[PATCH v9 1/1] virtio-media: Add virtio media device specification
Albert Esteve <[email protected]>
| Newsgroups | dev.linux.lists.virtio-comment |
|---|---|
| Message-ID | <[email protected]> |
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 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. Reviewed-by: Matias Ezequiel Vara Larsen <[email protected]> Reviewed-by: Alexandre Courbot <[email protected]> Signed-off-by: Albert Esteve <[email protected]> --- conformance.tex | 6 + content.tex | 1 + device-types/media/description.tex | 639 ++++++++++++++++++++++ device-types/media/device-conformance.tex | 15 + device-types/media/driver-conformance.tex | 11 + 5 files changed, 672 insertions(+) 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 d92a236..7220614 100644 --- a/conformance.tex +++ b/conformance.tex @@ -36,6 +36,8 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets} \ref{sec:Conformance / Driver Conformance / PMEM Driver Conformance}, \ref{sec:Conformance / Driver Conformance / CAN Driver Conformance} or \ref{sec:Conformance / Driver Conformance / SPI Controller 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} @@ -65,6 +67,8 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets} \ref{sec:Conformance / Device Conformance / PMEM Device Conformance}, \ref{sec:Conformance / Device Conformance / CAN Device Conformance} or \ref{sec:Conformance / Device Conformance / SPI Controller 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} @@ -162,6 +166,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets} \input{device-types/pmem/driver-conformance.tex} \input{device-types/can/driver-conformance.tex} \input{device-types/spi/driver-conformance.tex} +\input{device-types/media/driver-conformance.tex} \conformance{\section}{Device Conformance}\label{sec:Conformance / Device Conformance} @@ -254,6 +259,7 @@ \section{Conformance Targets}\label{sec:Conformance / Conformance Targets} \input{device-types/pmem/device-conformance.tex} \input{device-types/can/device-conformance.tex} \input{device-types/spi/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 09628ca..56b2cb1 100644 --- a/content.tex +++ b/content.tex @@ -778,6 +778,7 @@ \chapter{Device Types}\label{sec:Device Types} \input{device-types/pmem/description.tex} \input{device-types/can/description.tex} \input{device-types/spi/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..0527da7 --- /dev/null +++ b/device-types/media/description.tex @@ -0,0 +1,639 @@ +\section{Media Device}\label{sec:Device Types / Media Device} + +The virtio media device follows 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 device plays the +role of the kernel and the driver the role of user-space. + +The device is therefore responsible for presenting a virtual device that behaves +like an actual V4L2 device, which the driver can control. + +Note that virtio-media does not require the use of a V4L2 device driver or of +Linux on any side - V4L2 is only used as a transport protocol, +and both sides are free to convert it from/to any model that they wish to use. + +\subsection{Device ID}\label{sec:Device Types / Media Device / Device ID} + +48 + +\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} + +\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; + le8 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/latest/userspace-api/media/v4l/vidioc-querycap.html#c.V4L.v4l2_capability}{struct v4l2_capability}. +It 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. It 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. It 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} + +A driver executes the following sequence to initialize a device: + +\begin{enumerate} +\item Read the \field{device_caps} and \field{device_type} fields +from the configuration layout to identify the device. +\item Set up the \field{commandq} and \field{eventq} virtqueues. +\item May open a session (see Section \ref{sec:Device Types / Media Device / Device Operation / Open Device}) +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} + +The driver enqueues commands in the command queue 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 driver sending a command that contains invalid options will +receive \textit{EINVAL} in return, after the device tries to process it. + +The device enqueues events in the event queue for the driver to process. + +\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 { + le32 cmd; + le32 __reserved; +}; + +/* Header for all virtio responses from the device to the driver on the commandq. */ +struct virtio_media_resp_header { + le32 status; + le32 __reserved; +}; +\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} + +When