Re: [PATCH 1/1] virtio-media: Add virtio media device specification

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

On Fri, May 10, 2024 at 03:43:17PM +0200, Albert Esteve wrote:
> Virtio-media is an encapsulation of the V4L2 UAPI into
> virtio, able to virtualize any video device supported
> by V4L2
> 
> Note that virtio-media does not require the use of a
> V4L2 device driver or of Linux on the host or
> guest side - V4L2 is only used as a host-guest protocol,
> and both sides are free to convert it from/to any
> model that they wish to use.
> 
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  conformance.tex                           |  13 +-
>  content.tex                               |   1 +
>  device-types/media/description.tex        | 574 ++++++++++++++++++++++
>  device-types/media/device-conformance.tex |  11 +
>  device-types/media/driver-conformance.tex |   9 +
>  5 files changed, 604 insertions(+), 4 deletions(-)
>  create mode 100644 device-types/media/description.tex
>  create mode 100644 device-types/media/device-conformance.tex
>  create mode 100644 device-types/media/driver-conformance.tex
> 
> diff --git a/conformance.tex b/conformance.tex
> index dc00e84..c369da1 100644
> --- a/conformance.tex
> +++ b/conformance.tex
> @@ -32,8 +32,10 @@ \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 / Media Driver Conformance}.
> +
>  
>      \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
>    \end{itemize}
> @@ -59,8 +61,9 @@ \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 / Media Device Conformance}.
>  
>      \item Clause \ref{sec:Conformance / Legacy Interface: Transitional Device and Transitional Driver Conformance}.
>    \end{itemize}
> @@ -152,6 +155,7 @@ \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/media/driver-conformance.tex}
>  
>  \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance}
>  
> @@ -238,6 +242,7 @@ \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/media/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 0a62dce..59925ae 100644
> --- a/content.tex
> +++ b/content.tex
> @@ -767,6 +767,7 @@ \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/media/description.tex}
>  
>  \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
>  
> diff --git a/device-types/media/description.tex b/device-types/media/description.tex
> new file mode 100644
> index 0000000..d67cc96
> --- /dev/null
> +++ b/device-types/media/description.tex
> @@ -0,0 +1,574 @@
> +\section{Media Device}\label{sec:Device Types / Media Device}
> +
> +The virtio media device follow the same model (and structures) as V4L2. It
> +can be used to virtualize cameras, codec devices, or any other device
> +supported by V4L2. The complete definition of V4L2 structures and ioctls can
> +be found under the
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/index.html}{V4L2 UAPI documentation}.
> +
> +V4L2 is a UAPI that allows a less privileged entity (user-space) to use video
> +hardware exposed by a more privileged entity (the kernel). Virtio-media is an
> +encapsulation of this API into virtio, turning it into a virtualization API
> +for all classes of video devices supported by V4L2, where the host plays the
> +role of the kernel and the guest the role of user-space.
> +
> +The host is therefore responsible for presenting a virtual device that behaves
> +like an actual V4L2 device, which the guest can control.
> +
> +Note that virtio-media does not require the use of a V4L2 device driver or of
> +Linux on the host or guest side - V4L2 is only used as a host-guest protocol,
> +and both sides are free to convert it from/to any model that they wish to use.
> +
> +This section relies on definitions from
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/index.html}{V4L2 UAPI documentation}.
> +

We should generally avoid host/guest terminology, except as an example.

Also when encapsulating UAPIs, endian-ness generally becomes a problem.
What's the approach here?

> +\subsection{Device ID}\label{sec:Device Types / Media Device / Device ID}
> +
> +42
> +
> +\subsection{Virtqueues}\label{sec:Device Types / Media Device / Virtqueues}
> +
> +\begin{description}
> +\item[0] commandq - used for driver commands and device responses to these
> +commands.
> +\item[1] eventq - used for events sent by the device to the driver.
> +\end{description}
> +
> +\devicenormative{\subsubsection}{Virtqueues}{Device Types / Media Device / Virtqueues}
> +
> +The device MUST return the descriptor chains it receives on the commandq as
> +soon as possible, and must never hold them for indefinite periods of time.
