[virtio-dev] Re: [virtio-comment] [RFC PATCH v3] virtio-can: Device specification.

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

On Wed, Feb 21, 2024 at 04:42:18PM +0100, Mikhail Golubev-Ciuchea wrote:
> Hi Matias,
> 
> On 2/20/24 13:19, Matias Ezequiel Vara Larsen wrote:
> > Hello Mikail,
> > 
> > On Mon, Jan 08, 2024 at 06:18:50PM +0100, Mikhail Golubev-Ciuchea wrote:
> > > Hi all!
> > > 
> > > I kindly request a vote.
> > > 
> > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/186
> > > 
> > > 
> > > Best wishes,
> > > Mikhail Golubev-Ciuchea
> > > 
> > > 
> > > 
> > > On 6/9/23 16:22, Mikhail Golubev-Ciuchea wrote:
> > > > From: Harald Mommer <harald.mommer@opensynergy.com>
> > > > 
> > > > virtio-can is a virtual CAN device. It provides a way to give access to
> > > > a CAN controller from a driver guest. The device is aimed to be used by
> > > > driver guests running a HLOS as well as by driver guests running a
> > > > typical RTOS as used in controller environments.
> > > > 
> > > > Signed-off-by: Harald Mommer <Harald.Mommer@opensynergy.com>
> > > > Signed-off-by: Mikhail Golubev-Ciuchea <Mikhail.Golubev-Ciuchea@opensynergy.com>
> > > > ---
> > > > 
> > > > RFC v3:
> > > > * Add length fields in CAN RX and TX messages.
> > > > * Replace bus off indication queue with a config space bit.
> > > > * Clarify handling of unknown flag bits set in CAN frame.
> > > > * Remove MISRA C suffixes in constants.
> > > > * Reserve 16 bits in RX/TX messages for CAN XL priority.
> > > > * Reserve 8 bits in RX/TX messages for CAN classic DLC.
> > > > * Rework according to general virtio spec POV.
> > > > * Implementation:
> > > >     driver: https://lore.kernel.org/all/20230607145613.133203-1-Mikhail.Golubev-Ciuchea@opensynergy.com/
> > > >     QEmu device: https://github.com/OpenSynergy/qemu/tree/virtio-can-spec-rfc-v3
> > > > 
> > > > RFC v2:
> > > > * Add CAN classic feature flag.
> > > > * Add feature flag VIRTIO_CAN_F_LATE_TX_ACK.
> > > > * Add feature flag VIRTIO_CAN_F_RTR_FRAMES.
> > > > * Reserve 32 bits in RX/TX messages.
> > > > * Remove priorities of messages.
> > > > 
> > > >    conformance.tex                         |  12 +-
> > > >    content.tex                             |   1 +
> > > >    device-types/can/description.tex        | 249 ++++++++++++++++++++++++
> > > >    device-types/can/device-conformance.tex |   8 +
> > > >    device-types/can/driver-conformance.tex |   7 +
> > > >    introduction.tex                        |   2 +
> > > >    6 files changed, 275 insertions(+), 4 deletions(-)
> > > >    create mode 100644 device-types/can/description.tex
> > > >    create mode 100644 device-types/can/device-conformance.tex
> > > >    create mode 100644 device-types/can/driver-conformance.tex
> > > > 
> > > > diff --git a/conformance.tex b/conformance.tex
> > > > index 01ccd69..a07ef02 100644
> > > > --- a/conformance.tex
> > > > +++ b/conformance.tex
> > > > @@ -32,8 +32,9 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> > > >    \ref{sec:Conformance / Driver Conformance / Memory Driver Conformance},
> > > >    \ref{sec:Conformance / Driver Conformance / I2C Adapter Driver Conformance},
> > > >    \ref{sec:Conformance / Driver Conformance / SCMI Driver Conformance},
> > > > -\ref{sec:Conformance / Driver Conformance / GPIO Driver Conformance} or
> > > > -\ref{sec:Conformance / Driver Conformance / PMEM Driver Conformance}.
> > > > +\ref{sec:Conformance / Driver Conformance / GPIO Driver Conformance},
> > > > +\ref{sec:Conformance / Driver Conformance / PMEM Driver Conformance} or
> > > > +\ref{sec:Conformance / Driver Conformance / CAN Driver Conformance}.
> > > >        \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
> > > >      \end{itemize}
> > > > @@ -59,8 +60,9 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> > > >    \ref{sec:Conformance / Device Conformance / Memory Device Conformance},
> > > >    \ref{sec:Conformance / Device Conformance / I2C Adapter Device Conformance},
> > > >    \ref{sec:Conformance / Device Conformance / SCMI Device Conformance},
> > > > -\ref{sec:Conformance / Device Conformance / GPIO Device Conformance} or
> > > > -\ref{sec:Conformance / Device Conformance / PMEM Device Conformance}.
> > > > +\ref{sec:Conformance / Device Conformance / GPIO Device Conformance},
> > > > +\ref{sec:Conformance / Device Conformance / PMEM Device Conformance} or
> > > > +\ref{sec:Conformance / Device Conformance / CAN Device Conformance}.
