iwlwifi: mvm: add a loose synchronization of the NSSN across Rx queues

In order to support MSI-X efficiently, we want to avoid
communication across Rx queues. Each Rx queue should have
all the data it needs to process a packet.

The reordering buffer is a challenge in the MSI-X world
since we can have a single BA session whose packets are
directed to different queues. This is why each queue has
its own reordering buffer. The hardware is able to hint
the driver whether we have a hole or not, which allows
the driver to know whether it can release a packet or not.
This indication is called NSSN. Roughly, if the packet's
SN is lower than the NSSN, we can release the packet to
the stack. The NSSN is the SN of the newest packet received
without any holes + 1.

This is working as long as we don't have packets that we
release because of a timeout. When that happens, we could
have taken the decision to release a packet after we have
been waiting for its predecessor for too long. If this
predecessor comes later, we have to drop it because we
can't release packets out of order. In that case, the
hardware will give us an indication that we can we release
the packet (SN < NSSN), but the packet still needs to be
dropped.
This is why we sometimes need to ignore the NSSN and we
track the head_sn in software.
Here is a specific example of this:

1) Rx queue 1 got packets: 480, 482, 483
2) We release 480 to to the stack and wait for 481
3) NSSN is now 481
4) The timeout expires
5) We release 482 and 483, NSSN is still 480
6) 481 arrives its NSSN is 484.

We need to drop 481 even if 481 < 484. This is why we'll
update the head_sn to 484 at step 2. The flow now is:

1) Rx queue 1 got packets: 480, 482, 483
2) We release 480 to to the stack and wait for 481
3) NSSN is now 481 / head_sn is 481
4) The timeout expires
5) We release 482 and 483, NSSN is still 480 but head_sn is 484.
6) 481 arrives its NSSN is 484, but head_sn is 484 and we drop it.

This code introduces another problem in case all the traffic
goes well (no hole, no timeout):

Rx queue 1: 0   -> 483   (head_sn = 484)
Rx queue 2: 501 -> 4095  (head_sn = 0)
Rx queue 2: 0   -> 480   (head_sn = 481)
Rx queue 1: 481 but head_sn = 484 and we drop it.

At this point, the SN of queue 1 is far behind: more than
4040 packets behind. Queue 1 will consider 481 "old"
because 481 is in [501-64:501] whereas it is a very new
packet.

In order to fix that, send an Rx notification from time to
time (twice across the full set of 4096 packets) to make
sure no Rx queue is lagging too far behind.

What will happen then is:

Rx queue 1: 0    -> 483       (head_sn = 484)
Rx queue 2: 501  -> 2047      (head_sn = 2048)
Rx queue 1: Sync nofication   (head_sn = 2048)
Rx queue 2: 2048 -> 4095      (head_sn = 0)
Rx queue 1: Sync notification (head_sn = 0)
Rx queue 2: 1    -> 481       (head_sn = 482)
Rx queue 1: 481 and head_sn = 0.

In queue 1's data, head_sn is now 0, the packet coming in
is 481, it'll understand that the new packet is new and it
won't be dropped.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>