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

[Francesco Valla <francesco@valla.it>]

Hello Matias,

On Monday, 13 October 2025 at 11:52:49 Matias Ezequiel Vara Larsen <mvaralar@redhat.com> wrote:
> On Fri, Oct 10, 2025 at 11:20:22PM +0200, Francesco Valla wrote:
> > On Friday, 10 October 2025 at 17:46:25 Matias Ezequiel Vara Larsen <mvaralar@redhat.com> 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:
> > > > 
> > > 
> > > I am working on the v6 by addressing the comments in this thread.
> > > 
> > > > On Monday, 8 January 2024 at 14:10:35 Mikhail Golubev-Ciuchea <Mikhail.Golubev-Ciuchea@opensynergy.com> wrote:
> > > > 
> > > > [...]
> > > > 
> > > > > +
> > > > > +/* virtio_can private data structure */
> > > > > +struct virtio_can_priv {
> > > > > +	struct can_priv can;	/* must be the first member */
> > > > > +	/* NAPI for RX messages */
> > > > > +	struct napi_struct napi;
> > > > > +	/* NAPI for TX messages */
> > > > > +	struct napi_struct napi_tx;
> > > > > +	/* The network device we're associated with */
> > > > > +	struct net_device *dev;
> > > > > +	/* The virtio device we're associated with */
> > > > > +	struct virtio_device *vdev;
> > > > > +	/* The virtqueues */
> > > > > +	struct virtqueue *vqs[VIRTIO_CAN_QUEUE_COUNT];
> > > > > +	/* I/O callback function pointers for the virtqueues */
> > > > > +	vq_callback_t *io_callbacks[VIRTIO_CAN_QUEUE_COUNT];
> > > > > +	/* Lock for TX operations */
> > > > > +	spinlock_t tx_lock;
> > > > > +	/* Control queue lock. Defensive programming, may be not needed */
> > > > > +	struct mutex ctrl_lock;
> > > > > +	/* Wait for control queue processing without polling */
> > > > > +	struct completion ctrl_done;
> > > > > +	/* List of virtio CAN TX message */
> > > > > +	struct list_head tx_list;
> > > > > +	/* Array of receive queue messages */
> > > > > +	struct virtio_can_rx rpkt[128];
> > > > 
> > > > This array should probably be allocated dynamically at probe - maybe
> > > > using a module parameter instead of a hardcoded value as length? 
> > > > 
> > > 
> > > If I allocate this array in probe(), I would not know sdu[] in advance
> > > if I defined it as a flexible array. That made me wonder: can sdu[] be
> > > defined as flexible array for rx? 
> > > 
> > > Thanks.
> > > 
> > 
> > One thing that can be done is to define struct virtio_can_rx as:
> > 
> > struct virtio_can_rx {
> > #define VIRTIO_CAN_RX                   0x0101
> > 	__le16 msg_type;
> > 	__le16 length; /* 0..8 CC, 0..64 CAN-FD, 0..2048 CAN-XL, 12 bits */
> > 	__u8 reserved_classic_dlc; /* If CAN classic length = 8 then DLC can be 8..15 */
> > 	__u8 padding;
> > 	__le16 reserved_xl_priority; /* May be needed for CAN XL priority */
> > 	__le32 flags;
> > 	__le32 can_id;
> > 	__u8 sdu[] __counted_by(length);
> > };
> > 
> > and then allocate the rpkt[] array using the maximum length for SDU:
> > 
> > priv->rpkt = kcalloc(num_rx_buffers,
> > 		sizeof(struct virtio_can_rx) + VIRTIO_CAN_MAX_DLEN,
> > 		GFP_KERNEL);
> > 
> > In this way, the size of each member of rpkt[] is known and is thus
> > suitable for virtio_can_populate_vqs().
> > 
> > 
> 
> Thanks for your answer. What is the value of VIRTIO_CAN_MAX_DLEN? I
> can't find it nor in the code or in the spec. I guess is 64 bytes? Also,
> IIUC, using __counted_by() would not end up saving space but adding an
> extra check for the compiler. Am I right? In that case, can't I just use
> a fixed array of VIRTIO_CAN_MAX_DLEN bytes?

My bad, I forgot to say that VIRTIO_CAN_MAX_DLEN has to be defined, but:
given some more thoughts, maybe this can be a dynamic value based on
the features received from the virtio framework, to avoid wasting memory?

E.g.:

if (virtio_has_feature(VIRTIO_CAN_F_CAN_FD))
	sdu_len = CANFD_MAX_DLEN;
else
	sdu_len = CAN_MAX_DLEN;

priv->rpkt = kcalloc(num_rx_buffers, sizeof(struct virtio_can_rx) + sdu_len,
		GFP_KERNEL);


My understanding of __counted_by() is the same: additional checks but nothing
more.


CAN-XL appears to be not supported by the virtio specs v1.4 [1], but if/when
it will be, the addition of an additional case would be simple.

[1] https://github.com/oasis-tcs/virtio-spec/blob/virtio-1.4/device-types/can/description.tex#L33


> 
> Thanks, Matias.
> 
>

Many thanks to you for your effort!

Regards,
Francesco