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

[Arnd Bergmann <arnd@arndb.de>]

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.

> +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.

        Arnd