Subject: Re: [PATCH v5] can: virtio: Initial virtio CAN driver.

From: Matias Ezequiel Vara Larsen <[email-masked]>
Date: Tue, 21 Oct 2025 15:15:24 +0200

On Tue, Oct 21, 2025 at 02:08:35PM +0200, Francesco Valla wrote:
> On Tuesday, 21 October 2025 at 11:40:07 Matias Ezequiel Vara Larsen <[email-masked]> wrote:
> > On Mon, Oct 20, 2025 at 11:24:15PM +0200, Francesco Valla wrote:
> > > On Monday, 20 October 2025 at 16:56:08 Matias Ezequiel Vara Larsen <[email-masked]> wrote:
> > > > On Tue, Oct 14, 2025 at 06:01:07PM +0200, Francesco Valla wrote:
> > > > > On Tuesday, 14 October 2025 at 12:15:12 Matias Ezequiel Vara Larsen <[email-masked]> wrote:
> > > > > > On Thu, Sep 11, 2025 at 10:59:40PM +0200, Francesco Valla wrote:
> > > > > > > Hello Mikhail, Harald,
> > > > > > > 
> > > > > > > hoping there will be a v6 of this patch soon, a few comments:
> > > > > > > 
> > > > > > > On Monday, 8 January 2024 at 14:10:35 Mikhail Golubev-Ciuchea <[email-masked]> wrote:
> > > > > > > 
> > > > > > > [...]
> > > > > > > > +
> > > > > > > > +/* Compare with m_can.c/m_can_echo_tx_event() */
> > > > > > > > +static int virtio_can_read_tx_queue(struct virtqueue *vq)
> > > > > > > > +{
> > > > > > > > +	struct virtio_can_priv *can_priv = vq->vdev->priv;
> > > > > > > > +	struct net_device *dev = can_priv->dev;
> > > > > > > > +	struct virtio_can_tx *can_tx_msg;
> > > > > > > > +	struct net_device_stats *stats;
> > > > > > > > +	unsigned long flags;
> > > > > > > > +	unsigned int len;
> > > > > > > > +	u8 result;
> > > > > > > > +
> > > > > > > > +	stats = &dev->stats;
> > > > > > > > +
> > > > > > > > +	/* Protect list and virtio queue operations */
> > > > > > > > +	spin_lock_irqsave(&can_priv->tx_lock, flags);
> > > > > > > > +
> > > > > > > > +	can_tx_msg = virtqueue_get_buf(vq, &len);
> > > > > > > > +	if (!can_tx_msg) {
> > > > > > > > +		spin_unlock_irqrestore(&can_priv->tx_lock, flags);
> > > > > > > > +		return 0; /* No more data */
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	if (unlikely(len < sizeof(struct virtio_can_tx_in))) {
> > > > > > > > +		netdev_err(dev, "TX ACK: Device sent no result code\n");
> > > > > > > > +		result = VIRTIO_CAN_RESULT_NOT_OK; /* Keep things going */
> > > > > > > > +	} else {
> > > > > > > > +		result = can_tx_msg->tx_in.result;
> > > > > > > > +	}
> > > > > > > > +
> > > > > > > > +	if (can_priv->can.state < CAN_STATE_BUS_OFF) {
> > > > > > > > +		/* Here also frames with result != VIRTIO_CAN_RESULT_OK are
> > > > > > > > +		 * echoed. Intentional to bring a waiting process in an upper
> > > > > > > > +		 * layer to an end.
> > > > > > > > +		 * TODO: Any better means to indicate a problem here?
> > > > > > > > +		 */
> > > > > > > > +		if (result != VIRTIO_CAN_RESULT_OK)
> > > > > > > > +			netdev_warn(dev, "TX ACK: Result = %u\n", result);
> > > > > > > 
> > > > > > > Maybe an error frame reporting CAN_ERR_CRTL_UNSPEC would be better?
> > > > > > > 
> > > > > > I am not sure. In xilinx_can.c, CAN_ERR_CRTL_UNSPEC is indicated during
> > > > > > a problem in the rx path and this is the tx path. I think the comment
> > > > > > refers to improving the way the driver informs this error to the user
> > > > > > but I may be wrong.
> > > > > > 
> > > > > 
> > > > > Since we have no detail of what went wrong here, I suggested
> > > > > CAN_ERR_CRTL_UNSPEC as it is "unspecified error", to be coupled with a
> > > > > controller error with id CAN_ERR_CRTL; however, a different error might be
> > > > > more appropriate.
> > > > > 
> > > > > For sure, at least in my experience, having a warn printed to kmsg is *not*
> > > > > enough, as the application sending the message(s) would not be able to detect
> > > > > the error.
> > > > > 
> > > > > 
> > > > > > > For sure, counting the known errors as valid tx_packets and tx_bytes
> > > > > > > is misleading.
> > > > > > > 
> > > > > > 
> > > > > > I'll remove the counters below.
> > > > > > 
> > > > > 
> > > > > We don't really know what's wrong here - the packet might have been sent and
> > > > > and then not ACK'ed, as well as any other error condition (as it happens in the
> > > > > reference implementation from the original authors [1]). Echoing the packet
> > > > > only "to bring a waiting process in an upper layer to an end" and incrementing
> > > > > counters feels wrong, but maybe someone more expert than me can advise better
> > > > > here.
> > > > > 
> > > > > 
> > > > 
> > > > I agree. IIUC, in case there has been a problem during transmission, I
> > > > should 1) indicate this by injecting a CAN_ERR_CRTL_UNSPEC package with
> > > > netif_rx() and 2) use can_free_echo_skb() and increment the tx_error
> > > > stats. Is this correct?
> > > > 
> > > > Matias
> > > > 
> > > > 
> > > 
> > > That's my understanding too! stats->tx_dropped should be the right value to
> > > increment (see for example [1]).
> > > 
> > > [1] https://elixir.bootlin.com/linux/v6.17.3/source/drivers/net/can/ctucanfd/ctucanfd_base.c#L1035
> > > 
> > 
> > I think the counter to increment would be stats->tx_errors in this case ...
> > 
> 
> I don't fully agree. tx_errors is for CAN frames that got transmitted but then
> lead to an error (e.g.: no ACK), while here we might be dealing with frames
> that didn't even manage to reach the transmission queue [1].
> 
Let's use tx_dropped then, I honestly do not have an strong opinion
about it. We can change that later if we are not happy.

Matias