> +
> +\drivernormative{\subsubsection}{Virtqueues}{Device Types / Media Device / Virtqueues}
> +
> +The driver MUST re-queue the descriptor chains returned by the device on the
> +eventq as soon as possible, and must never hold them for indefinite periods
> +of time.
> +
> +\subsection{Feature Bits}\label{sec:Device Types / Media Device / Feature Bits}
> +
> +None
> +
> +\subsection{Device Configuration Layout}\label{sec:Device Types / Media Device / Device Configuration Layout}
> +
> +The video device configuration space uses the following layout:
> +
> +\begin{lstlisting}
> +struct virtio_media_config {
> +    le32 device_caps;
> +    le32 device_type;
> +    u8 card[32];
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{device_caps}] (driver-read-only) flags representing the device
> +capabilities as used in
> +\href{https://www.kernel.org/doc/html/v4.9/media/uapi/v4l/vidioc-querycap.html#c.v4l2_capability}{struct v4l2_capabilities}.
> +Corresponds with the \field{device_caps} field in the \textit{struct video_device}.
> +\item[\field{device_type}] (driver-read-only) informs the driver of the type
> +of the video device. Corresponds with the \field{vfl_devnode_type} field of the device.
> +\item[\field{card}] (driver-read-only) name of the device, a NUL-terminated
> +UTF-8 string. Corresponds with the \field{card} field of the \textit{struct v4l2_capability}.
> +If all the characters of the field are used, it does not need to be NUL-terminated.
> +\end{description}
> +
> +\subsection{Device Initialization}
> +
> +\begin{enumerate}
> +\item The driver reads the \field{device_caps} and \field{device_type} fields
> +from the configuration layout to identify the device.
> +\item The driver sets up the \field{commandq} and \field{eventq}.
> +\item The driver may open a session to use the device and send V4L2 ioctls in
> +order to receive more information about the device, such as supported
> +formats or controls.
> +\end{enumerate}
> +
> +\subsection{Device Operation}\label{sec:Device Types / Media Device / Device Operation}
> +
> +Commands are queued on the command queue by the driver for the device to
> +process. The errors returned by each command are standard
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/gen-errors.html}{Linux kernel error codes}.
> +For instance, a command that contains invalid options will return \textit{EINVAL}.
> +
> +Events are sent on the event queue by the device for the driver to handle.
> +
> +\subsubsection{Command Virtqueue}
> +
> +\paragraph{Device Operation: Command headers}
> +
> +\begin{lstlisting}
> +#define VIRTIO_MEDIA_CMD_OPEN 1
> +#define VIRTIO_MEDIA_CMD_CLOSE 2
> +#define VIRTIO_MEDIA_CMD_IOCTL 3
> +#define VIRTIO_MEDIA_CMD_MMAP 4
> +#define VIRTIO_MEDIA_CMD_MUNMAP 5
> +
> +/* Header for all virtio commands from the driver to the device on the commandq. */
> +struct virtio_media_cmd_header {
> +	u32 cmd;
> +	u32 __padding;
> +};
> +
> +/* Header for all virtio responses from the device to the driver on the commandq. */
> +struct virtio_media_resp_header {
> +	u32 status;
> +	u32 __padding;
> +};
> +\end{lstlisting}
> +
> +A command consists of a command header \textit{virtio_media_cmd_header}
> +containing the following device-readable field:
> +
> +\begin{description}
> +\item[\field{cmd}] specifies a device request type (VIRTIO_MEDIA_CMD_*).
> +\end{description}
> +
> +A response consists of a response header \textit{virtio_media_resp_header}
> +containing the following device-writable field:
> +
> +\begin{description}
> +\item[\field{status}] indicates a device request status.
> +\end{description}
> +
> +The status field can take 0 if the command was successful, or one of the
> +standard Linux error codes if it was not.
> +
> +\drivernormative{\paragraph}{Device Operation: Command Virtqueue: Sessions}{Device Types / Media Device / Device Operation / Command Virtqueue}
> +
> +Sessions are how the device is multiplexed, allowing several distinct works to
> +take place simultaneously. The driver needs to open a session before it can
> +perform any useful operation on the device.
