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Subject: Re: [PATCH v2 1/4] mac80211: Rearrange ieee80211_tx_info to make room
 for tx_time_est
To:     Johannes Berg <johannes@sipsolutions.net>,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        Kan Yan <kyan@google.com>
References: <157115993755.2500430.12214017471129215800.stgit@toke.dk>
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 <4bc17c32337888a57a78a0e2c874abd462faef21.camel@sipsolutions.net>
Cc:     linux-wireless@vger.kernel.org,
        make-wifi-fast@lists.bufferbloat.net, ath10k@lists.infradead.org,
        John Crispin <john@phrozen.org>,
        Lorenzo Bianconi <lorenzo@kernel.org>,
        Felix Fietkau <nbd@nbd.name>,
        Rajkumar Manoharan <rmanohar@codeaurora.org>,
        Kevin Hayes <kevinhayes@google.com>
From:   Ben Greear <greearb@candelatech.com>
Message-ID: <237a3110-69c1-6679-6ffb-892c3815049c@candelatech.com>
Date:   Fri, 18 Oct 2019 06:01:23 -0700
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On 10/18/2019 05:35 AM, Johannes Berg wrote:
> On Fri, 2019-10-18 at 12:15 +0200, Toke Høiland-Jørgensen wrote:
>> Kan Yan <kyan@google.com> writes:
>>
>>> The "tx_time_est" field, shared by control and status, is not able to
>>> survive until the skb returns to the mac80211 layer in some
>>> architectures. The same space is defined as driver_data and some
>>> wireless drivers use it for other purposes, as the cb in the sk_buff
>>> is free to be used by any layer.
>>>
>>> In the case of ath10k, the tx_time_est get clobbered by
>>> struct ath10k_skb_cb {
>>>         dma_addr_t paddr;
>>>         u8 flags;
>>>         u8 eid;
>>>         u16 msdu_id;
>>>         u16 airtime_est;
>>>         struct ieee80211_vif *vif;
>>>         struct ieee80211_txq *txq;
>>> } __packed;
>>
>> Ah, bugger, of course the driver that actually needs this is using the
>> full driver_data space :P
>
> Looks like you could shrink *this* fairly easily though.
>
> E.g. most likely vif == txq->vif unless txq==NULL, so it's down to 22
> bytes plus a bit/flag for knowing whether the pointer is a vif directly
> (if no TXQ) or a TXQ.

And of course you get two bits in every pointer (0x3) and likely the
dma addr too.  Plenty of space!

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com