Re: [RFC/RFT] minstrel_ht: new rate control module for 802.11n

[Felix Fietkau <nbd@openwrt.org>]

On 2010-06-28 2:01 AM, Björn Smedman wrote:
> 2010/6/23 Felix Fietkau <nbd@openwrt.org>:
>> On 2010-06-23 6:36 PM, Björn Smedman wrote:
>>> [snip] As
>>> far as I can tell, whenever the first subframe of an aggregate fails
>>> and is software retried, the rate control feedback for that aggregate
>>> is lost (ath_tx_rc_status() is never called with update_rc = true in
>>> xmit.c).
>> I think you misread that part. The loop iterates over all subframes in
>> the aggregate, and the first successful or swretry-expired frame will
>> trigger an AMPDU status report, which will update the RC. The first
>> subframe of the A-MPDU is not getting any special treatment here.
> 
> You're (still) right I misread that part. But I think there is another
> problem when the first subframe of an A-MPDU is not acked: if it has
> not expired yet it is (as I understand it) prepended to the tid queue
> for software retry and will therefore be the first subframe of the
> next aggregate as well, which will then be transmitted with the same
> "old" rates and counts as the previous aggregate. So the feedback from
> xmit to rc works, but the control information flow from rc to xmit is
> delayed.
> 
> I guess the real solution is your rewrite... But in the mean time
> perhaps we can memcpy the tx_info control from the last subframe to
> the first before calling ath_buf_set_rate() in ath_tx_sched_aggr()?
> Could that have any side effects? It could make the aggregate size go
> over the 4 ms limit I guess... How bad is that?
There's an easy solution which would take into account the 4ms frame
limit properly, and which could work without any memcpy() hacks:

I could just grab a pointer to the last buffer in the tid queue in the
ath_tx_sched_aggr() function, then pass it to ath_lookup_rate() via
ath_tx_form_aggr(), and also to ath_buf_set_rate(). Then I make those
functions use this last buffer as reference for the rate lookup.

- Felix