> +
> +\paragraph{Device Operation: Open device}
> +
> +\textbf{VIRTIO_MEDIA_CMD_OPEN} Command for creating a new session.
> +
> +This is the equivalent of calling \textit{open} on a V4L2 device node.
> +The driver uses \textit{virtio_media_cmd_open} to send an open request.
> +
> +\begin{lstlisting}
> +struct virtio_media_cmd_open {
> +    struct virtio_media_cmd_header hdr;
> +};
> +\end{lstlisting}
> +
> +The device responds to \textit{VIRTIO_MEDIA_CMD_OPEN} with \textit{virtio_media_resp_open}.
> +
> +\begin{lstlisting}
> +struct virtio_media_resp_open {
> +    struct virtio_media_resp_header hdr;
> +    u32 session_id;
> +    u32 __padding;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{session_id}] specifies an identifier for the current session. The
> +identifier can be used to perform other commands on the session, notably ioctls.
> +\end{description}
> +
> +\devicenormative{\subparagraph}{Device Operation: Open device}{Device Types / Media Device / Device Operation / Open device}
> +
> +Upon success, the device MUST set a \field{session_id} in \textit{virtio_media_resp_open}
> +to an integer that is NOT used by any other open session.
> +
> +\paragraph{Device Operation: Close device}
> +
> +\textbf{VIRTIO_MEDIA_CMD_CLOSE} Command for closing an active session.
> +
> +This is the equivalent of calling \textit{close} on a previously opened V4L2
> +device node. All resources associated with this session will be freed.
> +
> +This command does not require a response from the device.
> +
> +\begin{lstlisting}
> +struct virtio_media_cmd_close {
> +    struct virtio_media_cmd_header hdr;
> +    u32 session_id;
> +    u32 __padding;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{session_id}] specifies an identifier for the session to close.
> +\end{description}
> +
> +\drivernormative{\subparagraph}{Device Operation: Close device}{Device Types / Media Device / Device Operation / Close device}
> +
> +The session ID SHALL NOT be used again after queueing this command.
> +
> +\paragraph{Device Operation: V4L2 ioctls}
> +
> +\textbf{VIRTIO_MEDIA_CMD_IOCTL} Command for executing an ioctl on an open
> +session.
> +
> +This command asks the device to run one of the `VIDIOC_*` ioctls on the active
> +session.
> +
> +\begin{lstlisting}
> +struct virtio_media_cmd_ioctl {
> +    struct virtio_media_cmd_header hdr;
> +    u32 session_id;
> +    u32 code;
> +    /* Followed by the relevant ioctl payload as defined in the macro */
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{session_id}] specifies an identifier of thesession to run the ioctl on.
> +\item[\field{code}] specifies the code of the \field{VIDIOC_*} ioctl to run.
> +\end{description}
> +
> +The code is extracted from the
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/videodev.html}{videodev2.h},
> +header file. The file defines the ioctl's codes, type of payload, and
> +direction. The code consists of the second argument of the \field{_IO*} macro.
> +
> +For example, the \textit{VIDIOC_G_FMT} is defined as follows:
> +
> +\begin{lstlisting}
> +#define VIDIOC_G_FMT _IOWR('V',  4, struct v4l2_format)
> +\end{lstlisting}
> +
> +This means that its ioctl code is \textit{4}, that its payload is a
> +\textit{struct v4l2_format}, and that its direction is \textit{WR} (i.e., the
> +payload is written by both the driver and the device).
> +See Section \ref{sec:Device Types / Media Device / V4L2 ioctls / Ioctls payload}
> +for more information about the direction of ioctls.
> +
> +The payload layout is always a 64-bit representation of the corresponding
> +V4L2 structure, irrespective of the host and guest architecture.
> +
> +The device responds to \textit{VIRTIO_MEDIA_CMD_IOCTL} with \textit{virtio_media_resp_ioctl}.
