linux-kernel - Re: [PATCH net-next v2 9/9] net: ethernet: mtk-star-emac: separate tx/rx handling with two NAPIs

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Date:   Mon, 14 Mar 2022 08:57:05 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Biao Huang <biao.huang@...iatek.com>
Cc:     David Miller <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Fabien Parent <fparent@...libre.com>,
        Felix Fietkau <nbd@....name>, John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        "Matthias Brugger" <matthias.bgg@...il.com>,
        <netdev@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>,
        Yinghua Pan <ot_yinghua.pan@...iatek.com>,
        <srv_heupstream@...iatek.com>,
        Macpaul Lin <macpaul.lin@...iatek.com>
Subject: Re: [PATCH net-next v2 9/9] net: ethernet: mtk-star-emac: separate
 tx/rx handling with two NAPIs

On Mon, 14 Mar 2022 15:01:23 +0800 Biao Huang wrote:
> > Drivers are expected to stop their queues at the end of xmit routine
> > if
> > the ring can't accommodate another frame. It's more efficient to stop
> > the queues early than have to put skbs already dequeued from the
> > qdisc
> > layer back into the qdiscs.  
> Yes, if descriptors ring is full, it's meaningful to stop the queue 
> at the end of xmit; 
> But driver seems hard to know how many descriptors the next skb will
> request, e.g. 3 descriptors are available for next round send, but the
> next skb may need 4 descriptors, in this case, we still need judge
> whether descriptors are enough for skb transmission, then decide stop
> the queue or not, at the beginning of xmit routine.
> 
> Maybe we should judge ring is full or not at the beginning and the end
> of xmit routine(seems a little redundancy).

Assume the worst case scenario. You set the default ring size to 512,
skb can have at most MAX_SKB_FRAGS fragments (usually 17) so the max
number of descriptors should not be very high, hard to pre-compute,
or problematic compared to the total ring size.