Re: [RFC PATCH v3] can: virtio: Initial virtio CAN driver.

[Harald Mommer]

On 11.05.23 17:40, Simon Horman wrote:
> On Thu, May 11, 2023 at 05:14:44PM +0200, Mikhail Golubev-Ciuchea wrote:
> > From: Harald Mommer<harald.mommer@xxxxxxxxxxxxxxx>
> >
> > - CAN Control
> >
> >    - "ip link set up can0" starts the virtual CAN controller,
> >    - "ip link set up can0" stops the virtual CAN controller
> >
> > - CAN RX
> >
> >    Receive CAN frames. CAN frames can be standard or extended, classic or
> >    CAN FD. Classic CAN RTR frames are supported.
> >
> > - CAN TX
> >
> >    Send CAN frames. CAN frames can be standard or extended, classic or
> >    CAN FD. Classic CAN RTR frames are supported.
> >
> > - CAN BusOff indication
> >
> >    CAN BusOff is handled by a bit in the configuration space.
> >
> > Signed-off-by: Harald Mommer<Harald.Mommer@xxxxxxxxxxxxxxx>
> > Signed-off-by: Mikhail Golubev-Ciuchea<Mikhail.Golubev-Ciuchea@xxxxxxxxxxxxxxx>
> > Co-developed-by: Marc Kleine-Budde<mkl@xxxxxxxxxxxxxx>
> > Signed-off-by: Marc Kleine-Budde<mkl@xxxxxxxxxxxxxx>
> > Cc: Damir Shaikhutdinov<Damir.Shaikhutdinov@xxxxxxxxxxxxxxx>
> Hi Mikhail,
>
> thanks for your patch.
> Some minor feedback from my side.
>
> ...
>
> > diff --git a/drivers/net/can/virtio_can.c b/drivers/net/can/virtio_can.c
> ...
>
> > +/* Send a control message with message type either
> > + *
> > + * - VIRTIO_CAN_SET_CTRL_MODE_START or
> > + * - VIRTIO_CAN_SET_CTRL_MODE_STOP.
> > + *
> > + * Unlike AUTOSAR CAN Driver Can_SetControllerMode() there is no requirement
> > + * for this Linux driver to have an asynchronous implementation of the mode
> > + * setting function so in order to keep things simple the function is
> > + * implemented as synchronous function. Design pattern is
> > + * virtio_console.c/__send_control_msg() & virtio_net.c/virtnet_send_command().
> > + */
> > +static u8 virtio_can_send_ctrl_msg(struct net_device *ndev, u16 msg_type)
> > +{
> > +	struct virtio_can_priv *priv = netdev_priv(ndev);
> > +	struct device *dev = &priv->vdev->dev;
> > +	struct virtqueue *vq = priv->vqs[VIRTIO_CAN_QUEUE_CONTROL];
> > +	struct scatterlist sg_out[1];
> > +	struct scatterlist sg_in[1];
> > +	struct scatterlist *sgs[2];
> > +	int err;
> > +	unsigned int len;
> nit: For networking code please arrange local variables in reverse xmas
>       tree order - longest line to shortest.
>
>       You can check this using:https://github.com/ecree-solarflare/xmastree
>
>       In this case I think it would be:
>
> 	struct virtio_can_priv *priv = netdev_priv(ndev);
> 	struct device *dev = &priv->vdev->dev;
> 	struct scatterlist sg_out[1];
> 	struct scatterlist sg_in[1];
> 	struct scatterlist *sgs[2];
> 	struct virtqueue *vq;
> 	unsigned int len;
> 	int err;
>
> 	vq = priv->vqs[VIRTIO_CAN_QUEUE_CONTROL];
>
> ...

https://docs.kernel.org/process/maintainer-netdev.html

I see, I see...