> > > >        \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
> > > >      \end{itemize}
> > > > @@ -152,6 +154,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> > > >    \input{device-types/scmi/driver-conformance.tex}
> > > >    \input{device-types/gpio/driver-conformance.tex}
> > > >    \input{device-types/pmem/driver-conformance.tex}
> > > > +\input{device-types/can/driver-conformance.tex}
> > > >    \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
> > > > @@ -238,6 +241,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets}
> > > >    \input{device-types/scmi/device-conformance.tex}
> > > >    \input{device-types/gpio/device-conformance.tex}
> > > >    \input{device-types/pmem/device-conformance.tex}
> > > > +\input{device-types/can/device-conformance.tex}
> > > >    \conformance{\section}{Legacy Interface: Transitional Device and Transitional Driver Conformance}\label{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}
> > > >    A conformant implementation MUST be either transitional or
> > > > diff --git a/content.tex b/content.tex
> > > > index d2ab9eb..8806b57 100644
> > > > --- a/content.tex
> > > > +++ b/content.tex
> > > > @@ -765,6 +765,7 @@ \chapter{Device Types}\label{sec:Device Types}
> > > >    \input{device-types/scmi/description.tex}
> > > >    \input{device-types/gpio/description.tex}
> > > >    \input{device-types/pmem/description.tex}
> > > > +\input{device-types/can/description.tex}
> > > >    \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> > > > diff --git a/device-types/can/description.tex b/device-types/can/description.tex
> > > > new file mode 100644
> > > > index 0000000..2511d9c
> > > > --- /dev/null
> > > > +++ b/device-types/can/description.tex
> > > > @@ -0,0 +1,249 @@
> > > > +\section{CAN Device}\label{sec:Device Types / CAN Device}
> > > > +
> > > > +virtio-can is a virtio based CAN (Controller Area Network) controller.
> > > > +It is used to give a virtual machine access to a CAN bus. The CAN bus
> > > > +might either be a physical CAN bus or a virtual CAN bus between virtual
> > > > +machines or a combination of both.
> > > > +
> > > > +\subsection{Device ID}\label{sec:Device Types / CAN Device / Device ID}
> > > > +
> > > > +36
> > > > +
> > > > +\subsection{Virtqueues}\label{sec:Device Types / CAN Device / Virtqueues}
> > > > +
> > > > +\begin{description}
> > > > +\item[0] Txq
> > > > +\item[1] Rxq
> > > > +\item[2] Controlq
> > > > +\end{description}
> > > > +
> > > > +The \field{Txq} is used to send CAN packets to the CAN bus.
> > > > +
> > > > +The \field{Rxq} is used to receive CAN packets from the CAN bus.
> > > > +
> > > > +The \field{Controlq} is used to control the state of the CAN controller.
> > > > +
> > > > +\subsection{Feature bits}{Device Types / CAN Device / Feature bits}
> > > > +
> > > > +Actual CAN controllers support Extended CAN IDs with 29 bits (CAN~2.0B)
> > > > +as well as Standard CAN IDs with 11 bits (CAN~2.0A). The support of
> > > > +CAN~2.0B Extended CAN IDs is considered as mandatory for this
> > > > +specification.
> > > > +
> > > > +\begin{description}
> > > > +
> > > > +\item[VIRTIO_CAN_F_CAN_CLASSIC (0)]
> > > > +
> > > > +The device supports classic CAN frames with a maximum payload size of 8
> > > > +bytes.
> > > > +
> > > > +\item[VIRTIO_CAN_F_CAN_FD (1)]
> > > > +
> > > > +The device supports CAN FD frames with a maximum payload size of 64
> > > > +bytes.
> > > > +
> > > > +\item[VIRTIO_CAN_F_RTR_FRAMES (2)]
> > > > +
> > > > +The device supports RTR (remote transmission request) frames. RTR frames
> > > > +are only supported with classic CAN.
> > > > +
> > > > +\item[VIRTIO_CAN_F_LATE_TX_ACK (3)]
> > > > +
> > > > +The virtio CAN device marks transmission requests from the \field{Txq}
> > > > +as used after the CAN message has been transmitted on the CAN bus. If
> > > > +this feature bit has not been negotiated, the device is allowed to mark
> > > > +transmission requests already as used when the CAN message has been
> > > > +scheduled for transmission but might not yet have been transmitted on
> > > > +the CAN bus.
> > > > +
> > > > +\end{description}
> > > > +
> > > > +\subsubsection{Feature bit requirements}\label{sec:Device Types / CAN Device / Feature bits / Feature bit requirements}
> > > > +
> > > > +Some CAN feature bits require other CAN feature bits:
> > > > +\begin{description}
> > > > +\item[VIRTIO_CAN_F_RTR_FRAMES] Requires VIRTIO_CAN_F_CAN_CLASSIC.
> > > > +\end{description}
> > > > +
