Re: [PATCH V2] virtio: i2c: Allow zero-length transactions

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 16, 2021 at 03:17:23PM +0530, Viresh Kumar wrote:
> The I2C protocol allows zero-length requests with no data, like the
> SMBus Quick command, where the command is inferred based on the
> read/write flag itself.

So I wonder. What if we allow zero-length buffers in virtio? Would that
address the need?

> In order to allow such a request, allocate another bit,
> VIRTIO_I2C_FLAGS_M_RD(1), in the flags to pass the request type, as read
> or write. This was earlier done using the read/write permission to the
> buffer itself.
> 
> This still won't work well if multiple buffers are passed for the same
> request, i.e. the write-read requests, as the VIRTIO_I2C_FLAGS_M_RD flag
> can only be used with a single buffer.
> 
> Coming back to it, there is no need to send multiple buffers with a
> single request. All we need, is a way to group several requests
> together, which we can already do based on the
> VIRTIO_I2C_FLAGS_FAIL_NEXT flag.
> 
> Remove support for multiple buffers within a single request.
> 
> Since we are at very early stage of development currently, we can do
> these modifications without addition of new features or versioning of
> the protocol.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> 
> V1->V2:
> - Name the buffer-less request as zero-length request.
> 
> Hi Guys,
> 
> I did try to follow the discussion you guys had during V4, where we added
> support for multiple buffers for the same request, which I think is unnecessary
> now, after introduction of the VIRTIO_I2C_FLAGS_FAIL_NEXT flag.
> 
> https://lists.oasis-open.org/archives/virtio-comment/202011/msg00005.html
> 
> And so starting this discussion again, because we need to support stuff
> like: i2cdetect -q <i2c-bus-number>, which issues a zero-length SMBus
> Quick command.
> ---
>  virtio-i2c.tex | 60 +++++++++++++++++++++++++-------------------------
>  1 file changed, 30 insertions(+), 30 deletions(-)
> 
> diff --git a/virtio-i2c.tex b/virtio-i2c.tex
> index 949d75f44158..ae344b2bc822 100644
> --- a/virtio-i2c.tex
> +++ b/virtio-i2c.tex
> @@ -54,8 +54,7 @@ \subsubsection{Device Operation: Request Queue}\label{sec:Device Types / I2C Ada
>  \begin{lstlisting}
>  struct virtio_i2c_req {
>          struct virtio_i2c_out_hdr out_hdr;
> -        u8 write_buf[];
> -        u8 read_buf[];
> +        u8 buf[];
>          struct virtio_i2c_in_hdr in_hdr;
>  };
>  \end{lstlisting}
> @@ -84,16 +83,16 @@ \subsubsection{Device Operation: Request Queue}\label{sec:Device Types / I2C Ada
>      and sets it on the other requests. If this bit is set and a device fails
>      to process the current request, it needs to fail the next request instead
>      of attempting to execute it.
> +
> +\item[VIRTIO_I2C_FLAGS_M_RD(1)] is used to mark the request as READ or WRITE.
>  \end{description}
>  
>  Other bits of \field{flags} are currently reserved as zero for future feature
>  extensibility.
>  
> -The \field{write_buf} of the request contains one segment of an I2C transaction
> -being written to the device.
> -
> -The \field{read_buf} of the request contains one segment of an I2C transaction
> -being read from the device.
> +The \field{buf} of the request is optional and contains one segment of an I2C
> +transaction being read from or written to the device, based on the value of the
> +\field{VIRTIO_I2C_FLAGS_M_RD} bit in the \field{flags} field.
>  
>  The final \field{status} byte of the request is written by the device: either
>  VIRTIO_I2C_MSG_OK for success or VIRTIO_I2C_MSG_ERR for error.
> @@ -103,27 +102,27 @@ \subsubsection{Device Operation: Request Queue}\label{sec:Device Types / I2C Ada
>  #define VIRTIO_I2C_MSG_ERR    1
>  \end{lstlisting}
>  
> -If ``length of \field{read_buf}''=0 and ``length of \field{write_buf}''>0,
> -the request is called write request.