the device executes the command successfully, the value of the status +field is 0. Conversely, when the device fails to execute the command, the value +of the status fields corresponds with one of the standard Linux error codes. + +\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. Before starting operation, the driver needs to open +a session. This is equivalent to opening the \textit{/dev/videoX} file of the +V4L2 device. Each session gets a unique ID assigned, which can be then used +to perform actions on it. + +\paragraph{Device Operation: Open device}\label{sec:Device Types / Media Device / 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; + le32 session_id; + le32 __reserved; +}; +\end{lstlisting} + +\begin{description} +\item[\field{session_id}] identifies the current session, which is used for +other commands, predominantly 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; + le32 session_id; + le32 __reserved; +}; +\end{lstlisting} + +\begin{description} +\item[\field{session_id}] identifies 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, until it has +been obtained again through a subsequent \textit{VIRTIO_MEDIA_CMD_OPEN} call. + +\paragraph{Device Operation: V4L2 ioctls} + +\textbf{VIRTIO_MEDIA_CMD_IOCTL} Command for executing an ioctl on an open +session. + +This command tells the device to run one of the `VIDIOC_*` ioctls on the +session identified by \textit{session_id}. + +\begin{lstlisting} +struct virtio_media_cmd_ioctl { + struct virtio_media_cmd_header hdr; + le32 session_id; + le32 code; + /* Followed by the relevant ioctl command payload as defined in the macro */ +}; +\end{lstlisting} + +\begin{description} +\item[\field{session_id}] identifies the session 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}, its payload is a +\textit{struct v4l2_format}, and 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 struct layout always matches the 64-bit, little-endian +representation of the corresponding V4L2 structure. + +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 response 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. + +The payload of an ioctl in the descriptor chain follows the command structure, +the response 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 device-readable 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 device-readable section +of the descriptor chain, and the response in the device-writable section. +\end{itemize} + +A possible 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. + +\drivernormative{\subparagraph}{Device Operation: V4L2 ioctls}{Device Types / Media Device / Device Operation / V4L2 ioctls} + +For most V4L2 structures, the size is identical for both 32 and 64 bits versions. +If the payload struct layout size differs for 32 and 64 bits, the driver +MUST translate them to its the 64-bit, little-endian representation. + +If the device has not written the payload after an error (i.e., only the +header is returned), the driver MUST assume that the payload has not +been modified. + +\subparagraph{Handling of pointers to data in ioctl payload} +\label{sec:Device Types / Media Device / V4L2 ioctls / Pointers in V4l2 ioctls} + +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 +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/v4l/vidioc-g-ext-ctrls.html#c.V4L.v4l2_ext_controls}{struct v4l2_ext_controls}, +which size is determined by the count member. +\end{itemize} + +If the size of the pointed area is non-zero, then the main +payload is immediately followed by the pointed data in their order of +appearance in the structure. + +\devicenormative{\subparagraph}{Handling of pointers to data in ioctl payload}{Device Types / Media Device / Device Operation / Pointers in V4l2 ioctls} + +The pointer value, when its area is non-zero, is ignored by the device, +which MUST return the value initially passed by the driver. + + +\subparagraph{Handling of pointers to userspace memory in ioctl payload} +\label{sec:Device Types / Media Device / V4L2 ioctls / Userspace memory} + +A few pointers (used for \textit{SHARED_PAGES} memory type, +see \ref{sec:Device Types / Media Device / Device Operation / Shared Pages}) +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 may 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 { + le64 start; + le32 len; + le32 __reserved; +}; +\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} and \textit{VIDIOC_DQEVENT} are replaced by a dedicated event +(see \ref{sec:Device Types / Media Device / Device Operation / Event Virtqueue}). +\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 driver-only operation and shall