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

On 2021/7/1 12:04, Viresh Kumar wrote:
> On 01-07-21, 11:24, Jie Deng wrote:
>> Changes v10 -> v11
>> 	- Remove vi->adap.class = I2C_CLASS_DEPRECATED.
>> 	- Use #ifdef CONFIG_PM_SLEEP to replace the
"__maybe_unused".
>> 	- Remove "struct mutex lock" in "struct
virtio_i2c".
>> 	- Support zero-length request.
>> 	- Remove unnecessary logs.
>> 	- Remove vi->adap.timeout = HZ / 10, just use the default value.
>> 	- Use BIT(0) to define VIRTIO_I2C_FLAGS_FAIL_NEXT.
>> 	- Add the virtio_device index to adapter's naming mechanism.
> Thanks Jie.
>
> I hope you are going to send a fix for specification as well (for the
> zero-length request) ?

Yes. I will send that fix once this patch get merged.

>
>> diff --git a/drivers/i2c/busses/i2c-virtio.c
b/drivers/i2c/busses/i2c-virtio.c
>> +static int virtio_i2c_send_reqs(struct virtqueue *vq,
>> +				struct virtio_i2c_req *reqs,
>> +				struct i2c_msg *msgs, int nr)
>> +{
>> +	struct scatterlist *sgs[3], out_hdr, msg_buf, in_hdr;
>> +	int i, outcnt, incnt, err = 0;
>> +
>> +	for (i = 0; i < nr; i++) {
>> +		/*
>> +		 * Only 7-bit mode supported for this moment. For the address
format,
>> +		 * Please check the Virtio I2C Specification.
>> +		 */
>> +		reqs[i].out_hdr.addr = cpu_to_le16(msgs[i].addr << 1);
>> +
>> +		if (i != nr - 1)
>> +			reqs[i].out_hdr.flags = cpu_to_le32(VIRTIO_I2C_FLAGS_FAIL_NEXT);
>> +
>> +		outcnt = incnt = 0;
>> +		sg_init_one(&out_hdr, &reqs[i].out_hdr,
sizeof(reqs[i].out_hdr));
>> +		sgs[outcnt++] = &out_hdr;
>> +
>> +		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);
> The len can be zero here for zero-length transfers.
>
>> +
>> +		if (msgs[i].flags & I2C_M_RD)
>> +			sgs[outcnt + incnt++] = &msg_buf;
>> +		else
>> +			sgs[outcnt++] = &msg_buf;
>> +
>> +		sg_init_one(&in_hdr, &reqs[i].in_hdr,
sizeof(reqs[i].in_hdr));
>> +		sgs[outcnt + incnt++] = &in_hdr;
> Why are we still sending the msg_buf if the length is 0? Sending the
> buffer makes sense if you have some data to send, but otherwise it is
> just an extra sg element, which isn't required to be sent.

I think a fixed number of sgs will make things easier to develop backend.

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.

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
>> +static int virtio_i2c_freeze(struct virtio_device *vdev)
>> +{
>> +	virtio_i2c_del_vqs(vdev);
>> +	return 0;
>> +}
>> +
>> +static int virtio_i2c_restore(struct virtio_device *vdev)
>> +{
>> +	return virtio_i2c_setup_vqs(vdev->priv);
>> +}
>> +#endif
>> +
>> +static struct virtio_driver virtio_i2c_driver = {
>> +	.id_table	= id_table,
>> +	.probe		= virtio_i2c_probe,
>> +	.remove		= virtio_i2c_remove,
>> +	.driver	= {
>> +		.name	= "i2c_virtio",
>> +	},
>> +#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.

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