> +
> +\begin{lstlisting}
> +struct virtio_media_resp_ioctl {
> +    struct virtio_media_resp_header hdr;
> +    /* Followed by the ioctl payload as defined in the macro */
> +};
> +\end{lstlisting}
> +
> +\subparagraph{Ioctls payload}\label{sec:Device Types / Media Device / V4L2 ioctls / Ioctls payload}
> +
> +Each ioctl has a payload, which is defined by the third argument of the
> +\field{_IO*} macro defining it. 
> +
> +The payload of an ioctl in the descriptor chain follows the command structure,
> +the reponse structure, or both depending on the direction:
> +
> +\begin{itemize}
> +\item \textbf{_IOR} is read-only for the driver, meaning the payload
> +follows the response in the device-writable section of the descriptor chain.
> +\item \textbf{_IOW} is read-only for the device, meaning the payload
> +follows the command in the driver-writable section of the descriptor chain.
> +\item \textbf{_IOWR} is writable by both the device and driver,
> +meaning the payload must follow both the command in the driver-writable section
> +of the descriptor chain, and the response in the device-writable section.
> +\end{itemize}
> +
> +A common optimization for \textit{WR} ioctls is to provide the payload using
> +descriptors that both point to the same buffer. This mimics the behavior of
> +V4L2 ioctls where the data is only passed once and used as both input and
> +output by the kernel.
> +
> +\devicenormative{\subparagraph}{Device Operation: V4L2 ioctls}{Device Types / Media Device / Device Operation / V4L2 ioctls}
> +
> +In case of success of a device-writable ioctl, the device MUST always write the
> +payload in the device-writable part of the descriptor chain.
> +
> +In case of failure of a device-writable ioctl, the device is free to write the
> +payload in the device-writable part of the descriptor chain or not. Some errors
> +may still result in the payload being updated, and in this case the device is
> +expected to write the updated payload. If the device has not written the
> +payload after an error, the driver MUST assume that the payload has not been
> +modified.
> +
> +\subparagraph{Handling of pointers in ioctl payload}
> +
> +A few structures used as ioctl payloads contain pointers to further
> +data needed for the ioctl. There are notably:
> +
> +\begin{itemize}
> +\item The \field{planes} pointer of
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/buffer.html#struct-v4l2-buffer}{struct v4l2_buffer},
> +which size is determined by the length member.
> +\item The \field{controls} pointer of \textit{struct v4l2_ext_controls}, which
> +size is determined by the count member.
> +\end{itemize}
> +
> +If the size of the pointed area is determined to be non-zero, then the main
> +payload is immediately followed by the pointed data in their order of
> +appearance in the structure, and the pointer value itself is ignored by the
> +device, which must also return the value initially passed by the driver.
> +
> +\subparagraph{Handling of pointers to userspace memory}
> +\label{sec:Device Types / Media Device / V4L2 ioctls / Userspace memory}
> +
> +A few pointers are special in that they point to userspace memory in the
> +original V4L2 specification. They are:
> +
> +\begin{itemize}
> +\item The \field{m.userptr} member of \textit{struct v4l2_buffer} and
> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/buffer.html#struct-v4l2-plane}{struct v4l2_plane}
> +(technically an unsigned long, but designated a userspace address).
> +\item The \field{ptr} member of \textit{struct v4l2_ext_ctrl}.
> +\end{itemize}
> +
> +These pointers can cover large areas of scattered memory, which has the
> +potential to require more descriptors than the virtio queue can provide. For
> +these particular pointers only, a list of \textit{struct virtio_media_sg_entry}
> +that covers the needed amount of memory for the pointer is used instead of
> +using descriptors to map the pointed memory directly.
> +
> +\begin{lstlisting}
> +struct virtio_media_sg_entry {
> +	u64 start;
> +	u32 len;
> +	u32 __padding;
> +};
> +\end{lstlisting}
> +
> +For each such pointer to read, the device reads as many SG entries as needed
> +to cover the length of the pointed buffer, as described by its parent
> +structure (\field{length} member of \textit{struct v4l2_buffer} or
> +\textit{struct v4l2_plane} for buffer memory, and \field{size} member of
> +\textit{struct v4l2_ext_control} for control data).
