On Thu, Dec 17, 2020 at 03:00:55AM -0500, Michael S. Tsirkin wrote:
> On Thu, Dec 17, 2020 at 03:08:07PM +0800, Jie Deng wrote:
> >
> > On 2020/12/16 23:52, Stefan Hajnoczi wrote:
> >
> > On Wed, Nov 25, 2020 at 01:55:18PM +0800, Jie Deng wrote:
> >
> > diff --git a/virtio-i2c.tex b/virtio-i2c.tex
> > new file mode 100644
> > index 0000000..fdb0050
> > --- /dev/null
> > +++ b/virtio-i2c.tex
> > @@ -0,0 +1,139 @@
> > +\section{I2C Adapter Device}\label{sec:Device Types / I2C Adapter
> > Device}
> > +
> > +virtio-i2c is a virtual I2C adapter device. It provides a way to
> > flexibly
> > +organize and use the host I2C slave devices from the guest. By
> > attaching
> > +the host ACPI I2C slave nodes to the virtual I2C adapter device,
> > the guest can
> > +communicate with them without changing or adding extra drivers for
> > these
> > +slave I2C devices.
> >
> > Is there a way to identify I2C busses if more than one virtio-i2c device
> > is present? For example, imagine a host with 2 I2C busses. How does the
> > guest know which virtio-i2c device connects to which host bus?
> >
> > This virtio-i2c is a master device. The slave devices attached to it can be
> > configured
> > by the backend in the host. These slave devices can be under different host
> > I2C
> > master devices.
> > The guest will see this virtio-i2c master device and its slave devices.
> >
> > There is a example about the backend which shows how it works
> >
> > https://projectacrn.github.io/latest/developer-guides/hld/virtio-i2c.html?
> > highlight=i2c
> > https://github.com/projectacrn/acrn-hypervisor/blob/master/devicemodel/hw/pci/
> > virtio/virtio_i2c.c
> >
> >
> >
> > +\begin{lstlisting}
> > +struct virtio_i2c_req {
> > + le16 addr;
> > + le32 flags;
> >
> > What is the memory layout?
> >
> > 1. 0x0 addr, 0x2 flags
> >
> > or
> >
> > 2. 0x0 addr, 0x4 flags
> >
> > This is unclear to me. I don't see a general statement in the spec about
> > struct field alignment/padding and no details in this new spec change.
> >
> > Both are OK to me. I used to use "packed" but Michael said there was no
> > need to
> > pack it.ÃâÃÂ
> >
> > https://lkml.org/lkml/2020/9/3/339
> >
> > So you prefer it to be clear ?
> >
> > + le16 written;
> > + le16 read;
> > + u8 write_buf[];
> > + u8 read_buf[];
> > + u8 status;
> > +};
> > +\end{lstlisting}
> > +
> > +The \field{addr} of the request is the address of the I2C slave
> > device.
> >
> > https://en.wikipedia.org/wiki/I2c#Addressing_structure suggests there
> > are at least 7-bit and 10-bit addressing schemes in I2C. How does this
> > map to the little-endian 16-bit addr field?
> >
> > This field maps to the kernel struct "i2c_msg.addr".
> >
> > struct virtio_i2c_req *vmsg;
> > struct i2c_msg *msg;
> >
> > vmsg->addr = cpu_to_le16(msg->addr);
> >
> > The field in the request can be seen as host byte order
> > while the link can be seen as the I2C network byte order.
> > The host driver may take care this conversion.
> >
> > +The \field{flags} of the request is currently reserved as zero for
> > future
> > +feature extensibility.
> > +
> > +The \field{written} of the request is the number of data bytes in
> > the \field{write_buf}
> > +being written to the I2C slave address.
> >
> > This field seems redundant since the device can determine the size of
> > write_buf implicitly from the total out buffer size. virtio-blk takes
> > this approach.
> >
> > The read/write are the actual number of data bytes being read from or
> > written
> > to the device
> > which is not determined by the device. So I don't think it is redundant.
>
> I am still not sure I understand the difference.
> This point is unclear to multiple people.
I think I get it now. This is made clear by splitting the struct:
/* Driver->device fields */
struct virtio_i2c_out_hdr
{
le16 addr;
le16 padding;
le32 flags;
};
/* Device->driver fields */
struct virtio_i2c_in_hdr
{
le16 written;
le16 read;
u8 status;
};
/*
* Virtqueue element layout looks like this:
*
* struct virtio_i2c_out_hdr out_hdr; /* OUT */
* u8 write_buf[]; /* OUT */
* u8 read_buf[]; /* IN */
* struct virtio_i2c_in_hdr in_hdr; /* IN */
*/
This makes sense to me: a bi-directional request has both write_buf[]
and read_buf[] so the vring used.len field is not enough to report back
how many bytes were written and read. The virtio_i2c_in_hdr fields are
really needed.
Please split the struct in the spec so it's clear how this works.
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