Re: [virtio-comment] [PATCH v3] virtio-i2c: add the device specification

[Jie Deng <jie.deng@intel.com>]

On 2020/10/29 4:10, Paolo Bonzini wrote:
> On 27/10/20 07:00, Jie Deng wrote:
> > +The driver queues requests to the virtqueue, and they are used by the
> > +device. The request is the representation of one segment of an I2C
> > +transaction. Each request is of form:
> > +
> > +\begin{lstlisting}
> > +struct virtio_i2c_req {
> > +        le16 addr;
> > +        le16 flags;
> > +        le16 len;
> > +        u8 buf[];
> > +        u8 status;
> > +};
> > +\end{lstlisting}
> Hi,
>
> one extremely common two-segment I2C transaction is not reproducible
> with virtio-i2c.  This is when a write is followed by a read: the master
> starts off the transmission with a write, then sends a second START,
> then continues with a read from the same address.
>
> Would it make sense to define the request like
>
> struct virtio_i2c_req {
> 	le16 addr;
> 	le16 written;
> 	le16 read;
> 	u8 bufwrite[];
> 	u8 status;
> 	u8 bufread[];
> };
>
> instead?
>
> Thanks,
>
> Paolo
Hi Paolo,

I'm not sure why this two-segment I2C transaction is not reproducible ?

my understanding is that we don't need to care about the 
START/STOP/RESTART conditions
between virtio-i2c frontend and backend. These conditions only make 
sense when data
is exchanged between master and slave devices, rather than virtio 
frontend and backend.

Only the backend may need to consider these conditions when doing device 
emulation and
exchanging data with its slave devices. It has nothing to do with the 
frontend.

Is my understanding correct ?

Thanks