigger dispatch quantum than the write requests.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
block: compile ROW statically into the kernel

ROW is a new scheduling algorithm. Similar to the existing scheduling
algorithms it should be compiled to the kernel statically giving the user
the ability to switch to it without kernel recompilation.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
row: Adding support for reinsert already dispatched req

Add support for reinserting already dispatched request back to the
schedulers internal data structures.
The request will be reinserted back to the queue (head) it was
dispatched from as if it was never dispatched.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
row: Add support for urgent request handling

This patch add support for handling urgent requests.
ROW queue can be marked as "urgent". If an urgent queue was
un-served in a previous dispatch cycle and a request was added
to it - it will trigger issuing urgent request to the device driver.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
block: Add support for reinsert a dispatched req

Add support for reinserting a dispatched request back to the
scheduler's internal data structures.
This capability is used by the device driver when it chooses to
interrupt the current request transmission and execute another (more
urgent) pending request. For example: interrupting long write in order
to handle pending read. The device driver re-inserts the
remaining write request back to the scheduler, to be rescheduled
for transmission later on.

Add API for verifying whether the current scheduler
supports reinserting requests mechanism. If reinsert mechanism isn't
supported by the scheduler, this code path will never be activated.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
block: Add API for urgent request handling

This patch add support in block & elevator layers for handling
urgent requests. The decision if a request is urgent or not is taken
by the scheduler. Urgent request notification is passed to the underlying
block device driver (eMMC for example). Block device driver may decide to
interrupt the currently running low priority request to serve the new
urgent request. By doing so READ latency is greatly reduced in read&write
collision scenarios.

Note that if the current scheduler doesn't implement the urgent request
mechanism, this code path is never activated.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
block: row: Fix build for 3.0 kernel