not be +implemented by the device. +\end{itemize} + +\devicenormative{\subparagraph}{Device Operation: Unsupported ioctls}{Device Types / Media Device / Device Operation / Unsupported ioctls} + +When a request is not supported, the device MUST return \textit{ENOTTY}, +which corresponds to the response for unknown ioctls. + +\paragraph{Device Operation: Mapping a MMAP buffer} + +\textbf{VIRTIO_MEDIA_CMD_MMAP} Command for mapping a MMAP buffer into the +driver's address space. + +Shared memory region ID 0 is used to map MMAP buffers with +the \textit{VIRTIO_MEDIA_CMD_MMAP} command. + +\begin{lstlisting} +#define VIRTIO_MEDIA_MMAP_FLAG_RW (1 << 0) + +struct virtio_media_cmd_mmap { + struct virtio_media_cmd_header hdr; + le32 session_id; + le32 flags; + le32 offset; +}; +\end{lstlisting} + +\begin{description} +\item[\field{session_id}] identifies the session which the mapped buffer pertains to. +\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; + le64 driver_addr; + le64 len; +}; +\end{lstlisting} + +\begin{description} +\item[\field{driver_addr}] offset into SHM region ID 0 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} + +\devicenormative{\subparagraph}{Device Operation: Mapping a MMAP buffer}{Device Types / Media Device / Device Operation / Mapping a MMAP buffer} + +The \textit{len} parameter of the \textit{virtio_media_resp_mmap} response sent +by the device MUST always be equal to the length of the buffer. + +\paragraph{Device Operation: Unmapping a MMAP buffer} + +\textbf{VIRTIO_MEDIA_CMD_MUNMAP} unmaps a MMAP buffer previously mapped using \field{VIRTIO_MEDIA_CMD_MMAP}. + +\begin{lstlisting} +struct virtio_media_cmd_munmap { + struct virtio_media_cmd_header hdr; + le64 driver_addr; +}; +\end{lstlisting} + +\begin{description} +\item[\field{driver_addr}] offset into SHM region ID 0 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 driver. + +\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 both driver and device context. + +\begin{lstlisting} +enum virtio_media_memory { + VIRTIO_MEDIA_MMAP = V4L2_MEMORY_MMAP, + VIRTIO_MEDIA_SHARED_PAGES = V4L2_MEMORY_USERPTR, + VIRTIO_MEDIA_OBJECT = V4L2_MEMORY_DMABUF, +}; +\end{lstlisting} + +\subparagraph{MMAP} + +\textit{MMAP} memory type is the semantic equivalent of +\textit{V4L2_MEMORY_MMAP} in regular V4L2. + +In virtio-media, \textit{MMAP} buffers are provisioned by the device, 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 device buffers into the driver space by queueing the +\textit{struct virtio_media_cmd_mmap} and \textit{struct virtio_media_cmd_munmap} +commands to the commandq. + +\subparagraph{SHARED_PAGES} +\label{sec:Device Types / Media Device / Device Operation / Shared Pages} + +\textit{SHARED_PAGES} memory type is the semantic equivalent of +\textit{V4L2_MEMORY_USERPTR} in regular V4L2. + +In virtio-media, \textit{SHARED_PAGES} buffers are provisioned by the driver, +and use guest physical addresses. Instances of \textit{struct v4l2_buffer} +and \textit{struct v4l2_plane} of this memory 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} + +\devicenormative{\subparagraph}{Device Operation: Shared Pages}{Device Types / Media Device / Device Operation / Shared Pages} + +The device MUST not alter the pointer values provided by the driver, i.e. +the \field{m.userptr} member of \textit{struct v4l2_buffer} and +\textit{struct v4l2_plane} MUST be returned to the driver with the same value +as it was provided. + +\subparagraph{VIRTIO_OBJECT} +\label{sec:Device Types / Media Device / Device Operation / Virtio Object} + +\textit{VIRTIO_OBJECT} memory type is the semantic equivalent of +\textit{V4L2_MEMORY_DMABUF} in regular V4L2. + +In virtio-media, \textit{VIRTIO_OBJECT} 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. For example, in multi-planar +buffers, \textit{struct v4l2_plane} structures are located after the +\textit{struct v4l2_buffer} in the memory layout. Then, after the last plane, +the memory contains an array of UUIDs in which the first element corresponds +with the first plane, and so on. + +Conversely to \textit{SHARED_PAGES} buffers, which SG lists are never +modified by the device, UUIDs of virtio objects need to be added in both +the device-readable and device-writable section of the descriptor chain +so the device can update them if needed. + +Device-allocated buffers with the \textit{VIRTIO_MEDIA_MMAP} memory type may 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. + +\devicenormative{\subparagraph}{Device Operation: Virtio Object}{Device Types / Media Device / Device Operation / Virtio Object} + +The device MUST leave the \field{fd} placeholder of the V4L2 structure unchanged. + +\subsubsection{Event Virtqueue} +\label{sec:Device Types / Media Device / Device Operation / Event Virtqueue} + +Events are asynchronous notifications to the driver. 