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Subject[PATCH v15 1/6] media: Media Device Allocator API
Date
Media Device Allocator API to allows multiple drivers share a media device.
This API solves a very common use-case for media devices where one physical
device (an USB stick) provides both audio and video. When such media device
exposes a standard USB Audio class, a proprietary Video class, two or more
independent drivers will share a single physical USB bridge. In such cases,
it is necessary to coordinate access to the shared resource.

Using this API, drivers can allocate a media device with the shared struct
device as the key. Once the media device is allocated by a driver, other
drivers can get a reference to it. The media device is released when all
the references are released.

Signed-off-by: Shuah Khan <shuah@kernel.org>
---
Documentation/media/kapi/mc-core.rst | 41 ++++++++
drivers/media/Makefile | 6 ++
drivers/media/media-dev-allocator.c | 135 +++++++++++++++++++++++++++
include/media/media-dev-allocator.h | 63 +++++++++++++
4 files changed, 245 insertions(+)
create mode 100644 drivers/media/media-dev-allocator.c
create mode 100644 include/media/media-dev-allocator.h

diff --git a/Documentation/media/kapi/mc-core.rst b/Documentation/media/kapi/mc-core.rst
index f930725e0d6b..05bba0b61748 100644
--- a/Documentation/media/kapi/mc-core.rst
+++ b/Documentation/media/kapi/mc-core.rst
@@ -259,6 +259,45 @@ Subsystems should facilitate link validation by providing subsystem specific
helper functions to provide easy access for commonly needed information, and
in the end provide a way to use driver-specific callbacks.

+Media Controller Device Allocator API
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When the media device belongs to more than one driver, the shared media
+device is allocated with the shared struct device as the key for look ups.
+
+The shared media device should stay in registered state until the last
+driver unregisters it. In addition, the media device should be released when
+all the references are released. Each driver gets a reference to the media
+device during probe, when it allocates the media device. If media device is
+already allocated, the allocate API bumps up the refcount and returns the
+existing media device. The driver puts the reference back in its disconnect
+routine when it calls :c:func:`media_device_delete()`.
+
+The media device is unregistered and cleaned up from the kref put handler to
+ensure that the media device stays in registered state until the last driver
+unregisters the media device.
+
+**Driver Usage**
+
+Drivers should use the appropriate media-core routines to manage the shared
+media device life-time handling the two states:
+1. allocate -> register -> delete
+2. get reference to already registered device -> delete
+
+call :c:func:`media_device_delete()` routine to make sure the shared media
+device delete is handled correctly.
+
+**driver probe:**
+Call :c:func:`media_device_usb_allocate()` to allocate or get a reference
+Call :c:func:`media_device_register()`, if media devnode isn't registered
+
+**driver disconnect:**
+Call :c:func:`media_device_delete()` to free the media_device. Freeing is
+handled by the kref put handler.
+
+API Definitions
+^^^^^^^^^^^^^^^
+
.. kernel-doc:: include/media/media-device.h

.. kernel-doc:: include/media/media-devnode.h
@@ -266,3 +305,5 @@ in the end provide a way to use driver-specific callbacks.
.. kernel-doc:: include/media/media-entity.h

.. kernel-doc:: include/media/media-request.h
+
+.. kernel-doc:: include/media/media-dev-allocator.h
diff --git a/drivers/media/Makefile b/drivers/media/Makefile
index 985d35ec6b29..4a330d0e5e40 100644
--- a/drivers/media/Makefile
+++ b/drivers/media/Makefile
@@ -6,6 +6,12 @@
media-objs := media-device.o media-devnode.o media-entity.o \
media-request.o

