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Subject: Re: [PATCH 05/15] mt76: track rx airtime for airtime fairness and
 survey
To:     =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        linux-wireless@vger.kernel.org
References: <20190926174732.42375-1-nbd@nbd.name>
 <20190926174732.42375-5-nbd@nbd.name> <87v9teyw40.fsf@toke.dk>
From:   Felix Fietkau <nbd@nbd.name>
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Message-ID: <6af9b976-48e8-64c2-dffe-b3eaa03b9c73@nbd.name>
Date:   Fri, 27 Sep 2019 10:46:33 +0200
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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.

- Felix