Re: [PATCH v10] i2c: virtio: add a virtio i2c frontend driver

[Jie Deng]

On 2021/7/5 20:18, Viresh Kumar wrote:
> On 29-06-21, 12:43, Wolfram Sang wrote:
> > >  From the spec:
> > >
> > > The case when ``length of \field{write_buf}''=0, and at the same time,
> > > ``length of \field{read_buf}''=0 doesn't make any sense.
> > >
> > > I mentioned this in my first reply and to my understanding I did not get
> > > a reply that this has changed meanwhile.
> > Also, this code as mentioned before:
> >
> > > +             if (!msgs[i].len)
> > > +                     break;
> > I hope this can extended in the future to allow zero-length messages. If
> > this is impossible we need to set an adapter quirk instead.
> Wolfram,
>
> I stumbled again upon this while working at the backend implementation.
>
> If you look at i2c_smbus_xfer_emulated(), the command is always sent via
> msgbuf0[0]. Even in the case of I2C_SMBUS_QUICK, where we set msg[0].len = 0, we
> still send the buf. This is really confusing :(
>
> Do I understand correctly that we always need to send msg[0].buf even when
> msg[0].len is 0 ?
>
> If so, it would be difficult to implement this with the current i2c virtio
> specification, as the msg.len isn't really passed from guest to host, rather it
> is inferred using the length of the buffer itself. And so we can't really pass a
> buffer if length is 0.
>
> Moreover, the driver uses i2c_get_dma_safe_msg_buf(), which also depends on the
> length parameter here to allocate the buffer and copy data to it.
>
> All in all, the latest version of the driver doesn't work with "i2cdetect -q <bus>".
>
> To make it work, I had to add this:
>
> diff --git a/drivers/i2c/busses/i2c-virtio.c b/drivers/i2c/busses/i2c-virtio.c
> index 731267d42292..5b8bd98ae38e 100644
> --- a/drivers/i2c/busses/i2c-virtio.c
> +++ b/drivers/i2c/busses/i2c-virtio.c
> @@ -73,6 +73,9 @@ static int virtio_i2c_prepare_reqs(struct virtqueue *vq,
>                  sg_init_one(&out_hdr, &reqs[i].out_hdr, sizeof(reqs[i].out_hdr));
>                  sgs[outcnt++] = &out_hdr;
>
> +               if (!msgs[i].len)
> +                       msgs[i].len = 1;
> +
>                  if (msgs[i].len) {
>                          reqs[i].buf = i2c_get_dma_safe_msg_buf(&msgs[i], 1);
>                          if (!reqs[i].buf)
>
> which made it I2C_SMBUS_BYTE instead of I2C_SMBUS_QUICK.
>
> What should we do here Wolfram?
>
>
> Jie, while wolfram comes back and replies to this, I think you need to switch
> back to NOT supporting zero length transfer and set update virtio_i2c_func() to
> return:
>
>          I2C_FUNC_I2C | (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK);
>
> Support for zero-length transfers and I2C_FUNC_SMBUS_QUICK can be added
> separately.
>
> Thanks.


It's OK to me. Let's see what Wolfram says when he comes back.

I will send the updated version then.

Thanks.