netdev - Re: [PATCH 3/3] net: hisilicon: Add Fast Ethernet MAC driver

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Date:	Mon, 11 Jul 2016 10:16:25 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Dongpo Li <lidongpo@...ilicon.com>
Cc:	f.fainelli@...il.com, robh+dt@...nel.org, mark.rutland@....com,
	davem@...emloft.net, xuejiancheng@...ilicon.com,
	netdev@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] net: hisilicon: Add Fast Ethernet MAC driver

On Monday, July 11, 2016 11:44:23 AM CEST Dongpo Li wrote:
> Hi Arnd,
> 
> On 2016/6/28 17:34, Arnd Bergmann wrote:
> > On Tuesday, June 28, 2016 5:21:19 PM CEST Dongpo Li wrote:
> >> On 2016/6/15 5:20, Arnd Bergmann wrote:
> >>> On Tuesday, June 14, 2016 9:17:44 PM CEST Li Dongpo wrote:
> >>>> On 2016/6/13 17:06, Arnd Bergmann wrote:
> >>>>> On Monday, June 13, 2016 2:07:56 PM CEST Dongpo Li wrote:
> >>>>> You tx function uses BQL to optimize the queue length, and that
> >>>>> is great. You also check xmit reclaim for rx interrupts, so
> >>>>> as long as you have both rx and tx traffic, this should work
> >>>>> great.
> >>>>>
> >>>>> However, I notice that you only have a 'tx fifo empty'
> >>>>> interrupt triggering the napi poll, so I guess on a tx-only
> >>>>> workload you will always end up pushing packets into the
> >>>>> queue until BQL throttles tx, and then get the interrupt
> >>>>> after all packets have been sent, which will cause BQL to
> >>>>> make the queue longer up to the maximum queue size, and that
> >>>>> negates the effect of BQL.
> >>>>>
> >>>>> Is there any way you can get a tx interrupt earlier than
> >>>>> this in order to get a more balanced queue, or is it ok
> >>>>> to just rely on rx packets to come in occasionally, and
> >>>>> just use the tx fifo empty interrupt as a fallback?
> >>>>>
> >>>> In tx direction, there are only two kinds of interrupts, 'tx fifo empty'
> >>>> and 'tx one packet finish'. I didn't use 'tx one packet finish' because
> >>>> it would lead to high hardware interrupts rate. This has been verified in
> >>>> our chips. It's ok to just use tx fifo empty interrupt.
> >>>
> >>> I'm not convinced by the explanation, I don't think that has anything
> >>> to do with the hardware design, but instead is about the correctness
> >>> of the BQL logic with your driver.
> >>>
> >>> Maybe your xmit function can do something like
> >>>
> >>>       if (dql_avail(netdev_get_tx_queue(dev, 0)->dql) < 0)
> >>>               enable per-packet interrupt
> >>>       else
> >>>               use only fifo-empty interrupt
> >>>
> >>> That way, you don't get a lot of interrupts when the system is
> >>> in a state of packets being received and sent continuously,
> >>> but if you get to the point where your tx queue fills up
> >>> and no rx interrupts arrive, you don't have to wait for it
> >>> to become completely empty before adding new packets, and
> >>> BQL won't keep growing the queue.
> >>>
> >> Hi, Arnd
> >> I tried enable per-packet interrupt when tx queue full in xmit function
> >> and disable it in NAPI poll. But the number of interrupts are a little
> >> bigger than only using fifo-empty interrupt.
> > 
> > Right, I'd expect that to be the case, it basically means that the
> > algorithm works as expected.
> > 
> > Just to be sure you didn't have extra interrupts: you only enable the
> > per-packet interrupts if interrupts are currently enabled, not in
> > NAPI polling mode, right?
> > 
> Sorry so long to reply to you. I use the per-packet interrupt like this:
> In my xmit function,
> 	if (hardware tx fifo is full) {
> 		enable tx per-packet interrupt;
> 		netif_stop_queue(dev);
> 		return NETDEV_TX_BUSY;
> 	}
> 
> In interrupt handle function,
> 	if (interrupt is tx per-packet or tx fifo-empty or rx) {
> 		disable tx per-packet interrupt;
> 		napi_schedule(&priv->napi);
> 	}
> We disable tx per-packet interrupt anyway because the NAPI poll will reclaim
> the tx fifo.
> When the NAPI poll completed, it will only enable the tx fifo-empty interrupt
> and rx interrupt except the tx per-packet interrupt.
> 
> Is this solution okay?

Yes, this looks good to me.

	Arnd