Re: [PATCH V5 2/2] virtio: i2c: Allow zero-length transactions

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

On 2021/10/13 11:39, Viresh Kumar wrote:

> > > @@ -124,13 +140,23 @@ \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C
> > >
> > >  \drivernormative{\subsubsection}{Device Operation}{Device Types / I2C Adapter Device / Device Operation}
> > >
> > > +A driver SHOULD implement the VIRTIO_I2C_F_ZERO_LENGTH_REQUEST feature.
> > >         
> > I don't think this needs to be "SHOULD", as a driver may be written to only
> > talk to certain i2c clients on the device side, and they do not need zero
> > length requests. Maybe this could be
> >
> > "A driver MAY assume that the VIRTIO_I2C_F_ZERO_LENGTH_REQUEST
> > feature is available".
> >       
> The problem here is that "VIRTIO_I2C_FLAGS_M_RD" is supported only
> with VIRTIO_I2C_F_ZERO_LENGTH_REQUEST feature. If the feature isn't
> available, then the device/driver need to encode/decode the direction
> of the transfer (read/write) to/from the permissions of the buffer.
>

The "VIRTIO_I2C_FLAGS_M_RD" is consistent with the permissions of the buffer.

When this flag is set, the buffer is also marked as device write -only with VIRTQ_DESC_F_WRITE set.

So if this feature isn't available we can use the the latter. We only need this flag when there is no buffer.