> > > > +It is required that at least one of VIRTIO_CAN_F_CAN_CLASSIC and
> > > > +VIRTIO_CAN_F_CAN_FD is negotiated.
> > > > +
> > > > +\subsection{Device configuration layout}\label{sec:Device Types / CAN Device / Device configuration layout}
> > > > +
> > > > +Device configuration fields are listed below, they are read-only for a
> > > > +driver. The \field{status} always exists. A single read-only bit (for
> > > > +the driver) is currently defined for \field{status}:
> > > > +
> > > > +\begin{lstlisting}
> > > > +struct virtio_can_config {
> > > > +#define VIRTIO_CAN_S_CTRL_BUSOFF (1 << 0)
> > > > +        le16 status;
> > > > +};
> > > > +\end{lstlisting}
> > > > +
> > > > +The bit VIRTIO_CAN_S_CTRL_BUSOFF in \field{status} is used to indicate
> > > > +the unsolicited CAN controller state change from started to stopped due
> > > > +to a detected bus off condition.
> > > > +
> > > > +\drivernormative{\subsubsection}{Device Initialization}{Device Types / CAN Device / Device Operation / Initialization}
> > > > +
> > > > +The driver MUST populate the \field{Rxq} with empty device-writeable
> > > > +buffers of at least the size of struct virtio_can_rx, see section
> > > > +\ref{struct virtio_can_rx}.
> > > > +
> > > > +\subsection{Device Operation}\label{sec:Device Types / CAN Device / Device Operation}
> > > > +
> > > > +A device operation has an outcome which is described by one of the
> > > > +following values:
> > > > +
> > > > +\begin{lstlisting}
> > > > +#define VIRTIO_CAN_RESULT_OK     0
> > > > +#define VIRTIO_CAN_RESULT_NOT_OK 1
> > > > +\end{lstlisting}
> > > > +
> > > > +Other values are to be treated like VIRTIO_CAN_RESULT_NOT_OK.
> > > > +
> > > > +\subsubsection{Controller Mode}\label{sec:Device Types / CAN Device / Device Operation / Controller Mode}
> > > > +
> > > > +The general format of a request in the \field{Controlq} is
> > > > +
> > > > +\begin{lstlisting}
> > > > +struct virtio_can_control_out {
> > > > +#define VIRTIO_CAN_SET_CTRL_MODE_START  0x0201
> > > > +#define VIRTIO_CAN_SET_CTRL_MODE_STOP   0x0202
> > > > +        le16 msg_type;
> > > > +};
> > > > +\end{lstlisting}
> > > > +
> > > > +To participate in bus communication the CAN controller is started by
> > > > +sending a VIRTIO_CAN_SET_CTRL_MODE_START control message, to stop
> > > > +participating in bus communication it is stopped by sending a
> > > > +VIRTIO_CAN_SET_CTRL_MODE_STOP control message. Both requests are
> > > > +confirmed by the result of the operation.
> > > > +
> > > > +\begin{lstlisting}
> > > > +struct virtio_can_control_in {
> > > > +        u8 result;
> > > > +};
> > > > +\end{lstlisting}
> > > > +
> > > > +If the transition succeeded the \field{result} is VIRTIO_CAN_RESULT_OK
> > > > +otherwise it is VIRTIO_CAN_RESULT_NOT_OK. If a status update is
> > > > +necessary, the device updates the configuration \field{status} before
> > > > +marking the request used. As the configuration \field{status} change is
> > > > +caused by a request from the driver the device is allowed to omit the
> > 
> > Is this to indicate that when a driver requests a change, the device can
> > omit notifying of the change? As I understand the specification, the
> > device can omit the notification, but the change must still occur? Is it
> > possible for the device to omit the change as well? What else could have
> > triggered the status change if it wasn't the driver?
> > 
> > Thanks.
> > 
> 
> The change request notification OK/NOT_OK for every mode transition request
> is there in any case. In some cases the device would additionally notify the
> driver about the controller status via 'virtio_can_config' in config space
> (config space change notification mechanism), e.g. in case of a BusOff event
> on the device side. Although, it is possible to omit the config space
> notification e.g. when transitioning to stopped mode.
> 

Thanks for the clarification. I think I got confused with the following
sentence:

`As the configuration status change is caused by a request from the
driver the device is allowed to omit the configuration change
notification here.`

I think this could be rewritten as follows:

`The device may omit the configuration change notification as the
configuration status change is requested by the driver.`

This change is, however, minor, so feel free to ignore it.

Matias


---------------------------------------------------------------------
To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org