Re: [PATCH] i2c: virtio: add a virtio i2c frontend driver

[Jie Deng]

On 2020/9/4 12:06, Jason Wang wrote:
> > diff --git a/drivers/i2c/busses/Kconfig b/drivers/i2c/busses/Kconfig
> > index 293e7a0..70c8e30 100644
> > --- a/drivers/i2c/busses/Kconfig
> > +++ b/drivers/i2c/busses/Kconfig
> > @@ -21,6 +21,17 @@ config I2C_ALI1535
> >         This driver can also be built as a module.  If so, the module
> >         will be called i2c-ali1535.
> >   +config I2C_VIRTIO
> > +    tristate "Virtio I2C Adapter"
> > +    depends on VIRTIO
>
>
> I guess it should depend on some I2C module here.
The dependency of I2C is included in the Kconfig in its parent directory.
So there is nothing special to add here.


> > +struct virtio_i2c_msg {
> > +    struct virtio_i2c_hdr hdr;
> > +    char *buf;
> > +    u8 status;
>
>
> Any reason for separating status out of virtio_i2c_hdr?
The status is not from i2c_msg. So I put it out of virtio_i2c_hdr.

> > +};
> > +
> > +/**
> > + * struct virtio_i2c - virtio I2C data
> > + * @vdev: virtio device for this controller
> > + * @completion: completion of virtio I2C message
> > + * @adap: I2C adapter for this controller
> > + * @i2c_lock: lock for virtqueue processing
> > + * @vq: the virtio virtqueue for communication
> > + */
> > +struct virtio_i2c {
> > +    struct virtio_device *vdev;
> > +    struct completion completion;
> > +    struct i2c_adapter adap;
> > +    struct mutex i2c_lock;
> > +    struct virtqueue *vq;
> > +};
> > +
> > +static void virtio_i2c_msg_done(struct virtqueue *vq)
> > +{
> > +    struct virtio_i2c *vi = vq->vdev->priv;
> > +
> > +    complete(&vi->completion);
> > +}
> > +
> > +static int virtio_i2c_add_msg(struct virtqueue *vq,
> > +                  struct virtio_i2c_msg *vmsg,
> > +                  struct i2c_msg *msg)
> > +{
> > +    struct scatterlist *sgs[3], hdr, bout, bin, status;
> > +    int outcnt = 0, incnt = 0;
> > +
> > +    if (!msg->len)
> > +        return -EINVAL;
> > +
> > +    vmsg->hdr.addr = msg->addr;
> > +    vmsg->hdr.flags = msg->flags;
> > +    vmsg->hdr.len = msg->len;
>
>
> Missing endian conversion?
You are right. Need conversion here.
> > +
> > +    vmsg->buf = kzalloc(vmsg->hdr.len, GFP_KERNEL);
> > +    if (!vmsg->buf)
> > +        return -ENOMEM;
> > +
> > +    sg_init_one(&hdr, &vmsg->hdr, sizeof(struct virtio_i2c_hdr));
> > +    sgs[outcnt++] = &hdr;
> > +    if (vmsg->hdr.flags & I2C_M_RD) {
> > +        sg_init_one(&bin, vmsg->buf, msg->len);
> > +        sgs[outcnt + incnt++] = &bin;
> > +    } else {
> > +        memcpy(vmsg->buf, msg->buf, msg->len);
> > +        sg_init_one(&bout, vmsg->buf, msg->len);
> > +        sgs[outcnt++] = &bout;
> > +    }
> > +    sg_init_one(&status, &vmsg->status, sizeof(vmsg->status));
> > +    sgs[outcnt + incnt++] = &status;
> > +
> > +    return virtqueue_add_sgs(vq, sgs, outcnt, incnt, vmsg, GFP_KERNEL);
> > +}
> > +
> > +static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg 
> > *msgs, int num)
> > +{
> > +    struct virtio_i2c *vi = i2c_get_adapdata(adap);
> > +    struct virtio_i2c_msg *vmsg_o, *vmsg_i;
> > +    struct virtqueue *vq = vi->vq;
> > +    unsigned long time_left;
> > +    int len, i, ret = 0;
> > +
> > +    vmsg_o = kzalloc(sizeof(*vmsg_o), GFP_KERNEL);
> > +    if (!vmsg_o)
> > +        return -ENOMEM;
>
>
> It looks to me we can avoid the allocation by embedding virtio_i2c_msg 
> into struct virtio_i2c;
Yeah... That's better. Thanks.


> > +
> > +    mutex_lock(&vi->i2c_lock);
> > +    vmsg_o->buf = NULL;
> > +    for (i = 0; i < num; i++) {
> > +        ret = virtio_i2c_add_msg(vq, vmsg_o, &msgs[i]);
> > +        if (ret) {
> > +            dev_err(&adap->dev, "failed to add msg[%d] to 
> > virtqueue.\n", i);
> > +            goto err_unlock_free;
> > +        }
> > +
> > +        virtqueue_kick(vq);
> > +
> > +        time_left = wait_for_completion_timeout(&vi->completion, 
> > adap->timeout);
> > +        if (!time_left) {
> > +            dev_err(&adap->dev, "msg[%d]: addr=0x%x timeout.\n", i, 
> > msgs[i].addr);
> > +            ret = i;
> > +            goto err_unlock_free;
> > +        }
> > +
> > +        vmsg_i = (struct virtio_i2c_msg *)virtqueue_get_buf(vq, &len);
> > +        if (vmsg_i) {
> > +            /* vmsg_i should point to the same address with vmsg_o */
> > +            if (vmsg_i != vmsg_o) {
> > +                dev_err(&adap->dev, "msg[%d]: addr=0x%x virtqueue 
> > error.\n",
> > +                    i, vmsg_i->hdr.addr);
> > +                ret = i;
> > +                goto err_unlock_free;
> > +            }
>
>
> Does this imply in order completion of i2c device?  (E.g what happens 
> if multiple virtio i2c requests are submitted)
>
> Btw, this always use a single descriptor once a time which makes me 
> suspect if a virtqueue(virtio) is really needed. It looks to me we can 
> utilize the virtqueue by submit the request in a batch.
I'm afraid not all physical devices support batch. I'd like to keep the 
current design and consider
your suggestion as a possible optimization in the future.

Thanks.


> > +}
> > +
> > +static void virtio_i2c_del_vqs(struct virtio_device *vdev)
> > +{
> > +    vdev->config->reset(vdev);
>
>
> Why need reset here?
>
> Thanks
I'm following what other virtio drivers do.
They reset the devices before they clean up the queues.


> > +    vdev->config->del_vqs(vdev);
> > +}
> > +
> > +static int virtio_i2c_setup_vqs(struct virtio_i2c *vi)
> > +{
> > +    struct virtio_device *vdev = vi->vdev;
> > +
> > +    vi->vq = virtio_find_single_vq(vdev, virtio_i2c_msg_done, 
> > "i2c-msg");
>
>
> We've in the scope of ic2, so "msg" should be sufficient.
OK. Will change this name. Thanks.


> > +    return PTR_ERR_OR_ZERO(vi->vq);