linux-kernel - [PATCH 10/26] media: v4l2-dev: Add video_device_context_from

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Message-Id: <20250717-multicontext-mainline-2025-v1-10-81ac18979c03@ideasonboard.com>
Date: Thu, 17 Jul 2025 12:45:36 +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 10/26] media: v4l2-dev: Add
 video_device_context_from_queue()

Introduce an helper function to get a video_device_context from
a videobuf2 queue.

Device drivers that support multi-context operations will receive in
their vb2_ops callbacks implementation a vb2_queue argument that is
embedded in a video device context, either a context created by binding
the video device to a media device context or the video device default
context. This helper allows those drivers to retrieve the context
the vb2_queue is embedded with.

As the intended callers of this helpers are vb2_ops callbacks
implementation, called by the videobuf2 core in response to an operation
on a file handle, the returned context is guaranteed to be valid for
the whole duration of the callback.

Signed-off-by: Jacopo Mondi <jacopo.mondi@...asonboard.com>
---
 include/media/v4l2-dev.h | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/include/media/v4l2-dev.h b/include/media/v4l2-dev.h
index a41afb81663bc9cb3bfc06dcf9b11ceeaf7c3415..bab4b13b109362bec84d8d16440b6ea895206b60 100644
--- a/include/media/v4l2-dev.h
+++ b/include/media/v4l2-dev.h
@@ -819,6 +819,42 @@ void video_device_cleanup_context(struct video_device_context *ctx);
 struct video_device_context *
 video_device_context_from_file(struct file *filp, struct video_device *vdev);
 
+/**
+ * video_device_context_from_queue - Get a video device context associated with
+ *				     a vb2 queue
+ *
+ * @q: the videobuf2 queue
+ *
+ * Return the video device context this videobuf2 queue belongs to.
+ *
+ * Device drivers that support multi-context operations will always be provided
+ * with a device context to work with, either a context created by userspace
+ * by binding the video device to a media device context or the video device
+ * default context. The videobuf2 queue that context-aware drivers operates will
+ * always be contained in either one of those contexts.
+ *
+ * This function should be used by driver-specific callbacks implementation of
+ * the vb2_ops which receive a videobuf2 as argument. Being the vb2_ops callback
+ * called by the videobuf2 core in response to a userspace operation on an open
+ * file handle, callers of this function are guaranteed to always receive a
+ * valid context reference by this function. The reference will remain valid for
+ * the whole function scope of the vb2_ops callback that has received the
+ * videobuf2 queue as argument. Likewise, accessing the media device context
+ * from the video device context returned by this function is always safe within
+ * the same function scope.
+ *
+ * Drivers that do not support multi-context operations shall never use
+ * this function.
+ *
+ * This function does not increase the reference count of the returned video
+ * device context.
+ */
+static inline struct video_device_context *
+video_device_context_from_queue(struct vb2_queue *q)
+{
+	return container_of(q, struct video_device_context, queue);
+}
+
 #endif /* CONFIG_MEDIA_CONTROLLER */
 
 #endif /* _V4L2_DEV_H */

-- 
2.49.0