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.
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
I think zero length buffers are useful in their own right. But if you are doing it like this, then I think we should have conformance clauses that require that the flag is consistent with the buffer.
Paolo what are your thoughts on VIRTIO_I2C_FLAGS_FAIL_NEXT?
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