[PATCH V6 2/2] virtio: i2c: Allow zero-length transactions

[Viresh Kumar <viresh.kumar@linaro.org>]

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.

Add a new feature flag for zero length requests and make it mandatory
for it to be implemented, so we don't need to drag the old
implementation around.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/112
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jie Deng <jie.deng@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
---
 virtio-i2c.tex | 42 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 39 insertions(+), 3 deletions(-)

diff --git a/virtio-i2c.tex b/virtio-i2c.tex
index 5d634aec6e7c..5d407cb9e7c2 100644
--- a/virtio-i2c.tex
+++ b/virtio-i2c.tex
@@ -17,7 +17,21 @@ \subsection{Virtqueues}\label{sec:Device Types / I2C Adapter Device / Virtqueues
 
 \subsection{Feature bits}\label{sec:Device Types / I2C Adapter Device / Feature bits}
 
-None currently defined.
+\begin{description}
+\item[VIRTIO_I2C_F_ZERO_LENGTH_REQUEST (0)] The device supports zero-length I2C
+request and \field{VIRTIO_I2C_FLAGS_M_RD} flag. It is mandatory to implement
+this feature.
+\end{description}
+
+\begin{note}
+The \field{VIRTIO_I2C_FLAGS_M_RD} flag was not present in the initial
+implementation of the specification and the direction of the transfer (read or
+write) was inferred from the permissions (read-only or write-only) of the
+buffer itself. There is no need of having backwards compatibility for the older
+specification and so the \field{VIRTIO_I2C_FLAGS_FAIL_NEXT} feature is made
+mandatory. The driver should abort negotiation with the device, if the device
+doesn't offer this feature.
+\end{note}
 
 \subsection{Device configuration layout}\label{sec:Device Types / I2C Adapter Device / Device configuration layout}
 
@@ -83,13 +97,20 @@ \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.
+    If \field{VIRTIO_I2C_FLAGS_M_RD} bit is set in the \field{flags}, then the
+    request is called a read request. If \field{VIRTIO_I2C_FLAGS_M_RD} bit is
+    unset in the \field{flags}, then the request is called a write request.
 \end{description}
 
 Other bits of \field{flags} are currently reserved as zero for future feature
 extensibility.
 
-The \field{buf} of the request contains one segment of an I2C transaction
-being read from or written to the device.
+The \field{buf} is optional and will not be present for a zero-length request,
+like the SMBus "Quick" command. The \field{buf} 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.
@@ -124,13 +145,25 @@ \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C
 
 \drivernormative{\subsubsection}{Device Operation}{Device Types / I2C Adapter Device / Device Operation}
 
+A driver MUST accept the \field{VIRTIO_I2C_F_ZERO_LENGTH_REQUEST} feature and
+MUST abort negotiation with the device, if the device doesn't offer this
+feature.
+
 A driver MUST set \field{addr} and \field{flags} before sending the request.
 
 A driver MUST set the reserved bits of \field{flags} to be zero.
 
+A driver MUST NOT send the \field{buf}, for a zero-length request.
+
 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 set the \field{VIRTIO_I2C_FLAGS_M_RD} flag for a read operation,
+where the buffer is write-only for the device.
+
+A driver MUST NOT set the \field{VIRTIO_I2C_FLAGS_M_RD} flag for a write
+operation, where the buffer is read-only for the device.
+
 A driver MUST queue the requests in order if multiple requests are going to
 be sent at a time.
 
@@ -141,6 +174,9 @@ \subsubsection{Device Operation: Operation Status}\label{sec:Device Types / I2C
 
 \devicenormative{\subsubsection}{Device Operation}{Device Types / I2C Adapter Device / Device Operation}
 
+A device MUST offer the \field{VIRTIO_I2C_F_ZERO_LENGTH_REQUEST} feature and
+MUST reject any driver that doesn't negotiate this feature.
+
 A device SHOULD keep consistent behaviors with the hardware as described in
 \hyperref[intro:I2C]{I2C}.
 
-- 
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