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    SubjectRe: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver
    On 29-06-21, 10:27, Wolfram Sang wrote:
    > > While we are at it, this has been replaced by a Rust counterpart [1]
    > > (as that makes it hypervisor agnostic, which is the goal of my work
    > > here) and I need someone with I2C knowledge to help review it. It
    > > should be okay even if you don't understand Rust a lot, just review
    > > this file[2] which is where most of i2c specific stuff lies.
    > Can't promise I can do this before my holidays, but I will try.


    > > I am not sure why you say I2C_RDWR isn't supported. The spec and Linux
    > This is how I interpreted Arnd's response. I said mulitple times that I
    > might be missing something so I double check.
    > > SMBUS. To clarify on an earlier point, every virtio transfer may
    > > contain one or more struct i2c_msg instances, all processed together
    > > (as expected).
    > That was the information missing for me so far becasue...
    > > If you see virtio_i2c_send_reqs() in this patch, you will see that it
    > > converts a stream of i2c_req messages to their virtio counterparts and
    > > send them together, consider it a single transaction.
    > ... when I checked virtio_i2c_send_reqs(), I also saw
    > virtqueue_add_sgs() but I had no idea if this will end up as REP_START
    > on the physical bus or not. But it definately should.

    Just think of virtqueue_add_sgs() as something that setups the
    structures for transfer. The actual stuff at the other end (host)
    happens only after virtqueue_kick() is called at the guest (this
    notifies the host that data is present now), in response the backend
    running at host will re-create the struct i2c_msg and issue:

    struct i2c_rdwr_ioctl_data data;
    data.nmsgs = count;
    data.msgs = msgs;

    return ioctl(adapter->fd, I2C_RDWR, &data);

    So we will end up recreating the exact situation as when
    virtio_i2c_xfer() is called.


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