linux-kernel - Re: [PATCH] i2c: virtio: add a virtio i2c frontend driver

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Date:   Fri, 4 Sep 2020 14:08:52 +0800
From:   Jie Deng <jie.deng@...el.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linux-i2c@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, mst@...hat.com, jasowang@...hat.com,
        wsa+renesas@...g-engineering.com, wsa@...nel.org,
        jarkko.nikula@...ux.intel.com, jdelvare@...e.de,
        Sergey.Semin@...kalelectronics.ru, krzk@...nel.org,
        rppt@...nel.org, loic.poulain@...aro.org, tali.perry1@...il.com,
        bjorn.andersson@...aro.org, shuo.a.liu@...el.com,
        conghui.chen@...el.com, yu1.wang@...el.com
Subject: Re: [PATCH] i2c: virtio: add a virtio i2c frontend driver


On 2020/9/3 18:20, Andy Shevchenko wrote:
> On Thu, Sep 03, 2020 at 01:34:45PM +0800, Jie Deng wrote:
>> Add an I2C bus driver for virtio para-virtualization.
>>
>> The controller can be emulated by the backend driver in
>> any device model software by following the virtio protocol.
>>
>> This driver communicates with the backend driver through a
>> virtio I2C message structure which includes following parts:
>>
>> - Header: i2c_msg addr, flags, len.
>> - Data buffer: the pointer to the i2c msg data.
>> - Status: the processing result from the backend.
>>
>> People may implement different backend drivers to emulate
>> different controllers according to their needs. A backend
>> example can be found in the device model of the open source
>> project ACRN. For more information, please refer to
>> https://projectacrn.org.
>>
>> The virtio device ID 34 is used for this I2C adpter since IDs
>> before 34 have been reserved by other virtio devices.
> Seems it's slightly different version to what I have reviewed internally.
> My comments below. (I admit that some of them maybe new)
>
> ...

Yeah... Some new devices have been added into the virtio spec during 
these days.


>> +/**
>> + * struct virtio_i2c_hdr - the virtio I2C message header structure
>> + * @addr: i2c_msg addr, the slave address
>> + * @flags: i2c_msg flags
>> + * @len: i2c_msg len
>> + */
>> +struct virtio_i2c_hdr {
>> +	__virtio16 addr;
>> +	__virtio16 flags;
>> +	__virtio16 len;
>> +} __packed;
> As Misha noticed and somewhere I saw 0-day reports these should be carefully
> taken care of.
>
> ...

Sure. Will fix these.


>> +static int virtio_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg *msgs, int num)
>> +{
>> +	struct virtio_i2c *vi = i2c_get_adapdata(adap);
>> +	struct virtio_i2c_msg *vmsg_o, *vmsg_i;
>> +	struct virtqueue *vq = vi->vq;
>> +	unsigned long time_left;
>> +	int len, i, ret = 0;
>> +
>> +	vmsg_o = kzalloc(sizeof(*vmsg_o), GFP_KERNEL);
>> +	if (!vmsg_o)
>> +		return -ENOMEM;
>> +
>> +	mutex_lock(&vi->i2c_lock);
>> +	vmsg_o->buf = NULL;
>> +	for (i = 0; i < num; i++) {
>> +		ret = virtio_i2c_add_msg(vq, vmsg_o, &msgs[i]);
>> +		if (ret) {
>> +			dev_err(&adap->dev, "failed to add msg[%d] to virtqueue.\n", i);
>> +			goto err_unlock_free;
> break;

OK.


>> +		}
>> +
>> +		virtqueue_kick(vq);
>> +
>> +		time_left = wait_for_completion_timeout(&vi->completion, adap->timeout);
>> +		if (!time_left) {
>> +			dev_err(&adap->dev, "msg[%d]: addr=0x%x timeout.\n", i, msgs[i].addr);
>> +			ret = i;
>> +			goto err_unlock_free;
> break;
>
> And so on.
OK.
>> +		}
>> +
>> +		vmsg_i = (struct virtio_i2c_msg *)virtqueue_get_buf(vq, &len);
>> +		if (vmsg_i) {
>> +			/* vmsg_i should point to the same address with vmsg_o */
>> +			if (vmsg_i != vmsg_o) {
>> +				dev_err(&adap->dev, "msg[%d]: addr=0x%x virtqueue error.\n",
>> +					i, vmsg_i->hdr.addr);
>> +				ret = i;
>> +				goto err_unlock_free;
>> +			}
>> +			if (vmsg_i->status != VIRTIO_I2C_MSG_OK) {
>> +				dev_err(&adap->dev, "msg[%d]: addr=0x%x error=%d.\n",
>> +					i, vmsg_i->hdr.addr, vmsg_i->status);
>> +				ret = i;
>> +				goto err_unlock_free;
>> +			}
>> +			if ((vmsg_i->hdr.flags & I2C_M_RD) && vmsg_i->hdr.len)
>> +				memcpy(msgs[i].buf, vmsg_i->buf, vmsg_i->hdr.len);
>> +
>> +			kfree(vmsg_i->buf);
>> +			vmsg_i->buf = NULL;
>> +		}
>> +		reinit_completion(&vi->completion);
>> +	}
>> +	if (i == num)
>> +		ret = num;
> And this conditional seems a dup of the for-loop successfully iterating over
> entire queue.
You are right.
We may save several lines of code by using "Return (ret<0) ? ret : i" at 
the end.


>> +err_unlock_free:
> Redundant.
OK.
>> +	mutex_unlock(&vi->i2c_lock);
>> +	kfree(vmsg_o->buf);
>> +	kfree(vmsg_o);
>> +	return ret;
>> +}
> ...
>
>> +	vi->adap.timeout = HZ / 10;
> + Blank line.
OK.
>> +	ret = i2c_add_adapter(&vi->adap);
>> +	if (ret) {
>> +		dev_err(&vdev->dev, "failed to add virtio-i2c adapter.\n");
>> +		virtio_i2c_del_vqs(vdev);
> Usually we do clean up followed by message.
I will change the order. Thank you.
>> +	}
>> +
>> +	return ret;
>