On Mon, Feb 8, 2016 at 11:54 AM, Krishna Chaitanya <chaitanya.mgit@xxxxxxxxx> wrote: > On Mon, Feb 8, 2016 at 2:56 PM, Emmanuel Grumbach <egrumbach@xxxxxxxxx> wrote: >> On Mon, Feb 8, 2016 at 10:38 AM, Felix Fietkau <nbd@xxxxxxxxxxx> wrote: >>> Requires software tx queueing support. frag_list support (for zero-copy) >>> is optional. >>> >>> Signed-off-by: Felix Fietkau <nbd@xxxxxxxxxxx> >>> --- >> >> >> Ok - looks fine, but... and here comes the hard stuff. >> The frame size in the PLCP is limited in a way that you can't - from a >> spec POV - enable A-MSDU for low rates. Of course, you don't want to >> do that for low rates at all regardless of the spec. >> Since you build the A-MSDU in the mac80211 Tx queue which is not aware >> of the link quality, how do we prevent A-MSDU if the rate is low / >> dropping. >> I'd even argue that when the rates get lower, you'll have more >> packets piling up in the software queue and ... even more chance to >> get A-MSDU in the exact case where you really want to avoid it? > > Similar to triggering AMPDU setup, we should put this control > in RC (minstrel) to start/stop AMSDU based on link quality/if the rates > drop below a pre-defined MCS (or) only for best-throughput rates. Ok - but the size of A-MPDU is determined in the firmware / hardware typically (at least for Intel devices and I have to admit my ignorance about other designs...). So that if you get bad link conditions, the firmware can play for that size. A-MSDU built in the host is a different story. I remember that athXk devices attach the rate on the Tx packet itself so that the rate control is serialized with the data path. iwlwifi works differently: we have a map of rates in the firmware. This map is maintained in the driver updated based on the feedback we get from the device. When an update is needed, iwlwifi sends a command to the firmware and that command bypasses all the Tx packets currently queue so that a packet that is already waiting in the queue will be sent with the updated map. This allows iwlwifi to react downgrade faster when needed. So - for athXk (and maybe the MediaTek devices Felix is working on), this may not be a big problem: you'd need to add another input to the A-MSDU building code coming from the rate control. FWIW: in iwlwifi I simply decided to be on the safe side and allowed the driver to build A-MSDUs only if the rate is fairly high and we are not downgrading. But you saw (and commented on) that patch already I think :) -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
