[PATCH v11 1/5] media: Media Device Allocator API

[Shuah Khan]

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@xxxxxxxxxx>
---
 Documentation/media/kapi/mc-core.rst |  41 ++++++++
 drivers/media/Makefile               |   4 +
 drivers/media/media-dev-allocator.c  | 142 +++++++++++++++++++++++++++
 include/media/media-dev-allocator.h  |  53 ++++++++++
 4 files changed, 240 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 0bcfeadbc52d..07f2a6a90af2 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..1d7653318af6 100644
--- a/drivers/media/Makefile
+++ b/drivers/media/Makefile
@@ -6,6 +6,10 @@
 media-objs	:= media-device.o media-devnode.o media-entity.o \
 		   media-request.o
 
+ifeq ($(CONFIG_USB),y)
+	media-objs += media-dev-allocator.o
+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..a37515254009
--- /dev/null
+++ b/drivers/media/media-dev-allocator.c
@@ -0,0 +1,142 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * media-dev-allocator.c - Media Controller Device Allocator API
+ *
+ * Copyright (c) 2018 Shuah Khan <shuah@xxxxxxxxxx>
+ *
+ * Credits: Suggested by Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
+ */
+
+/*
+ * 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>
+
+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: mdev=%p\n", __func__, &mdi->mdev);
+
+	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 *modp)
+{
+	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 (modp != mdi->owner && !try_module_get(mdi->owner))
+			dev_err(dev, "%s: try_module_get() error\n", __func__);
+		dev_dbg(dev, "%s: get mdev=%p module_name %s\n",
+			__func__, &mdi->mdev, module_name);
+		return &mdi->mdev;
+	}
+
+	mdi = kzalloc(sizeof(*mdi), GFP_KERNEL);
+	if (!mdi)
+		return NULL;
+
+	mdi->owner = modp;
+	kref_init(&mdi->refcount);
+	list_add_tail(&mdi->list, &media_device_list);
+
+	dev_dbg(dev, "%s: alloc mdev=%p module_name %s\n", __func__,
+		&mdi->mdev, module_name);
+	return &mdi->mdev;
+}
+
+struct media_device *media_device_usb_allocate(struct usb_device *udev,
+					       const char *module_name)
+{
+	struct media_device *mdev;
+	struct module *modptr;
+
+	mutex_lock(&module_mutex);
+	modptr = find_module(module_name);
+	mutex_unlock(&module_mutex);
+
+	mutex_lock(&media_device_lock);
+	mdev = __media_device_get(&udev->dev, module_name, modptr);
+	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 media_device_instance *mdi = to_media_device_instance(mdev);
+	struct module *modptr;
+
+	dev_dbg(mdi->mdev.dev, "%s: mdev=%p module_name %s\n",
+		__func__, &mdi->mdev, module_name);
+
+	mutex_lock(&module_mutex);
+	modptr = find_module(module_name);
+	mutex_unlock(&module_mutex);
+
+	mutex_lock(&media_device_lock);
+	/* put module reference if media_device owner is not THIS_MODULE */
+	if (mdi->owner != modptr) {
+		module_put(mdi->owner);
+		dev_dbg(mdi->mdev.dev,
+			"%s decremented owner module reference\n", __func__);
+	}
+	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..9164795e911c
--- /dev/null
+++ b/include/media/media-dev-allocator.h
@@ -0,0 +1,53 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * media-dev-allocator.h - Media Controller Device Allocator API
+ *
+ * Copyright (c) 2018 Shuah Khan <shuah@xxxxxxxxxx>
+ *
+ * Credits: Suggested by Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>
+ */
+
+/*
+ * 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_ALLOCTOR_H
+#define _MEDIA_DEV_ALLOCTOR_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
+ *
+ * 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,
+					       char *module_name);
+/**
+ * media_device_delete() - Release media device. Calls kref_put().
+ *
+ * @mdev:		struct &media_device pointer
+ * @module_name:	should be filled with %KBUILD_MODNAME
+ *
+ * This interface should be called to put Media Device Instance kref.
+ */
+void media_device_delete(struct media_device *mdev, char *module_name);
+#else
+static inline struct media_device *media_device_usb_allocate(
+			struct usb_device *udev, char *module_name)
+			{ return NULL; }
+static inline void media_device_delete(
+			struct media_device *mdev, char *module_name) { }
+#endif /* CONFIG_MEDIA_CONTROLLER */
+#endif
-- 
2.17.1