Virtualization - Jul 2021 - [PATCH v11] i2c: virtio: add a virtio i2c frontend driver

On 01-07-21, 14:10, Jie Deng wrote:
> I think a fixed number of sgs will make things easier to develop backend.
Yeah, but it looks awkward to send a message buffer which isn't used
at all. From protocol's point of view, it just looks wrong/buggy.

The backend can just look at the number of elements received, they
can either be 2 (in case of zero-length) transfer, or 3 (for
read/write) and any other number is invalid.
> If you prefer to parse the number of descriptors instead of using the msg
> length to
> 
> distinguish the zero-length request from other requests, I'm OK to set
a
> limit.
My concern is more about the specification here first.
> if (!msgs[i].len) {
> ??? sg_init_one(&msg_buf, reqs[i].buf, msgs[i].len);
> 
> ??? if (msgs[i].flags & I2C_M_RD)
> ??? ??? sgs[outcnt + incnt++] = &msg_buf;
> ??? else
> ??? ??? sgs[outcnt++] = &msg_buf;
> }
> > > +#ifdef CONFIG_PM_SLEEP
> > You could avoid this pair of ifdef by creating dummy versions of below
> > routines for !CONFIG_PM_SLEEP case. Up to you.
> 
> 
> Thank you. I'd like to keep the same.
Sure.

-- 
viresh

On 2021/7/1 14:18, Viresh Kumar wrote:
> On 01-07-21, 14:10, Jie Deng wrote:
>> I think a fixed number of sgs will make things easier to develop
backend.
> Yeah, but it looks awkward to send a message buffer which isn't used
> at all. From protocol's point of view, it just looks wrong/buggy.
>
> The backend can just look at the number of elements received, they
> can either be 2 (in case of zero-length) transfer, or 3 (for
> read/write) and any other number is invalid.
>
OK. Let's add the following two lines to make sure that msg_buf is only
sent when the msgs len is not zero. And backend judges whether it is
a zero-length request by checking the number of elements received.

 ?+ if (msgs[i].len) {
 ?????????? reqs[i].buf = i2c_get_dma_safe_msg_buf(&msgs[i], 1);
 ?????????? if (!reqs[i].buf)
 ?????????????????? break;

 ????????? sg_init_one(&msg_buf, reqs[i].buf, msgs[i].len);

 ????????? if (msgs[i].flags & I2C_M_RD)
 ????????????????? sgs[outcnt + incnt++] = &msg_buf;
 ????????? else
 ????????????????? sgs[outcnt++] = &msg_buf;
+}