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

[Jason Wang <jasowang@redhat.com>]

On 2020/9/4 下午9:21, Jie Deng wrote:
>
> On 2020/9/4 12:06, Jason Wang wrote:
>>
>>> diff --git a/drivers/i2c/busses/Kconfig b/drivers/i2c/busses/Kconfig
>>> index 293e7a0..70c8e30 100644
>>> --- a/drivers/i2c/busses/Kconfig
>>> +++ b/drivers/i2c/busses/Kconfig
>>> @@ -21,6 +21,17 @@ config I2C_ALI1535
>>>         This driver can also be built as a module.  If so, the module
>>>         will be called i2c-ali1535.
>>>   +config I2C_VIRTIO
>>> +    tristate "Virtio I2C Adapter"
>>> +    depends on VIRTIO
>>
>>
>> I guess it should depend on some I2C module here.
>>
> The dependency of I2C is included in the Kconfig in its parent directory.
> So there is nothing special to add here.


Ok.


>
>
>>
>>>
>>> +struct virtio_i2c_msg {
>>> +    struct virtio_i2c_hdr hdr;
>>> +    char *buf;
>>> +    u8 status;
>>
>>
>> Any reason for separating status out of virtio_i2c_hdr?
>>
> The status is not from i2c_msg. 


You meant ic2_hdr? You embed status in virtio_i2c_msg anyway.


> So I put it out of virtio_i2c_hdr.


Something like status or response is pretty common in virtio request 
(e.g net or scsi), if no special reason, it's better to keep it in the hdr.


>
>>
>>> +};
>>> +
>>> +/**
>>> + * struct virtio_i2c - virtio I2C data
>>> + * @vdev: virtio device for this controller
>>> + * @completion: completion of virtio I2C message
>>> + * @adap: I2C adapter for this controller
>>> + * @i2c_lock: lock for virtqueue processing
>>> + * @vq: the virtio virtqueue for communication
>>> + */
>>> +struct virtio_i2c {
>>> +    struct virtio_device *vdev;
>>> +    struct completion completion;
>>> +    struct i2c_adapter adap;
>>> +    struct mutex i2c_lock;
>>> +    struct virtqueue *vq;
>>> +};
>>> +
>>> +static void virtio_i2c_msg_done(struct virtqueue *vq)
>>> +{
>>> +    struct virtio_i2c *vi = vq->vdev->priv;
>>> +
>>> +    complete(&vi->completion);
>>> +}
>>> +
>>> +static int virtio_i2c_add_msg(struct virtqueue *vq,
>>> +                  struct virtio_i2c_msg *vmsg,
>>> +                  struct i2c_msg *msg)
>>> +{
>>> +    struct scatterlist *sgs[3], hdr, bout, bin, status;
>>> +    int outcnt = 0, incnt = 0;
>>> +
>>> +    if (!msg->len)
>>> +        return -EINVAL;
>>> +
>>> +    vmsg->hdr.addr = msg->addr;
>>> +    vmsg->hdr.flags = msg->flags;
>>> +    vmsg->hdr.len = msg->len;
>>
>>
>> Missing endian conversion?
>>
> You are right. Need conversion here.
>>
>>> +
>>> +    vmsg->buf = kzalloc(vmsg->hdr.len, GFP_KERNEL);
>>> +    if (!vmsg->buf)
>>> +        return -ENOMEM;
>>> +
>>> +    sg_init_one(&hdr, &vmsg->hdr, sizeof(struct virtio_i2c_hdr));
>>> +    sgs[outcnt++] = &hdr;
>>> +    if (vmsg->hdr.flags & I2C_M_RD) {
>>> +        sg_init_one(&bin, vmsg->buf, msg->len);
>>> +        sgs[outcnt + incnt++] = &bin;
>>> +    } else {
>>> +        memcpy(vmsg->buf, msg->buf, msg->len);
>>> +        sg_init_one(&bout, vmsg->buf, msg->len);
>>> +        sgs[outcnt++] = &bout;
>>> +    }
>>> +    sg_init_one(&status, &vmsg->status, sizeof(vmsg->status));
>>> +    sgs[outcnt + incnt++] = &status;
>>> +
>>> +    return virtqueue_add_sgs(vq, sgs, outcnt, incnt, vmsg, 
>>> GFP_KERNEL);
>>> +}
>>> +
>>> +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;
>>
>>
>> It looks to me we can avoid the allocation by embedding 
>> virtio_i2c_msg into struct virtio_i2c;
>>
> Yeah... That's better. Thanks.
>
>
>>
>>> +
>>> +    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;
>>> +        }
>>> +
>>> +        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;
>>> +        }
>>> +
>>> +        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;
>>> +            }
>>
>>
>> Does this imply in order completion of i2c device?  (E.g what happens 
>> if multiple virtio i2c requests are submitted)
>>
>> Btw, this always use a single descriptor once a time which makes me 
>> suspect if a virtqueue(virtio) is really needed. It looks to me we 
>> can utilize the virtqueue by submit the request in a batch.
>>
> I'm afraid not all physical devices support batch. 


Yes but I think I meant for the virtio device not the physical one. It's 
impossible to forbid batching if you have a queue anyway ...


> I'd like to keep the current design and consider
> your suggestion as a possible optimization in the future.
>
> Thanks.
>
>
>>>
>>> +}
>>> +
>>> +static void virtio_i2c_del_vqs(struct virtio_device *vdev)
>>> +{
>>> +    vdev->config->reset(vdev);
>>
>>
>> Why need reset here?
>>
>> Thanks
>>
> I'm following what other virtio drivers do.
> They reset the devices before they clean up the queues.


You're ring.

Thanks


>
>
>>
>>> +    vdev->config->del_vqs(vdev);
>>> +}
>>> +
>>> +static int virtio_i2c_setup_vqs(struct virtio_i2c *vi)
>>> +{
>>> +    struct virtio_device *vdev = vi->vdev;
>>> +
>>> +    vi->vq = virtio_find_single_vq(vdev, virtio_i2c_msg_done, 
>>> "i2c-msg");
>>
>>
>> We've in the scope of ic2, so "msg" should be sufficient.
>>
>>
> OK. Will change this name. Thanks.
>
>
>>> +    return PTR_ERR_OR_ZERO(vi->vq);
>