> > +static netdev_tx_t virtio_can_start_xmit(struct sk_buff *skb,
> > +					 struct net_device *dev)
> > +{
> > +	struct virtio_can_priv *priv = netdev_priv(dev);
> > +	struct canfd_frame *cf = (struct canfd_frame *)skb->data;
> > +	struct virtio_can_tx *can_tx_msg;
> > +	struct virtqueue *vq = priv->vqs[VIRTIO_CAN_QUEUE_TX];
> > +	struct scatterlist sg_out[1];
> > +	struct scatterlist sg_in[1];
> > +	struct scatterlist *sgs[2];
> > +	unsigned long flags;
> > +	u32 can_flags;
> > +	int err;
> > +	int putidx;
> > +	netdev_tx_t xmit_ret = NETDEV_TX_OK;
> > +	const unsigned int hdr_size = offsetof(struct virtio_can_tx_out, sdu);
> > +
> > +	if (can_dev_dropped_skb(dev, skb))
> > +		goto kick; /* No way to return NET_XMIT_DROP here */
> > +
> > +	/* No local check for CAN_RTR_FLAG or FD frame against negotiated
> > +	 * features. The device will reject those anyway if not supported.
> > +	 */
> > +
> > +	can_tx_msg = kzalloc(sizeof(*can_tx_msg), GFP_ATOMIC);
> > +	if (!can_tx_msg)
> > +		goto kick; /* No way to return NET_XMIT_DROP here */
> > +
> > +	can_tx_msg->tx_out.msg_type = cpu_to_le16(VIRTIO_CAN_TX);
> > +	can_flags = 0;
> > +
> > +	if (cf->can_id & CAN_EFF_FLAG) {
> > +		can_flags |= VIRTIO_CAN_FLAGS_EXTENDED;
> > +		can_tx_msg->tx_out.can_id = cpu_to_le32(cf->can_id & CAN_EFF_MASK);
> > +	} else {
> > +		can_tx_msg->tx_out.can_id = cpu_to_le32(cf->can_id & CAN_SFF_MASK);
> > +	}
> > +	if (cf->can_id & CAN_RTR_FLAG)
> > +		can_flags |= VIRTIO_CAN_FLAGS_RTR;
> > +	else
> > +		memcpy(can_tx_msg->tx_out.sdu, cf->data, cf->len);
> > +	if (can_is_canfd_skb(skb))
> > +		can_flags |= VIRTIO_CAN_FLAGS_FD;
> > +
> > +	can_tx_msg->tx_out.flags = cpu_to_le32(can_flags);
> > +	can_tx_msg->tx_out.length = cpu_to_le16(cf->len);
> > +
> > +	/* Prepare sending of virtio message */
> > +	sg_init_one(&sg_out[0], &can_tx_msg->tx_out, hdr_size + cf->len);
> > +	sg_init_one(&sg_in[0], &can_tx_msg->tx_in, sizeof(can_tx_msg->tx_in));
> > +	sgs[0] = sg_out;
> > +	sgs[1] = sg_in;
> > +
> > +	putidx = virtio_can_alloc_tx_idx(priv);
> > +
> > +	if (unlikely(putidx < 0)) {
> > +		netif_stop_queue(dev);
> > +		kfree(can_tx_msg);
> > +		netdev_warn(dev, "TX: Stop queue, no putidx available\n");
> If I understand things correctly, this code is on the datapath.
> So perhaps these should be rate limited, or only logged once.
> Likewise elsewhere in this function.

This block is almost a never. Like all blocks returning NETDEV_TX_BUSY.

virtio_can_alloc_tx_idx() may return on error -ENOSPC or -ENOMEM.

The queue is stopped under "/* Normal queue stop when no transmission 
slots are left */" before -ENOSPC is seen here. When -ENOMEM is returned 
we have more severe problems as some trace, considered as a theoretical 
issue only. So if the trace is seen most likely flow control got broken, 
needs to be fixed and I won't want to miss this.

> > +		xmit_ret = NETDEV_TX_BUSY;
> > +		goto kick;
> > +	}
> > +
> > +	can_tx_msg->putidx = (unsigned int)putidx;
> > +
> > +	/* Protect list operation */
> > +	spin_lock_irqsave(&priv->tx_lock, flags);
> > +	list_add_tail(&can_tx_msg->list, &priv->tx_list);
> > +	spin_unlock_irqrestore(&priv->tx_lock, flags);
> > +
> > +	/* Push loopback echo. Will be looped back on TX interrupt/TX NAPI */
> > +	can_put_echo_skb(skb, dev, can_tx_msg->putidx, 0);
> > +
> > +	/* Protect queue and list operations */
> > +	spin_lock_irqsave(&priv->tx_lock, flags);
> > +	err = virtqueue_add_sgs(vq, sgs, 1u, 1u, can_tx_msg, GFP_ATOMIC);
> > +	if (err != 0) { /* unlikely when vq->num_free was considered */
> > +		list_del(&can_tx_msg->list);
> > +		can_free_echo_skb(dev, can_tx_msg->putidx, NULL);
> > +		virtio_can_free_tx_idx(priv, can_tx_msg->putidx);
> > +		spin_unlock_irqrestore(&priv->tx_lock, flags);
> > +		netif_stop_queue(dev);
> > +		kfree(can_tx_msg);
> > +		if (err == -ENOSPC)
> > +			netdev_dbg(dev, "TX: Stop queue, no space left\n");
> > +		else
> > +			netdev_warn(dev, "TX: Stop queue, reason = %d\n", err);
> > +		xmit_ret = NETDEV_TX_BUSY;
> > +		goto kick;
> > +	}
Below is the "normal flow control" block which is likely to be entered 
under load. Everything else returning NETDEV_TX_BUSY is "never" or at 
least "almost never".
> > +	/* Normal queue stop when no transmission slots are left */
> > +	if (atomic_read(&priv->tx_inflight) >= priv->can.echo_skb_max ||
> > +	    vq->num_free == 0 || (vq->num_free < 2 &&
> > +	    !virtio_has_feature(vq->vdev, VIRTIO_RING_F_INDIRECT_DESC))) {
> > +		netif_stop_queue(dev);
> > +		netdev_dbg(dev, "TX: Normal stop queue\n");
> > +	}
> > +
> > +	spin_unlock_irqrestore(&priv->tx_lock, flags);
> > +
> > +kick:
> > +	if (netif_queue_stopped(dev) || !netdev_xmit_more()) {
> > +		if (!virtqueue_kick(vq))
> > +			netdev_err(dev, "%s(): Kick failed\n", __func__);
> > +	}
> > +
> > +	return xmit_ret;
> > +}
> ...