> +If \field{VIRTIO_I2C_FLAGS_M_RD} bit is set in the \field{flags}, then the
> +request is called a read request.
>  
> -If ``length of \field{read_buf}''>0 and ``length of \field{write_buf}''=0,
> -the request is called read request.
> +If \field{VIRTIO_I2C_FLAGS_M_RD} bit is unset in the \field{flags}, then the
> +request is called a write request.
>  
> -If ``length of \field{read_buf}''>0 and ``length of \field{write_buf}''>0,
> -the request is called write-read request. It means an I2C write segment followed
> -by a read segment. Usually, the write segment provides the number of an I2C
> -controlled device register to be read.
> +The \field{buf} is optional and will not be present for a zero-length request,
> +like SMBus Quick.
>  
> -The case when ``length of \field{write_buf}''=0, and at the same time,
> -``length of \field{read_buf}''=0 doesn't make any sense.
> +The virtio I2C protocol supports write-read requests, i.e. an I2C write segment
> +followed by a read segment (usually, the write segment provides the number of an
> +I2C controlled device register to be read), by grouping a list of requests
> +together using the \field{VIRTIO_I2C_FLAGS_FAIL_NEXT} flag.
>  
>  \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C Adapter Device / Device Operation: Operation Status}
>  
> -\field{addr}, \field{flags}, ``length of \field{write_buf}'' and ``length of \field{read_buf}''
> -are determined by the driver, while \field{status} is determined by the processing
> -of the device. A driver puts the data written to the device into \field{write_buf}, while
> -a device puts the data of the corresponding length into \field{read_buf} according to the
> -request of the driver.
> +\field{addr}, \field{flags}, and ``length of \field{buf}'' are determined by the
> +driver, while \field{status} is determined by the processing of the device.  A
> +driver, for a write request, puts the data to be written to the device into the
> +\field{buf}, while a device, for a read request, puts the data read from device
> +into the \field{buf} according to the request from the driver.
>  
>  A driver may send one request or multiple requests to the device at a time.
>  The requests in the virtqueue are both queued and processed in order.
> @@ -141,11 +140,10 @@ \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C
>  
>  A driver MUST set the reserved bits of \field{flags} to be zero.
>  
> -The driver MUST NOT send a request with ``length of \field{write_buf}''=0 and
> -``length of \field{read_buf}''=0 at the same time.
> +A driver MUST NOT send the \field{buf}, for a zero-length request.
>  
> -A driver MUST NOT use \field{read_buf} if the final \field{status} returned
> -from the device is VIRTIO_I2C_MSG_ERR.
> +A driver MUST NOT use \field{buf}, for a read request, if the final
> +\field{status} returned from the device is VIRTIO_I2C_MSG_ERR.
>  
>  A driver MUST queue the requests in order if multiple requests are going to
>  be sent at a time.
> @@ -160,11 +158,13 @@ \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C
>  A device SHOULD keep consistent behaviors with the hardware as described in
>  \hyperref[intro:I2C]{I2C}.
>  
> -A device MUST NOT change the value of \field{addr}, reserved bits of \field{flags}
> -and \field{write_buf}.
> +A device MUST NOT change the value of \field{addr}, and reserved bits of
> +\field{flags}.
> +
> +A device MUST not change the value of the \field{buf} for a write request.
>  
> -A device MUST place one I2C segment of the corresponding length into \field{read_buf}
> -according the driver's request.
> +A device MUST place one I2C segment of the ``length of \field{buf}'', for the
> +read request, into the \field{buf} according the driver's request.
>  
>  A device MUST guarantee the requests in the virtqueue being processed in order
>  if multiple requests are received at a time.
> -- 
> 2.31.1.272.g89b43f80a514