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

[Harald Mommer <harald.mommer@opensynergy.com>]

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@opensynergy.com>
>>
>> - 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@opensynergy.com>
>> Signed-off-by: Mikhail Golubev-Ciuchea<Mikhail.Golubev-Ciuchea@opensynergy.com>
>> Co-developed-by: Marc Kleine-Budde<mkl@pengutronix.de>
>> Signed-off-by: Marc Kleine-Budde<mkl@pengutronix.de>
>> Cc: Damir Shaikhutdinov<Damir.Shaikhutdinov@opensynergy.com>
> 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;
>> +}
> ...