Re: [PATCH v3] i2c: virtio: add a virtio i2c frontend driver

[Jason Wang]

On 2020/10/12 上午10:45, Jie Deng wrote:
> On 2020/10/10 11:14, Jason Wang wrote:
> > > +
> > > +        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);
> > > +            break;
> > > +        }
> >
> >
> > You don't set error number here. Is this intended?
> >
> > And using a timeout here is not good, and if the request is finished 
> > just after the timeout, in the next xfer you may hit the following 
> > check.
> >
> > It's better to use either interrupt here.
> Could you check the I2C drivers in the kernel ? The 
> "wait_for_completion_timeout" mechanism
> is commonly used by I2C bus drivers in their i2c_algorithm.master_xfer.


There's a major difference between virtio-i2c and other drivers. In the 
case of virtio, the device could be a software device emulated by a 
remote process. This means the timeout might not be rare.

I don't see how timeout is properly handled in this patch (e.g did you 
notice that you don't set any error when timeout? or is this intended?)


> > > +
> > > +        vmsg = (struct virtio_i2c_msg *)virtqueue_get_buf(vq, &len);
> > > +        /* vmsg should point to the same address with &vi->vmsg */
> > > +        if ((!vmsg) || (vmsg != &vi->vmsg)) {
> > > +            dev_err(&adap->dev, "msg[%d]: addr=0x%x virtqueue 
> > > error.\n",
> > > +                i, msgs[i].addr);
> > > +            break;
> > > +        }
> >
> >
> > So I think we can remove this check. Consider only one descriptor 
> > will be used at most, unless there's a bug in the device (and no 
> > other driver to the similar check), we should not hit this.
> >
> > Btw, as I replied in the previous version, the device should be 
> > cacpable of dealing of a batch of requests through the virtqueue, 
> > otherwise it's meaningless to use a queue here.
> We should not assume there is no bug in the device. I don't think we 
> can remove this check if we want our code to be robust.


Can you tell when at which case you may hit !vmsg or vmsg != vi->vmsg?



> As I said, currently, we are using the virtqueue to send the msg one 
> by one to the backend. The mechanism is described in the spec. 


Which part of the spec describes such "one by one" mechanism? If there 
is one, I'd happily give a NACK since it doesn't require a queue to work 
which is conflict with the concept of the virtqueue.


> Thanks.
>
>
> > > +
> > >
> > > +
> > > +#ifndef _UAPI_LINUX_VIRTIO_I2C_H
> > > +#define _UAPI_LINUX_VIRTIO_I2C_H
> > > +
> > > +#include <linux/types.h>
> > > +#include <linux/virtio_ids.h>
> > > +#include <linux/virtio_config.h>
> > > +
> > > +/**
> > > + * 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 {
> > > +    __le16 addr;
> > > +    __le16 flags;
> > > +    __le16 len;
> > > +};
> >
> >
> > I'm afraid this is not complete. E.g the status is missed.
> >
> > I suspect what virtio-scsi use is better. Which split the in from the 
> > out instead of reusing the same buffer. And it can ease the uAPI 
> > header export.
> >
> > Thanks
>
> I think following definition in uAPI for the status is enough.
> There is no need to provide a "u8" status in the structure.
>
> /* The final status written by the device */
> #define VIRTIO_I2C_MSG_OK    0
> #define VIRTIO_I2C_MSG_ERR    1
>
> You can see an example in virtio_blk.
>
> In the spec:
>
> struct virtio_blk_req {
> le32 type;
> le32 reserved;
> le64 sector;
> u8 data[];
> u8 status;
> };
>
> In virtio_blk.h, there is only following definitions.
>
> #define VIRTIO_BLK_S_OK        0
> #define VIRTIO_BLK_S_IOERR    1
> #define VIRTIO_BLK_S_UNSUPP    2

virtio-blk is a bad example, it's just too late to fix. For any new 
introduced uAPI it should be a complete one.

Thanks


> Thanks.