Re: [PATCH v5] i2c: virtio: add a virtio i2c frontend driver

[Jie Deng]

On 2021/3/2 15:24, Viresh Kumar wrote:
> On 02-03-21, 14:24, Jie Deng wrote:
> > Not for the full duplex. As Paolo explained in those links.
> > We defined a combined request called "write-read request"
> >
> > "
> > 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.
> I think above talks about the real I2C protocol at bus level ?
>
> > In theory there's no difference between one multi-segment transaction
> > and many single-segment transactions _in a single-master scenario_.
> >
> > However, it is a plausible configuration to have multiple guests sharing
> > an I2C host device as if they were multiple master.
> >
> > So the spec should provide a way at least to support for transactions with
> > 1 write and 1 read segment in one request to the same address.
> > "
> >  From the perspective of specification design, it hopes to provide more
> > choices
> > while from the perspective of specific implementation, we can choose what we
> > need
> > as long as it does not violate the specification.
> That is fine, but what I was not able to understand was how do we get
> to know if we should expect both write and read bufs after the out
> header or only one of them ?
>
> I think I have understood that part now, we need to look at incnt and
> outcnt and make out what kind of transfer we need to do.
>
> - If outcnt == 1 and incnt == 2, then it is read operation.
> - If outcnt == 2 and incnt == 1, then it is write operation.
> - If outcnt == 2 and incnt == 2, then it is read-write operation.
>
> Anything else is invalid. Is my understanding correct here ?
Correct.  By checking the sequences of descriptor's R/W in the virtqueue,
You can know the type of request. A simple state machine can be used to
classify in this case.

O I  I   : read request.

O O I  : write request.

O O I I : read-write request.

But if only using the first two types like in this driver, the backend 
will be much easier to
implement since the request is fixed to 3 descriptors and we only need 
to check the second
descriptor to know the type.
> > Since the current Linux driver doesn't use this mechanism. I'm considering
> > to move
> > the "struct virtio_i2c_req" into the driver and use one "buf" instead.
> Linux can very much have its own definition of the structure and so I
> am not in favor of any such structure in the spec as well, it confuses
> people (like me) :).