media: Media Device Allocator API

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>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

2 files changed, 141 insertions, 0 deletions

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);