+ifeq ($(CONFIG_MEDIA_CONTROLLER),y)
+ ifeq ($(CONFIG_USB),y)
+ media-objs += media-dev-allocator.o
+ endif
+endif
+
#
# I2C drivers should come before other drivers, otherwise they'll fail
# when compiled as builtin drivers
diff --git a/drivers/media/media-dev-allocator.c b/drivers/media/media-dev-allocator.c
new file mode 100644
index 000000000000..ae17887dec59
--- /dev/null
+++ b/drivers/media/media-dev-allocator.c
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * media-dev-allocator.c - Media Controller Device Allocator API
+ *
+ * Copyright (c) 2019 Shuah Khan <shuah@kernel.org>
+ *
+ * Credits: Suggested by Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+ */
+
+/*
+ * This file adds a global refcounted Media Controller Device Instance API.
+ * A system wide global media device list is managed and each media device
+ * includes a kref count. The last put on the media device releases the media
+ * device instance.
+ *
+ */
+
+#include <linux/kref.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/usb.h>
+
+#include <media/media-device.h>
+#include <media/media-dev-allocator.h>
+
+static LIST_HEAD(media_device_list);
+static DEFINE_MUTEX(media_device_lock);
+
+struct media_device_instance {
+ struct media_device mdev;
+ struct module *owner;
+ struct list_head list;
+ struct kref refcount;
+};
+
+static inline struct media_device_instance *
+to_media_device_instance(struct media_device *mdev)
+{
+ return container_of(mdev, struct media_device_instance, mdev);
+}
+
+static void media_device_instance_release(struct kref *kref)
+{
+ struct media_device_instance *mdi =
+ container_of(kref, struct media_device_instance, refcount);
+
+ dev_dbg(mdi->mdev.dev, "%s: releasing Media Device\n", __func__);
+
+ mutex_lock(&media_device_lock);
+
+ media_device_unregister(&mdi->mdev);
+ media_device_cleanup(&mdi->mdev);
+
+ list_del(&mdi->list);
+ mutex_unlock(&media_device_lock);
+
+ kfree(mdi);
+}
+
+/* Callers should hold media_device_lock when calling this function */
+static struct media_device *__media_device_get(struct device *dev,
+ const char *module_name,
+ struct module *owner)
+{
+ struct media_device_instance *mdi;
+
+ list_for_each_entry(mdi, &media_device_list, list) {
+ if (mdi->mdev.dev != dev)
+ continue;
+
+ kref_get(&mdi->refcount);
+
+ /* get module reference for the media_device owner */
+ if (owner != mdi->owner && !try_module_get(mdi->owner))
+ dev_err(dev,
+ "%s: module %s get owner reference error\n",
+ __func__, module_name);
+ else
+ dev_dbg(dev, "%s: module %s got owner reference\n",
+ __func__, module_name);
+ return &mdi->mdev;
+ }
+
+ mdi = kzalloc(sizeof(*mdi), GFP_KERNEL);
+ if (!mdi)
+ return NULL;
+
+ mdi->owner = owner;
+ kref_init(&mdi->refcount);
+ list_add_tail(&mdi->list, &media_device_list);
+
+ dev_dbg(dev, "%s: Allocated media device for owner %s\n",
+ __func__, module_name);
+ return &mdi->mdev;
+}
+
+struct media_device *media_device_usb_allocate(struct usb_device *udev,
+ const char *module_name,
+ struct module *owner)
+{
+ struct media_device *mdev;
+
+ mutex_lock(&media_device_lock);
+ mdev = __media_device_get(&udev->dev, module_name, owner);
+ if (!mdev) {
+ mutex_unlock(&media_device_lock);
+ return ERR_PTR(-ENOMEM);
+ }
+
+ /* check if media device is already initialized */
+ if (!mdev->dev)
+ __media_device_usb_init(mdev, udev, udev->product,
+ module_name);
+ mutex_unlock(&media_device_lock);
+ return mdev;
+}
+EXPORT_SYMBOL_GPL(media_device_usb_allocate);
+
+void media_device_delete(struct media_device *mdev, const char *module_name,
+ struct module *owner)
+{
+ struct media_device_instance *mdi = to_media_device_instance(mdev);
+
+ mutex_lock(&media_device_lock);
+ /* put module reference for the media_device owner */
+ if (mdi->owner != owner) {
+ module_put(mdi->owner);
+ dev_dbg(mdi->mdev.dev,
+ "%s: module %s put owner module reference\n",
+ __func__, module_name);
+ }
+ mutex_unlock(&media_device_lock);
+ kref_put(&mdi->refcount, media_device_instance_release);
+}
+EXPORT_SYMBOL_GPL(media_device_delete);
diff --git a/include/media/media-dev-allocator.h b/include/media/media-dev-allocator.h
new file mode 100644
index 000000000000..b35ea6062596
--- /dev/null
+++ b/include/media/media-dev-allocator.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * media-dev-allocator.h - Media Controller Device Allocator API
+ *
+ * Copyright (c) 2019 Shuah Khan <shuah@kernel.org>
+ *
+ * Credits: Suggested by Laurent Pinchart <laurent.pinchart@ideasonboard.com>
+ */
+
+/*
+ * This file adds a global ref-counted Media Controller Device Instance API.
+ * A system wide global media device list is managed and each media device
+ * includes a kref count. The last put on the media device releases the media
+ * device instance.
+ */
+
+#ifndef _MEDIA_DEV_ALLOCATOR_H
+#define _MEDIA_DEV_ALLOCATOR_H
+
+struct usb_device;
+
+#if defined(CONFIG_MEDIA_CONTROLLER) && defined(CONFIG_USB)
+/**
+ * media_device_usb_allocate() - Allocate and return struct &media device
+ *
+ * @udev: struct &usb_device pointer
+ * @module_name: should be filled with %KBUILD_MODNAME
+ * @owner: struct module pointer %THIS_MODULE for the driver.
+ * %THIS_MODULE is null for a built-in driver.
+ * It is safe even when %THIS_MODULE is null.
+ *
+ * This interface should be called to allocate a Media Device when multiple
+ * drivers share usb_device and the media device. This interface allocates
+ * &media_device structure and calls media_device_usb_init() to initialize
+ * it.
+ *
+ */
+struct media_device *media_device_usb_allocate(struct usb_device *udev,
+ const char *module_name,
+ struct module *owner);
+/**
+ * media_device_delete() - Release media device. Calls kref_put().
+ *
+ * @mdev: struct &media_device pointer
+ * @module_name: should be filled with %KBUILD_MODNAME
+ * @owner: struct module pointer %THIS_MODULE for the driver.
+ * %THIS_MODULE is null for a built-in driver.
+ * It is safe even when %THIS_MODULE is null.
+ *
+ * This interface should be called to put Media Device Instance kref.
+ */
+void media_device_delete(struct media_device *mdev, const char *module_name,
+ struct module *owner);
+#else
+static inline struct media_device *media_device_usb_allocate(
+ struct usb_device *udev, const char *module_name,
+ struct module *owner)
+ { return NULL; }
+static inline void media_device_delete(
+ struct media_device *mdev, const char *module_name,
+ struct module *owner) { }
+#endif /* CONFIG_MEDIA_CONTROLLER && CONFIG_USB */
+#endif /* _MEDIA_DEV_ALLOCATOR_H */
--
2.17.1
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