> +
> +Since the device never needs to modify the list of SG entries, it is only
> +provided by the driver in the device-readable section of the descriptor chain,
> +and not repeated in the device-writable section, even for WR ioctls.
> +
> +\subparagraph{Unsupported ioctls}
> +
> +A few ioctls are replaced by other, more suitable mechanisms.
> +
> +\begin{itemize}
> +\item \textit{VIDIOC_QUERYCAP} is replaced by reading the configuration area
> +(see \ref{sec:Device Types / Media Device / Device Configuration Layout}).
> +\item \textit{VIDIOC_DQBUF} is replaced by a dedicated event
> +(see \ref{sec:Device Types / Media Device / Device Operation / Dequeue buffer}).
> +\item \textit{VIDIOC_DQEVENT} is replaced by a dedicated event
> +(see \ref{sec:Device Types / Media Device / Device Operation / Emit an event}).
> +\item \textit{VIDIOC_G_JPEGCOMP} and \textit{VIDIOC_S_JPEGCOMP} are deprecated
> +and replaced by the controls of the JPEG class.
> +\item \textit{VIDIOC_LOG_STATUS} is a guest-only operation and shall not be
> +implemented by the host.
> +\end{itemize}
> +
> +\devicenormative{\subparagraph}{Device Operation: Unsupported ioctls}{Device Types / Media Device / Device Operation / Unsupported ioctls}
> +
> +If being requested an unsupported ioctl, the device MUST return the same
> +error response as it would for an unknown ioctl, i.e. \textit{ENOTTY}.
> +
> +\paragraph{Device Operation: Mapping a MMAP buffer}
> +
> +\textbf{VIRTIO_MEDIA_CMD_MMAP} Command for mapping a MMAP buffer into the
> +guest's address space.
> +
> +\begin{lstlisting}
> +#define VIRTIO_MEDIA_MMAP_FLAG_RW (1 << 0)
> +
> +struct virtio_media_cmd_mmap {
> +	struct virtio_media_cmd_header hdr;
> +	u32 session_id;
> +	u32 flags;
> +	u64 offset;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{flags}] is the set of flags for the mapping. \field{VIRTIO_MEDIA_MMAP_FLAG_RW}
> +can be set if a read-write mapping is desired. Without this flag the mapping
> +will be read-only.
> +\item[\field{offset}] corresponds to the \field{mem_offset} field of the
> +\textit{union v4l2_plane} for the plane to map. This field can be obtained
> +using the \textit{VIDIOC_QUERYBUF} ioctl.
> +\end{description}
> +
> +The device responds to \textit{VIRTIO_MEDIA_CMD_MMAP} with \textit{virtio_media_resp_mmap}.
> +
> +\begin{lstlisting}
> +struct virtio_media_resp_mmap {
> +    struct virtio_media_resp_header hdr;
> +    u64 addr;
> +    u64 len;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{addr}] device physical address of the start of the mapping.
> +\item[\field{len}] length of the mapping as indicated by the \textit{struct v4l2_plane}
> +the buffer belongs to.
> +\end{description}
> +
> +\paragraph{Device Operation: Unmapping a MMAP buffer}
> +
> +\textbf{VIRTIO_MEDIA_CMD_MUNMAP} Unmap a MMAP buffer previously mapped using \field{VIRTIO_MEDIA_CMD_MMAP}.
> +
> +\begin{lstlisting}
> +struct virtio_media_cmd_munmap {
> +    struct virtio_media_cmd_header hdr;
> +    u64 guest_addr;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{guest_addr}] guest physical address previously returned by 
> +\textit{VIRTIO_MEDIA_CMD_MMAP} at which the buffer has been previously mapped.
> +\end{description}
> +
> +The device responds to \textit{VIRTIO_MEDIA_CMD_MUNMAP} with \textit{virtio_media_resp_munmap}.
