Re: [PATCH] virtio: i2c: Allow buffer less transactions

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

On 2021/7/29 13:47, Viresh Kumar wrote:
> On 29-07-21, 13:39, Jie Deng wrote:
>
> > Well, then can we also add a flag to mark the zero-length request.
> >
> > I think it will be convenient for backend developers.
> Hmm, I think that information is already present at the device since
> only two elements will be present in the received request, one out
> buffer and one in buffer. And since the middle (data) buffer is not
> present, the device can easily know that it is a zero-length request.
>
> I don't see a real advantage of adding a flag for this.
>   


The number of descriptors required for a request will not be fixed.

If there is a descriptor sequence in the virtqueue, then you may need a 
state machine

to parse the sequence. For example, when you have


WRÂÂ ÂÂ RR Â ÂÂÂÂ ZR

O O IÂÂ O I IÂÂÂÂ O I

* WR:(write request), RR (read request), ZR (zero request), O: output 
descriptor, I: input descriptor


When you encounter "O I" combination, you have to read the element after 
it to distinguish

RR and ZR. Will this bring unnecessary difficulties for the backend coding ?