Re: [PATCH v11] i2c: virtio: add a virtio i2c frontend driver

[Jie Deng]

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.