netdev - Re: [PATCH 2/2] net: Add driver for LiteX's LiteETH network interface

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Date:   Mon, 9 Aug 2021 12:03:36 +0000
From:   Joel Stanley <joel@....id.au>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Rob Herring <robh+dt@...nel.org>,
        Stafford Horne <shorne@...il.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Anton Blanchard <anton@...abs.org>,
        Gabriel Somlo <gsomlo@...il.com>, David Shah <dave@....me>,
        Karol Gugala <kgugala@...micro.com>,
        Mateusz Holenko <mholenko@...micro.com>,
        devicetree <devicetree@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] net: Add driver for LiteX's LiteETH network interface

Thanks for the review Jakub.

On Fri, 6 Aug 2021 at 23:10, Jakub Kicinski <kuba@...nel.org> wrote:
> > +static int liteeth_start_xmit(struct sk_buff *skb, struct net_device *netdev)
> > +{
> > +     struct liteeth *priv = netdev_priv(netdev);
> > +     void __iomem *txbuffer;
> > +     int ret;
> > +     u8 val;
> > +
> > +     /* Reject oversize packets */
> > +     if (unlikely(skb->len > MAX_PKT_SIZE)) {
> > +             if (net_ratelimit())
> > +                     netdev_dbg(netdev, "tx packet too big\n");
> > +             goto drop;
> > +     }
> > +
> > +     txbuffer = priv->tx_base + priv->tx_slot * LITEETH_BUFFER_SIZE;
> > +     memcpy_toio(txbuffer, skb->data, skb->len);
> > +     writeb(priv->tx_slot, priv->base + LITEETH_READER_SLOT);
> > +     writew(skb->len, priv->base + LITEETH_READER_LENGTH);
> > +
> > +     ret = readl_poll_timeout_atomic(priv->base + LITEETH_READER_READY, val, val, 5, 1000);
>
> Why the need for poll if there is an interrupt?
> Why not stop the Tx queue once you're out of slots and restart
> it when the completion interrupt comes?

That makes sense.

In testing I have not been able to hit the LITEETH_READER_READY
not-ready state. I assume it's there to say that the slots are full.

>
> > +     if (ret == -ETIMEDOUT) {
> > +             netdev_err(netdev, "LITEETH_READER_READY timed out\n");
>
> ratelimit this as well, please
>
> > +             goto drop;
> > +     }
> > +
> > +     writeb(1, priv->base + LITEETH_READER_START);
> > +
> > +     netdev->stats.tx_bytes += skb->len;
>
> Please count bytes and packets in the same place

AFAIK we don't know the length when the interrupt comes in, so we need
to count both here in xmit?

>
> > +     priv->tx_slot = (priv->tx_slot + 1) % priv->num_tx_slots;
> > +     dev_kfree_skb_any(skb);
> > +     return NETDEV_TX_OK;
> > +drop:
> > +     /* Drop the packet */
> > +     dev_kfree_skb_any(skb);
> > +     netdev->stats.tx_dropped++;
> > +
> > +     return NETDEV_TX_OK;
> > +}