> +
> +\begin{lstlisting}
> +struct virtio_media_resp_munmap {
> +    struct virtio_media_resp_header hdr;
> +};
> +\end{lstlisting}
> +
> +\devicenormative{\subparagraph}{Device Operation: Unmapping a MMAP buffer}{Device Types / Media Device / Device Operation / Unmapping a MMAP buffer}
> +
> +The device MUST keep mappings performed using \textit{VIRTIO_MEDIA_CMD_MMAP}
> +valid until \textit{VIRTIO_MEDIA_CMD_MUNMAP} is called, even if the buffers or
> +session they belong to are released or closed by the guest.
> +
> +\paragraph{Device Operation: Memory Types}
> +
> +The semantics of the three V4L2 memory types (\textit{MMAP}, \textit{USERPTR}
> +and \textit{DMABUF}) can easily be mapped to a guest/host context.
> +
> +\subparagraph{MMAP}
> +
> +In virtio-media, \textit{MMAP} buffers are provisioned by the host, just like
> +they are by the kernel in regular V4L2. Similarly to how userspace can map a
> +\textit{MMAP} buffer into its address space using mmap and munmap, the
> +virtio-media driver can map host buffers into the guest space by queueing the
> +\textit{struct virtio_media_cmd_mmap} and \textit{struct virtio_media_cmd_munmap}
> +commands to the commandq.
> +
> +\subparagraph{USERPTR}
> +
> +In virtio-media, \textit{USERPTR} buffers are provisioned by the guest, just
> +like they are by userspace in regular V4L2. Instances of \textit{struct v4l2_buffer}
> +and \textit{struct v4l2_plane} of this type are followed by a list of
> +\textit{struct virtio_media_sg_entry}. For more information, see
> +\ref{sec:Device Types / Media Device / V4L2 ioctls / Userspace memory}
> +
> +The host must not alter the pointer values provided by the guest, i.e.
> +\field{the m.userptr} member of \textit{struct v4l2_buffer} and
> +\textit{struct v4l2_plane} must be returned to the guest with the same value
> +as it was provided.
> +
> +\subparagraph{DMABUF}
> +
> +In virtio-media, \textit{DMABUF} buffers are provisioned by a virtio object,
> +just like they are by a \textit{DMABUF} in regular V4L2. Virtio objects are
> +16-bytes UUIDs and do not fit in the placeholders for file descriptors, so
> +they follow their embedding data structure as needed and the device must
> +leave the V4L2 structure placeholder unchanged.
> +
> +Contrary to \textit{USERPTR} buffers, virtio objects UUIDs need to be added in
> +both the device-readable and device-writable section of the descriptor chain.
> +
> +Host-allocated buffers with the \textit{V4L2_MEMORY_MMAP} memory type can also
> +be exported as virtio objects for use with another virtio device using the
> +\textit{VIDIOC_EXPBUF} ioctl. The fd placefolder of \textit{v4l2_exportbuffer}
> +means that space for the UUID needs to be reserved right after that structure
> +
> +\subsubsection{Event Virtqueue}
> +
> +Events are a way for the device to inform the driver about asynchronous events
> +that it should know about. In virtio-media, they are used as a replacement for
> +the \textit{VIDIOC_DQBUF} and \textit{VIDIOC_DQEVENT} ioctls and the polling
> +mechanism, which would be impractical to implement on top of virtio.
> +
> +\paragraph{Device Operation: Event header}
> +
> +\begin{lstlisting}
> +#define VIRTIO_MEDIA_EVT_ERROR 0
> +#define VIRTIO_MEDIA_EVT_DQBUF 1
> +#define VIRTIO_MEDIA_EVT_EVENT 2
> +
> +/* Header for events queued by the device for the driver on the eventq. */
> +struct virtio_media_event_header {
> +    u32 event;
> +    u32 session_id;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{event}] one of \field{VIRTIO_MEDIA_EVT_*}.
> +\item[\field{session_id}] ID of the session the event applies to.
> +\end{description}
> +
> +\paragraph{Device Operation: Device-side error}
> +
> +\textbf{VIRTIO_MEDIA_EVT_ERROR} Upon receiving this event, the session
> +mentioned in the header is considered corrupted and automatically closed by
> +the device.
> +
> +\begin{lstlisting}
> +struct virtio_media_event_error {
> +    struct virtio_media_event_header hdr;
> +    u32 errno;
> +    u32 __padding;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{errno}] error code describing the kind of error that occurred.
