Re: [PATCH 05/15] mt76: track rx airtime for airtime fairness and survey

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Felix Fietkau <nbd@nbd.name> writes:

> On 2019-09-27 10:35, Toke Høiland-Jørgensen wrote:
>> Felix Fietkau <nbd@nbd.name> writes:
>>> +	switch (status->encoding) {
>>> +	case RX_ENC_LEGACY:
>>> +		if (WARN_ON_ONCE(status->band > NL80211_BAND_5GHZ))
>>> +			return 0;
>>> +
>>> +		sband = dev->hw->wiphy->bands[status->band];
>>> +		if (!sband || status->rate_idx > sband->n_bitrates)
>>> +			return 0;
>>> +
>>> +		rate = &sband->bitrates[status->rate_idx];
>>> +
>>> +		return mt76_calc_legacy_rate_duration(rate, sp, len);
>>> +	case RX_ENC_VHT:
>>> +		streams = status->nss;
>>> +		idx = status->rate_idx;
>>> +		group = VHT_GROUP_IDX(streams, sgi, bw);
>>> +		break;
>>> +	case RX_ENC_HT:
>>> +		streams = ((status->rate_idx >> 3) & 3) + 1;
>>> +		idx = status->rate_idx & 7;
>>> +		group = HT_GROUP_IDX(streams, sgi, bw);
>>> +		break;
>>> +	default:
>>> +		WARN_ON_ONCE(1);
>>> +		return 0;
>>> +	}
>>> +
>>> +	if (WARN_ON_ONCE(streams > 4))
>>> +		return 0;
>>> +
>>> +	duration = airtime_mcs_groups[group].duration[idx];
>>> +	duration <<= airtime_mcs_groups[group].shift;
>>> +	duration *= len;
>>> +	duration /= AVG_PKT_SIZE;
>>> +	duration /= 1024;
>> 
>> On an earlier patch of mine you expressed concern over divisions in the
>> fast path. Does this mean this is no longer a concern? Or is the
>> compiler doing fancy things with the constant division here? :)
> I made the constants power-of-2, so that the compiler can turn them into
> bit shifts. I verified the disassembly: no divisions left in the code.

I see. Neat :)

There's still a regular division in the legacy rate calc further up,
though, right? But I guess that doesn't happen so often?

-Toke