netdev - Re: [PATCH 3/3] net: hisilicon: new hip04 ethernet driver

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Date:	Tue, 25 Mar 2014 18:05:33 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	Zhangfei Gao <zhangfei.gao@...il.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	netdev <netdev@...r.kernel.org>,
	Zhangfei Gao <zhangfei.gao@...aro.org>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 3/3] net: hisilicon: new hip04 ethernet driver

On Tuesday 25 March 2014 10:00:30 Florian Fainelli wrote:
> 2014-03-25 1:12 GMT-07:00 Arnd Bergmann <arnd@...db.de>:
> > On Tuesday 25 March 2014 12:06:31 Zhangfei Gao wrote:
> >> Dear Arnd
> >>
> >> On Mon, Mar 24, 2014 at 11:18 PM, Arnd Bergmann <arnd@...db.de> wrote:
> >> > On Monday 24 March 2014 22:14:56 Zhangfei Gao wrote:
> >> >
> >> >> +
> >> >> +static void hip04_tx_reclaim(struct net_device *ndev, bool force)
> >> >> +{
> >> >> +     struct hip04_priv *priv = netdev_priv(ndev);
> >> >> +     unsigned tx_head = priv->tx_head;
> >> >> +     unsigned tx_tail = priv->tx_tail;
> >> >> +     struct tx_desc *desc = &priv->tx_desc[priv->tx_tail];
> >> >> +
> >> >> +     while (tx_tail != tx_head) {
> >> >> +             if (desc->send_addr != 0) {
> >> >> +                     if (force)
> >> >> +                             desc->send_addr = 0;
> >> >> +                     else
> >> >> +                             break;
> >> >> +             }
> >> >> +             if (priv->tx_phys[tx_tail]) {
> >> >> +                     dma_unmap_single(&ndev->dev, priv->tx_phys[tx_tail],
> >> >> +                             priv->tx_skb[tx_tail]->len, DMA_TO_DEVICE);
> >> >> +                     priv->tx_phys[tx_tail] = 0;
> >> >> +             }
> >> >> +             dev_kfree_skb_irq(priv->tx_skb[tx_tail]);
> >> >> +             priv->tx_skb[tx_tail] = NULL;
> >> >> +             tx_tail = TX_NEXT(tx_tail);
> >> >> +             priv->tx_count--;
> >> >> +     }
> >> >> +     priv->tx_tail = tx_tail;
> >> >> +}
> >> >
> >> > I think you still need to find a solution to ensure that the tx reclaim is
> >> > called eventually through a method other than start_xmit.
> >>
> >> In the iperf stress test, if move reclaim to poll, there is some
> >> error, sometimes sending zero packets.
> >> While keep reclaim in the xmit to reclaim transmitted packets looks
> >> stable in the test,
> >> There TX_DESC_NUM desc can be used.
> >
> > What I meant is that you need a correct implementation, presumably
> > you added a bug when you moved the function to poll(), and also you
> > forgot to add a timer.
> 
> Using a timer to ensure completion of TX packets is a trick that
> worked in the past, but now that the networking stack got smarter,
> this might artificially increase the processing time of packets in the
> transmit path, and this will defeat features like TCP small queues
> etc.. as could be seen with the mvneta driver [1]. The best way really
> is to rely on TX completion interrupts when those exist as they cannot
> lie about the hardware status (in theory) and they should provide the
> fastest way to complete TX packets.

By as Zhangfei Gao pointed out, this hardware does not have a working
TX completion interrupt. Using timers to do this has always just been
a workaround for broken hardware IMHO.

	Arnd
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