On Thu, Jun 20, 2024 at 9:45 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
>
> On 08/06/2024 03:24, Alexandre Courbot wrote:
> > Hi Hans,
> >
> > On Fri, Jun 7, 2024 at 5:43 PM Hans Verkuil <hverkuil@xxxxxxxxx> wrote:
> >>
> >> Hi Albert,
> >>
> >>
> >> On 07/06/2024 10:00, 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.
> >>
> >> I just have two notes, one minor, one more substantial:
> >>
> >>>
> >>> Signed-off-by: Albert Esteve <aesteve@xxxxxxxxxx>
> >>> ---
> >>> conformance.tex | 13 +-
> >>> content.tex | 1 +
> >>> device-types/media/description.tex | 578 ++++++++++++++++++++++
> >>> device-types/media/device-conformance.tex | 11 +
> >>> device-types/media/driver-conformance.tex | 9 +
> >>> 5 files changed, 608 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..887eacf
> >>> --- /dev/null
> >>> +++ b/device-types/media/description.tex
> >>> @@ -0,0 +1,578 @@
> >>> +\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 device assumes 64-bit little-endian V4L2 structures
> >>> +are exchanged. 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.
> >>> +
> >>> +This section relies on definitions from
> >>> +\href{https://www.kernel.org/doc/html/latest/userspace-api/media/index.html}{V4L2 UAPI documentation}.
> >>> +
> >>> +\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];
> >>
> >> Use char instead of u8. It's always been a pain that struct v4l2_capability
> >> used u8 instead of char for the character arrays. I never understood why, and
> >> if you are making a new struct I would recommend using char.
> >>
> >> Up to you, though.
> >>
> >>> +};
> >>> +\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.
> >>> +
> >>> +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
> >>
> >> Actually, only m.userptr can cover a large area. The other two are limited.
> >> See my comment about USERPTR support below.
> >>
> >>> +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 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}
> >>> +
> >>> +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
> >>> +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;
> >>> + 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 offset;
> >>> + u64 len;
> >>> +};
> >>> +\end{lstlisting}
> >>> +
> >>> +\begin{description}
> >>> +\item[\field{offset}] 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}
> >>> +
> >>> +\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 offset;
> >>> +};
> >>> +\end{lstlisting}
> >>> +
> >>> +\begin{description}
> >>> +\item[\field{offset}] 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.
> >>> +
> >>> +\subparagraph{MMAP}
> >>> +
> >>> +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{USERPTR}
> >>> +
> >>> +In virtio-media, \textit{USERPTR} buffers are provisioned by the driver, 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 device must not alter the pointer values provided by the driver, i.e.
> >>> +\field{the 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.
> >>
> >> I very, very strongly recommend that you drop USERPTR support for virtio.
> >> MMAP and DMABUF are sufficient.
> >>
> >> It's a pain to support, and we discourage it for new drivers.
> >>
> >> If memory serves, there is at least one chromeos driver that is stuck to
> >> USERPTR support for some reason (I can't remember the details), and that is
> >> (I think) the only reason USERPTR support is part of virtio. But that is
> >> really a chromeos issue that they need to fix, IMHO.
> >
> > I am wondering if the concerns about USERPTR should apply to the
> > host/guest context.
> >
> > In virtio-media, the USERPTR memory type just means that the buffer
> > memory has been allocated by the guest and the addresses passed are
> > guest physical addresses. It doesn't necessarily mean that the memory
> > has been allocated with the USERPTR type by the guest user-space.
>
> Ah, OK. But then USERPTR is a really confusing name for people with a
> V4L2 background.
>
> Regardless, this needs to be explained better.
We can certainly do that and use a different name, as Albert suggested
we do for DMABUF. I agree it can be confusing,
>
> > For instance, say the guest user-space allocated memory using memfd,
> > and is passing the FDs to the guest driver as DMABUF buffers. From the
> > guest kernel point of view, these buffers will still resolve into
> > guest physical memory, so the USERPTR memory type will be used by
> > guest/host communication to indicate that fact. I don't know of any
> > formal way to resolve guest DMABUFs into proper virtio resources
> > unfortunately.
> >
> > Without the ability to communicate buffers as guest physical memory,
> > the only kinds of DMABUFs supported by the guest driver would be those
> > coming from another virtio driver, like virtio-gpu. Granted, in
> > practice that's probably where these will come from anyway, but I
> > think it would be nice to be able to support memfd, at least for
> > testing purposes.
> >
> > But to answer what I think your concern is, if you prefer that the
> > guest driver does not advertise support for USERPTR memory type to the
> > guest userspace, we can certainly enforce that (either absolutely or
> > through an option).
>
> I think I would like that.
Acknowledged - memfd + DMABUF can cover the same use-cases anyway.
Just for my erudition, is the reason for eschewing USERPTR documented
somewhere?
Thanks,
Alex.
>
> Regards,
>
> Hans
>
> >
> > Cheers,
> > Alex.
> >
> >>
> >> Regards,
> >>
> >> Hans
> >>
> >>> +
> >>> +\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.
> >>> +
> >>> +Device-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 device 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
> >>
>
