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

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Date:   Tue, 15 Mar 2022 08:59:35 +0800
From:   Biao Huang <biao.huang@...iatek.com>
To:     Jakub Kicinski <kuba@...nel.org>
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

Dear Jakub,
	Thanks for your comments~

On Mon, 2022-03-14 at 08:57 -0700, Jakub Kicinski wrote:
> 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.
Yes, we'll check the available descriptor number at the end of xmit
routine, and ensure it will larger than (MAX_SKB_FRAGS + 1) in next
send. (refer to stmmac_main.c)

Regards!