Re: [PATCH v5] i2c: virtio: add a virtio i2c frontend driver

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

On 2021/3/3 15:54, Viresh Kumar wrote:

> On 01-03-21, 14:41, Jie Deng wrote:
>> 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;
>> +	u8 *buf;
>> +
>> +	for (i = 0; i < nr; i++) {
>> +		if (!msgs[i].len)
>> +			break;
>> +
>> +		reqs[i].out_hdr.addr = cpu_to_le16(msgs[i].addr << 1);
>> +
>> +		if (i != nr - 1)
>> +			reqs[i].out_hdr.flags |= 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;
>> +
>> +		buf = kzalloc(msgs[i].len, GFP_KERNEL);
>> +		if (!buf)
>> +			break;
>> +
>> +		if (msgs[i].flags & I2C_M_RD) {
>> +			reqs[i].read_buf = buf;
>> +			sg_init_one(&msg_buf, reqs[i].read_buf, msgs[i].len);
>> +			sgs[outcnt + incnt++] = &msg_buf;
>> +		} else {
>> +			reqs[i].write_buf = buf;
>> +			memcpy(reqs[i].write_buf, msgs[i].buf, msgs[i].len);
>> +			sg_init_one(&msg_buf, reqs[i].write_buf, msgs[i].len);
>> +			sgs[outcnt++] = &msg_buf;
>> +		}
>> +
>> +		sg_init_one(&in_hdr, &reqs[i].in_hdr, sizeof(reqs[i].in_hdr));
>> +		sgs[outcnt + incnt++] = &in_hdr;
>> +
>> +		err = virtqueue_add_sgs(vq, sgs, outcnt, incnt, &reqs[i], GFP_KERNEL);
>> +		if (err < 0) {
>> +			pr_err("failed to add msg[%d] to virtqueue.\n", i);
>> +			if (msgs[i].flags & I2C_M_RD) {
>> +				kfree(reqs[i].read_buf);
>> +				reqs[i].read_buf = NULL;
>> +			} else {
>> +				kfree(reqs[i].write_buf);
>> +				reqs[i].write_buf = NULL;
>> +			}
>> +			break;
>> +		}
>> +	}
>> +
>> +	return i;
>> +}
>> diff --git a/include/uapi/linux/virtio_i2c.h b/include/uapi/linux/virtio_i2c.h
>> +/**
>> + * struct virtio_i2c_out_hdr - the virtio I2C message OUT header
>> + * @addr: the controlled device address
>> + * @padding: used to pad to full dword
>> + * @flags: used for feature extensibility
>> + */
>> +struct virtio_i2c_out_hdr {
>> +	__le16 addr;
>> +	__le16 padding;
>> +	__le32 flags;
>> +};
> Both this code and the virtio spec (which is already merged) are
> missing msgs[i].flags and they are never sent to backend. The only
> flags available here are the ones defined by virtio spec and these are
> not i2c flags.
>
> I also looked at your i2c backend for acrn and it mistakenly copies
> the hdr.flag, which is the virtio flag and not i2c flag.
>
> https://github.com/projectacrn/acrn-hypervisor/blob/master/devicemodel/hw/pci/virtio/virtio_i2c.c#L539
>
> I will send a fix for the specs if you agree that there is a problem
> here.
>
> what am I missing here ? This should have been caught in your testing
> and so I feel I must be missing something.
This is not a problem. My original proposal was to mirror the struct 
i2c_msg.
The code you looked at was based on that.
However, the virtio TC prefer not to mirror it. They have some concerns.
For example, there is a bit I2C_M_RD in i2c_msg.flag which has the same 
meaning with
the R/W in virtio descriptor. This is a repetition which may cause problems.
So the virtio_i2c_out_hdr.flags is used to instead of i2c_msg.flags for 
extension.

You can check this link 
https://github.com/oasis-tcs/virtio-spec/issues/88 to learn the whole story.
There are discussions about the spec (v1 ~ v7).

I'm updating these drivers step by step to make sure they finally follow 
the spec.