Re: [PATCH 3/5] tools/vhost-user-i2c: Add backend driver

[Viresh Kumar]

On 25-03-21, 17:16, Arnd Bergmann wrote:
> On Wed, Mar 24, 2021 at 8:33 AM Viresh Kumar <viresh.kumar@linaro.org> wrote:
> 
> > +static uint8_t vi2c_xfer(VuDev *dev, struct i2c_msg *msg)
> > +{
> > +    VuI2c *i2c = container_of(dev, VuI2c, dev.parent);
> > +    struct i2c_rdwr_ioctl_data data;
> > +    VI2cAdapter *adapter;
> > +
> > +    adapter = vi2c_find_adapter(i2c, msg->addr);
> > +    if (!adapter) {
> > +        g_printerr("Failed to find adapter for address: %x\n", msg->addr);
> > +        return VIRTIO_I2C_MSG_ERR;
> > +    }
> > +
> > +    data.nmsgs = 1;
> > +    data.msgs = msg;
> > +
> > +    if (ioctl(adapter->fd, I2C_RDWR, &data) < 0) {
> > +        g_printerr("Failed to transfer data to address %x : %d\n", 
> > msg->addr, errno);
> > +        return VIRTIO_I2C_MSG_ERR;
> > +    }
> 
> As you found during testing, this doesn't work for host kernels
> that only implement the SMBUS protocol. Since most i2c clients
> only need simple register read/write operations, I think you should
> at least handle the common ones (and one two byte read/write)
> here to make it more useful.

I am thinking if that is what we really want to support, then
shouldn't the i2c virtio spec be updated first to support SMBUS type
transfers as well?

> > +static void vi2c_handle_ctrl(VuDev *dev, int qidx)
> > +{
> > +    VuVirtq *vq = vu_get_queue(dev, qidx);
> > +    struct i2c_msg msg;
> > +    struct virtio_i2c_out_hdr *out_hdr;
> > +    struct virtio_i2c_in_hdr *in_hdr;
> > +    bool fail_next = false;
> > +    size_t len, in_hdr_len;
> > +
> > +    for (;;) {
> > +        VuVirtqElement *elem;
> > +
> > +        elem = vu_queue_pop(dev, vq, sizeof(VuVirtqElement));
> > +        if (!elem) {
> > +            break;
> > +        }
> > +
> > +        g_debug("%s: got queue (in %d, out %d)", __func__, elem->in_num,
> > +                elem->out_num);
> > +
> > +        /* Validate size of out header */
> > +        if (elem->out_sg[0].iov_len != sizeof(*out_hdr)) {
> > +            g_warning("%s: Invalid out hdr %zu : %zu\n", __func__,
> > +                      elem->out_sg[0].iov_len, sizeof(*out_hdr));
> > +            continue;
> > +        }
> > +
> > +        out_hdr = elem->out_sg[0].iov_base;
> > +
> > +        /* Bit 0 is reserved in virtio spec */
> > +        msg.addr = out_hdr->addr >> 1;
> > +
> > +        /* Read Operation */
> > +        if (elem->out_num == 1 && elem->in_num == 2) {
> > +            len = elem->in_sg[0].iov_len;
> > +            if (!len) {
> > +                g_warning("%s: Read buffer length can't be zero\n", 
> > __func__);
> > +                continue;
> > +            }
> 
> 
> It looks like you are not handling endianness conversion here. As far as I
> can tell, the protocol requires little-endian data, but the code might
> run on a big-endian CPU.
> 
> Jie Deng also pointed out the type differences, but actually handling
> them correctly is more important that describing them the right way.

Right, I missed that.

-- 
viresh