Re: [PATCH 1/5] ethernet: add sun8i-emac driver

LABBE Corentin
2016-06-09T09:44:02Z

Hello

I agree to all your comments, but for some I have additionnal questions

On Mon, Jun 06, 2016 at 11:25:15AM -0700, Florian Fainelli wrote:
> On 06/03/2016 02:56 AM, LABBE Corentin wrote:
>
> [snip]
>
> > +
> > +/* The datasheet said that each descriptor can transfers up to 4096bytes
> > + * But latter, a register documentation reduce that value to 2048
> > + * Anyway using 2048 cause strange behaviours and even BSP driver use 2047
> > + */
> > +#define DESC_BUF_MAX 2044
> > +#if (DESC_BUF_MAX < (ETH_FRAME_LEN + 4))
> > +#error "DESC_BUF_MAX must be set at minimum to ETH_FRAME_LEN + 4"
> > +#endif
>
> You can probably drop that, it would not make much sense to enable
> fragments and a buffer size smaller than ETH_FRAME_LEN + ETH_FCS_LEN anyway.
>

I has added this test for preventing someone who want to "optimize" DESC_BUF_MAX to doing mistake.
But I agree that it is of low use.

> > +/* Return the number of contiguous free descriptors
> > + * starting from tx_slot
> > + */
> > +static int rb_tx_numfreedesc(struct net_device *ndev)
> > +{
> > + struct sun8i_emac_priv *priv = netdev_priv(ndev);
> > +
> > + if (priv->tx_slot < priv->tx_dirty)
> > + return priv->tx_dirty - priv->tx_slot;
>
> Does this work with if tx_dirty wraps around?
>

The tx_dirty cannot wrap since I always keep an empty slot. (tx_slot cannot go equal or after tx_dirty)

> > +/* Grab a frame into a skb from descriptor number i */
> > +static int sun8i_emac_rx_from_ddesc(struct net_device *ndev, int i)
> > +{
> > + struct sk_buff *skb;
> > + struct sun8i_emac_priv *priv = netdev_priv(ndev);
> > + struct dma_desc *ddesc = priv->dd_rx + i;
> > + int frame_len;
> > + int crc_checked = 0;
> > +
> > + if (ndev->features & NETIF_F_RXCSUM)
> > + crc_checked = 1;
>
> Assuming CRC here refers to the Ethernet frame's FCS, then this is
> absolutely not how NETIF_F_RXCSUM works. NETIF_F_RXCSUM is about your
> Ethernet adapter supporting L3/L4 checksum offloads, while the Ethernet
> FCS is pretty much mandatory for the frame to be properly received in
> the first place. Can you clarify which way it is?
>

No CRC here is RXCSUM. I understand the misnaming.
I will rename the variable to rxcsum_done.

> > +
> > + priv->ndev->stats.rx_packets++;
> > + priv->ndev->stats.rx_bytes += frame_len;
> > + priv->rx_sk[i] = NULL;
> > +
> > + /* this frame is not the last */
> > + if ((ddesc->status & BIT(8)) == 0) {
> > + dev_warn(priv->dev, "Multi frame not implemented currlen=%d\n",
> > + frame_len);
> > + }
> > +
> > + sun8i_emac_rx_sk(ndev, i);
> > +
> > + netif_rx(skb);
>
> netif_receive_skb() at the very least, or if you implement NAPI, like
> you shoud napi_gro_receive().
>

netif_receive_skb documentation say
"This function may only be called from softirq context and interrupts should be enabled."
but the calling functions is in hardirq context.

> > + return 0;
> > +}
> > +
> > +/* Cycle over RX DMA descriptors for finding frame to receive
> > + */
> > +static int sun8i_emac_receive_all(struct net_device *ndev)
> > +{
> > + struct sun8i_emac_priv *priv = netdev_priv(ndev);
> > + struct dma_desc *ddesc;
> > +
> > + ddesc = priv->dd_rx + priv->rx_dirty;
> > + while (!(ddesc->status & BIT(31))) {
> > + sun8i_emac_rx_from_ddesc(ndev, priv->rx_dirty);
> > + rb_inc(&priv->rx_dirty, nbdesc_rx);
> > + ddesc = priv->dd_rx + priv->rx_dirty;
> > + };
>
> So, what if we ping flood your device here, is not there a remote chance
> that we keep the RX interrupt so busy we can't break out of this loop,
> and we are executing from hard IRQ context, that's bad.
>

I have added a start variable for preventing to do more than a full loop.

> > +
> > + return 0;
> > +}
> > +
> > +/* iterate over dma_desc for finding completed xmit.
> > + * Called from interrupt context, so no need to spinlock tx
>
> Humm, well it depends if what you are doing here may race with
> ndo_start_xmit(), and usually it does.
>

I believe that how it is designed it cannot race each over (access the same descriptor slot) since I keep a free slot between each other.

> Also, you should consider completing TX packets in NAPI context (soft
> IRQ) instead of hard IRQs like here.
>

I wanted to finish this driver the "old" way (with hard IRQ) and implementing NAPI after as a Kconfig choice.
Does NAPI is mandatory now ? (or really recommended)
For resuming my understanding, NAPI is good when expecting high traffic. (so my Kconfig idea)
If you say that NAPI is really preferable, I will do it.

> > + /* last descriptor point back to first one */
> > + ddesc--;
> > + ddesc->next = (u32)priv->dd_rx_phy;
>
> So is there a limitation of this hardware that can only do DMA within
> the first 4GB of the system?
>

Yes, I have added all DMA stuff for handling that after apritzel review.

> > +static int sun8i_emac_check_if_running(struct net_device *ndev)
> > +{
> > + if (!netif_running(ndev))
> > + return -EBUSY;
>
> This does not seem like the intended usage of a
>

I have changed the return code after reading other drivers.
But could you end your sentence for be sure that the problem is that.

> > +
> > +static int sun8i_emac_remove(struct platform_device *pdev)
> > +{
> > + struct net_device *ndev = platform_get_drvdata(pdev);
> > +
> > + unregister_netdev(ndev);
> > + platform_set_drvdata(pdev, NULL);
>
> Missing unregister_netdevice() here.
>

Does I need to replace unregister_netdev by it ?
They seems to to the same job.

> > + free_netdev(ndev);
> > +
> > + return 0;
> > +}
> --
> Florian

Thanks for your review

Regards

LABBE Corentin