idpf: replace flow scheduling buffer ring with buffer pool

Replace the TxQ buffer ring with one large pool/array of buffers (only
for flow scheduling). This eliminates the tag generation and makes it
impossible for a tag to be associated with more than one packet.

The completion tag passed to HW through the descriptor is the index into
the array. That same completion tag is posted back to the driver in the
completion descriptor, and used to index into the array to quickly
retrieve the buffer during cleaning.  In this way, the tags are treated
as a fix sized resource. If all tags are in use, no more packets can be
sent on that particular queue (until some are freed up). The tag pool
size is 64K since the completion tag width is 16 bits.

For each packet, the driver pulls a free tag from the refillq to get the
next free buffer index. When cleaning is complete, the tag is posted
back to the refillq. A multi-frag packet spans multiple buffers in the
driver, therefore it uses multiple buffer indexes/tags from the pool.
Each frag pulls from the refillq to get the next free buffer index.
These are tracked in a next_buf field that replaces the completion tag
field in the buffer struct. This chains the buffers together so that the
packet can be cleaned from the starting completion tag taken from the
completion descriptor, then from the next_buf field for each subsequent
buffer.

In case of a dma_mapping_error occurs or the refillq runs out of free
buf_ids, the packet will execute the rollback error path. This unmaps
any buffers previously mapped for the packet. Since several free
buf_ids could have already been pulled from the refillq, we need to
restore its original state as well. Otherwise, the buf_ids/tags
will be leaked and not used again until the queue is reallocated.

Descriptor completions only advance the descriptor ring index to "clean"
the descriptors. The packet completions only clean the buffers
associated with the given packet completion tag and do not update the
descriptor ring index.

When operating in queue based scheduling mode, the array still acts as a
ring and will only have TxQ descriptor count entries. The tx_bufs are
still associated 1:1 with the descriptor ring entries and we can use the
conventional indexing mechanisms.

Fixes: c2d548c ("idpf: add TX splitq napi poll support")
Signed-off-by: Luigi Rizzo <[email protected]>
Signed-off-by: Brian Vazquez <[email protected]>
Signed-off-by: Joshua Hay <[email protected]>
Reviewed-by: Madhu Chittim <[email protected]>
Reviewed-by: Aleksandr Loktionov <[email protected]>
Tested-by: Samuel Salin <[email protected]>
Signed-off-by: Tony Nguyen <[email protected]>

Author: jahay1