• Emmanuel Grumbach's avatar
    iwlwifi: mvm: add a loose synchronization of the NSSN across Rx queues · 3c514bf8
    Emmanuel Grumbach authored
    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: default avatarEmmanuel Grumbach <emmanuel.grumbach@intel.com>
    Signed-off-by: default avatarLuca Coelho <luciano.coelho@intel.com>
    Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
    3c514bf8
mac80211.c 145 KB