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 { + le32 event; + le32 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} Unrecoverable session error. Upon +emitting this event, the device considers the session mentioned in the +header to be invalid, and returns an error to all future commands +referring to it. Upon receiving this event, the driver stops using the +session, and shall close it as soon as possible. + +\begin{lstlisting} +struct virtio_media_event_error { + struct virtio_media_event_header hdr; + le32 errno; + le32 __reserved; +}; +\end{lstlisting} + +\begin{description} +\item[\field{errno}] error code describing the kind of error that occurred. +\end{description} + +\devicenormative{\subparagraph}{Device Operation: Device-side error}{Device Types / Media Device / Device Operation / Device-side error} + +After an error is signaled, when the device considers the session as +non-existing, the device MUST NOT recycle the session ID until the driver has +explicitly closed it. + +\drivernormative{\subparagraph}{Device Operation: Device-side error}{Device Types / Media Device / Device Operation / Device-side error} + +Upon receiving an error event for a session, the driver MUST +explicitly close the session using a \textit{VIRTIO_MEDIA_CMD_CLOSE} command. + +\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. + +Every time a buffer previously queued (i.e., using the +\textit{VIDIOC_QBUF} ioctl) is done being processed, the +device queues a \textit{struct virtio_media_event_dqbuf} event on +the eventq, signifying that the buffer may be used again by the driver. +This is like an implicit \textit{VIDIOC_DQBUF} ioctl. + +\begin{lstlisting} +#define VIRTIO_MEDIA_MAX_PLANES 8 + +struct virtio_media_event_dqbuf { + struct virtio_media_event_header hdr; + struct v4l2_buffer buffer; + struct v4l2_plane planes[VIRTIO_MEDIA_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} + +Note that in the case of a \field{SHARED_PAGES} buffer, the \textit{struct v4l2_buffer} +used as event payload is not followed by the buffer's SG entries: since that memory +is the same that the driver submitted with the \textit{VIDIOC_QBUF}, it would +be redundant to have it here. + +\drivernormative{\subparagraph}{Device Operation: Dequeue buffer}{Device Types / Media Device / Device Operation / Dequeue buffer} + +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 device addresses. + +\paragraph{Device Operation: Emit an event} + +\textbf{VIRTIO_MEDIA_EVT_EVENT} Signals that a V4L2 event has been emitted for a session. + +Every time an event for which the driver has been previously subcribed to +(i.e., using the \textit{VIDIOC_SUBSCRIBE_EVENT} ioctl) is signaled, the +device queues a \textit{struct virtio_media_event_event} event on the eventq. +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..99112ff --- /dev/null +++ b/device-types/media/device-conformance.tex @@ -0,0 +1,15 @@ +\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 / Device Operation / Open device} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / V4L2 ioctls} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Pointers in V4l2 ioctls} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Unsupported ioctls} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Mapping a MMAP buffer} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Unmapping a MMAP buffer} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Shared Pages} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Virtio Object} +\item \ref{devicenormative:Device Types / Media Device / Device Operation / Device-side error} +\end{itemize} diff --git a/device-types/media/driver-conformance.tex b/device-types/media/driver-conformance.tex new file mode 100644 index 0000000..eb380ad --- /dev/null +++ b/device-types/media/driver-conformance.tex @@ -0,0 +1,11 @@ +\conformance{\subsection}{Media Driver Conformance}\label{sec:Conformance / Driver Conformance / Media Driver Conformance} + +A Media driver MUST conform to the following normative statements: + +\begin{itemize} +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Command Virtqueue} +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Close device} +\item \ref{drivernormative:Device Types / Media Device / Device Operation / V4L2 ioctls} +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Device-side error} +\item \ref{drivernormative:Device Types / Media Device / Device Operation / Dequeue buffer} +\end{itemize} -- 2.49.0