> +\end{description}
> +
> +\paragraph{Device Operation: Dequeue buffer}
> +\label{sec:Device Types / Media Device / Device Operation / Dequeue buffer}
> +
> +\textbf{VIRTIO_MEDIA_EVT_DQBUF} Signals that a buffer is not being used anymore
> +by the device and is returned to the driver.
> +
> +A \textit{struct virtio_media_event_dqbuf} event is queued on the eventq by the
> +device every time a buffer previously queued using the \textit{VIDIOC_QBUF}
> +ioctl is done being processed and can be used by the driver again. This is like
> +an implicit \textit{VIDIOC_DQBUF} ioctl.
> +
> +\begin{lstlisting}
> +struct virtio_media_event_dqbuf {
> +    struct virtio_media_event_header hdr;
> +    struct v4l2_buffer buffer;
> +    struct v4l2_plane planes[VIDEO_MAX_PLANES];
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{buffer}] \textit{struct v4l2_buffer} describing the buffer that has been dequeued.
> +\item[\field{planes}] array of \textit{struct v4l2_plane} containing the plane information for multi-planar buffers.
> +\end{description}
> +
> +Pointer values in the \textit{struct v4l2_buffer} and \textit{struct v4l2_plane}
> +are meaningless and must be ignored by the driver. It is recommended that the
> +device sets them to NULL in order to avoid leaking potential host addresses.
> +
> +Note that in the case of a \field{USERPTR} buffer, the \textit{struct v4l2_buffer}
> +used as event payload is not followed by the buffer memory: since that memory
> +is the same that the driver submitted with the \textit{VIDIOC_QBUF}, it would
> +be redundant to have it here.
> +
> +\paragraph{Device Operation: Emit an event}
> +\label{sec:Device Types / Media Device / Device Operation / Emit an event}
> +
> +\textbf{VIRTIO_MEDIA_EVT_EVENT} Signals that a V4L2 event has been emitted for a session.
> +
> +A \textit{struct virtio_media_event_event} event is queued on the eventq by the
> +device every time an event the driver previously subscribed to using the
> +\textit{VIDIOC_SUBSCRIBE_EVENT} ioctl has been signaled. This is like an
> +implicit \textit{VIDIOC_DQEVENT} ioctl.
> +
> +\begin{lstlisting}
> +struct virtio_media_event_event {
> +    struct virtio_media_event_header hdr;
> +    struct v4l2_event event;
> +};
> +\end{lstlisting}
> +
> +\begin{description}
> +\item[\field{event}] \textit{struct v4l2_event} describing the event that occurred.
> +\end{description}
> diff --git a/device-types/media/device-conformance.tex b/device-types/media/device-conformance.tex
> new file mode 100644
> index 0000000..3338822
> --- /dev/null
> +++ b/device-types/media/device-conformance.tex
> @@ -0,0 +1,11 @@
> +\conformance{\subsection}{Media Device Conformance}\label{sec:Conformance / Device Conformance / Media Device Conformance}
> +
> +A Media device MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\item \ref{devicenormative:Device Types / Media Device / Virtqueues}
> +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Open device}
> +\item \ref{devicenormative:Device Types / Media Device / Device Operation / V4L2 ioctls}
> +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Unsupported ioctls}
> +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Unmapping a MMAP buffer}
> +\end{itemize}
> \ No newline at end of file
> diff --git a/device-types/media/driver-conformance.tex b/device-types/media/driver-conformance.tex
> new file mode 100644
> index 0000000..058b812
> --- /dev/null
> +++ b/device-types/media/driver-conformance.tex
> @@ -0,0 +1,9 @@
> +\conformance{\subsection}{Media Device Conformance}\label{sec:Conformance / Driver Conformance / Media Driver Conformance}
> +
> +A Media device MUST conform to the following normative statements:
> +
> +\begin{itemize}
> +\item \ref{drivernormative:Device Types / Media Device / Virtqueues}
> +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Command Virtqueue}
> +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Close device}
> +\end{itemize}
> \ No newline at end of file
> -- 
> 2.44.0