Message-Id: <20250717-multicontext-mainline-2025-v1-14-81ac18979c03@ideasonboard.com>
Date: Thu, 17 Jul 2025 12:45:40 +0200
From: Jacopo Mondi <jacopo.mondi@...asonboard.com>
To: Sakari Ailus <sakari.ailus@...ux.intel.com>, 
 Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
 Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, 
 Kieran Bingham <kieran.bingham@...asonboard.com>, 
 Nicolas Dufresne <nicolas.dufresne@...labora.com>, 
 Mauro Carvalho Chehab <mchehab@...nel.org>, 
 Tomasz Figa <tfiga@...omium.org>, 
 Marek Szyprowski <m.szyprowski@...sung.com>, 
 Raspberry Pi Kernel Maintenance <kernel-list@...pberrypi.com>, 
 Florian Fainelli <florian.fainelli@...adcom.com>, 
 Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>, 
 Hans Verkuil <hverkuil@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, 
 linux-rpi-kernel@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
 Jacopo Mondi <jacopo.mondi@...asonboard.com>
Subject: [PATCH 14/26] media: Documentation: Add VIDIOC_SUBDEV_BIND_CONTEXT

Document the newly introduced VIDIOC_SUBDEV_BIND_CONTEXT ioctl.

Signed-off-by: Jacopo Mondi <jacopo.mondi@...asonboard.com>
---
 .../userspace-api/media/v4l/user-func.rst          |  1 +
 .../media/v4l/vidioc-subdev-bind-context.rst       | 81 ++++++++++++++++++++++
 2 files changed, 82 insertions(+)

diff --git a/Documentation/userspace-api/media/v4l/user-func.rst b/Documentation/userspace-api/media/v4l/user-func.rst
index 0d9aff56ab653b2a4f6afe4828f88bc5637addf1..236847a1d6cb2a266bea30ecf7583979099b0343 100644
--- a/Documentation/userspace-api/media/v4l/user-func.rst
+++ b/Documentation/userspace-api/media/v4l/user-func.rst
@@ -66,6 +66,7 @@ Function Reference
     vidioc-remove-bufs
     vidioc-s-hw-freq-seek
     vidioc-streamon
+    vidioc-subdev-bind-context
     vidioc-subdev-enum-frame-interval
     vidioc-subdev-enum-frame-size
     vidioc-subdev-enum-mbus-code
diff --git a/Documentation/userspace-api/media/v4l/vidioc-subdev-bind-context.rst b/Documentation/userspace-api/media/v4l/vidioc-subdev-bind-context.rst
new file mode 100644
index 0000000000000000000000000000000000000000..5cba529a3cdcb63c7257f871d667fa792c0ca382
--- /dev/null
+++ b/Documentation/userspace-api/media/v4l/vidioc-subdev-bind-context.rst
@@ -0,0 +1,81 @@
+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: V4L
+
+.. _vidioc_subdev_bind_context:
+
+********************************
+ioctl VIDIOC_SUBDEV_BIND_CONTEXT
+********************************
+
+Name
+====
+
+VIDIOC_SUBDEV_BIND_CONTEXT - Bind a subdevice file handle to a media device
+context
+
+Synopsis
+========
+
+.. c:macro:: VIDIOC_SUBDEV_BIND_CONTEXT
+
+``int ioctl(int fd, VIDIOC_SUBDEV_BIND_CONTEXT, struct v4l2_subdev_bind_context *argp)``
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``argp``
+    Pointer to struct :c:type:`v4l2_subdev_bind_context`.
+
+Description
+===========
+
+Applications call the ``VIDIOC_SUBDEV_BIND_CONTEXT`` ioctl to bind a subdevice
+file handle to a media device  context. Binding a subdevice file handle to a
+media device context creates an isolated execution context which allows to
+multiplex the usage of a video device. This means, in practice, that the
+subdevice configuration (format, sizes etc) applied on a file handle bound to a
+media device context won't be visible on file handles bound to a different media
+device context (or not bound at all).
+
+By opening a media device applications create a media device context to which
+video devices and subdevices file handles can be bound to. The file descriptor
+returned by a call to :c:func:`open()` on the media device identifies uniquely
+the media device context. Application populates the ``context_fd`` field of
+:c:type:`v4l2_subdev_bind_context` with the file descriptor of an open media
+device to identify the media context to which they want to bind a subdevice
+to.
+
+Applications can open a subdevice node multiple times, and call
+``VIDIOC_BIND_CONTEXT`` on each file handle returned by a successful call to
+:c:func:`open()` to isolate the operations performed on that file handle from
+any operation performed on other file handles bound to different contexts. This
+means, in example, that the subdevice format and sizes are isolated from the
+ones associated with a file descriptor, obtained by opening the same subdevice
+but bound to a different media device context (or not bound at all).
+
+The bounding operation realizes a permanent association valid until the
+subdevice context is released by closing the file handle.
+
+A subdevice file handle can be bound to the same media device context once
+only. Trying to bind the same file handle to the same media device context a
+second time, without releasing the already established context by closing the
+bound file descriptor first, will result in an error.
+
+Bounding is an opt-in feature that applications are free to ignore. Any
+operation directed to a non bound file handle will continue to work as it used
+to, and the video device configuration (formats, sizes etc) will be visible
+across all the other non-bound file handles.
+
+Return Value
+============
+
+On success 0 is returned, on error -1 and the ``errno`` variable is set
+appropriately. The generic error codes are described at the
+:ref:`Generic Error Codes <gen-errors>` chapter.
+
+EINVAL
+    The media device context file handle ``context_fd`` is not valid or the
+    subdevice file handle is already bound to a context.

-- 
2.49.0

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