Re: [virtio-comment] [PATCH v5] virtio-i2c: add the device specification

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 22 Dec 2020 14:11:24 +0800
Jie Deng <jie.deng@intel.com> wrote:

> On 2020/12/20 3:05, Michael S. Tsirkin wrote:
> > On Fri, Dec 18, 2020 at 10:06:45AM +0800, Jie Deng wrote:  
> >> On 2020/12/17 18:26, Stefan Hajnoczi wrote:  
> >>> On Thu, Dec 17, 2020 at 03:00:55AM -0500, Michael S. Tsirkin wrote:  
> >>>> On Thu, Dec 17, 2020 at 03:08:07PM +0800, Jie Deng wrote:  
> >>>>>           +The \field{flags} of the request is currently reserved as zero for future
> >>>>>           +feature extensibility.
> >>>>>           +
> >>>>>           +The \field{written} of the request is the number of data bytes in the \field{write_buf}
> >>>>>           +being written to the I2C slave address.
> >>>>>
> >>>>>       This field seems redundant since the device can determine the size of
> >>>>>       write_buf implicitly from the total out buffer size. virtio-blk takes
> >>>>>       this approach.
> >>>>>
> >>>>> The read/write are the actual number of data bytes being read from or written
> >>>>> to the device
> >>>>> which is not determined by the device. So I don't think it is redundant.  
> >>>> I am still not sure I understand the difference.
> >>>> This point is unclear to multiple people.  
> >>> I think I get it now. This is made clear by splitting the struct:
> >>>
> >>>     /* Driver->device fields */
> >>>     struct virtio_i2c_out_hdr
> >>>     {
> >>>         le16 addr;
> >>>         le16 padding;
> >>>         le32 flags;
> >>>     };
> >>>
> >>>     /* Device->driver fields */
> >>>     struct virtio_i2c_in_hdr
> >>>     {
> >>>         le16 written;
> >>>         le16 read;
> >>>         u8 status;
> >>>     };  
> >> written/read are not device->driver fields. They are driver->device fields.
> >> They are not determined by the device but the driver(user).
> >>
> >> However, Michael said that the two fields may duplicate buf size available
> >> in the descriptor. He intended to remove them.
> >>
> >> "
> >> I note that read and written actually duplicate buf size
> >> available in the descriptor.
> >> Given we no longer mirror i2c_msg 1:1 do we still want to do this?
> >> It will be trivial for the host device to populate these fields
> >> correctly for linux.
> >> Duplication of information iten leads to errors ...
> >> "
> >>
> >> But there is a corner case I'm not sure if you have noticed.
> >>
> >> read and written can be 0. I think we may not put a buf with size 0 into the
> >> virtqueue.  
> > You always have the header and the status, right?
> > E.g. with the below, the total buffer size is virtio_i2c_out_hdr size +
> > write size for writes and read size + virtio_i2c_in_hdr size for reads.
> > Neither result is ever 0.  
> 
> Then how to distinguish the request type the buffer contains.

I have read through the thread and I remain confused.

> 
> Each type will have both virtio_i2c_out_hdr and virtio_i2c_in_hdr.
> the backend can know the type by checking the read/written.
> 
> If the read=0 and the written>0, the request is a write request
> The buffer may contains 3 scatterlist:
> 
> virtio_i2c_out_hdr // scatterlist[0]

So, what does virtio_i2c_{out,in}_hdr contain here? If it is the one from
above, ...

> 
>      buf[/* this is write data, since read = 0 */] // scatterlist[1]
> 
>      virtio_i2c_in_hdr // scatterlist[2]

...we do not know whether there's read data, write data, or what their
length is, until we've actually consumed the whole buffer, and then we
have to go backwards.

> 
> If the read>0 and the written=0, the request is a read request.
> The buffer may contains 3 scatterlist:
> 
> virtio_i2c_out_hdr // scatterlist[0]
> 
>      buf[/* This is read data, since written = 0 */] // scatterlist[1]
> 
>      virtio_i2c_in_hdr // scatterlist[2]
> 
> If the read>0 and the written>0, the request is a write-read request.
> The buffer may contains 4 scatterlist:
> 
> virtio_i2c_out_hdr   // scatterlist[0]
> 
>      buf[/*write data*/]  // scatterlist[1]
> 
>      buf[/*read data*/] // scatterlist[2]
> 
>      virtio_i2c_in_hdr // scatterlist[3]

Is there any reason why we need to infer the type of the request by
checking some lengths? Can't we just specify explicit flags for read
and write? What am I missing?

> 
> >> @Stefan @Paolo
> >>
> >> So what's your opinion about these two fields ?
> >>  
> >>>     /*
> >>>      * Virtqueue element layout looks like this:
> >>>      *
> >>>      * struct virtio_i2c_out_hdr out_hdr; /* OUT */
> >>>      * u8 write_buf[]; /* OUT */
> >>>      * u8 read_buf[]; /* IN */
> >>>      * struct virtio_i2c_in_hdr in_hdr; /* IN */
> >>>      */
> >>>
> >>> This makes sense to me: a bi-directional request has both write_buf[]
> >>> and read_buf[] so the vring used.len field is not enough to report back
> >>> how many bytes were written and read. The virtio_i2c_in_hdr fields are
> >>> really needed.
> >>>
> >>> Please split the struct in the spec so it's clear how this works.  


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