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

From: Matias Ezequiel Vara Larsen <[email-masked]>
Date: Mon, 13 Oct 2025 11:52:49 +0200

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 <[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:
> > > 
> > 
> > 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 <[email-masked]> 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?

Thanks, Matias.