LinuxTV Patchwork [v10,1/4] media: Media Device Allocator API

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Submitter Shuah Khan
Date Jan. 24, 2019, 8:32 p.m.
Message ID <df3aae79629e5c0ab5a57dfaf5e2d815a134cd2d.1548360791.git.shuah@kernel.org>
Download mbox | patch
Permalink /patch/54159/
State New
Delegated to: Hans Verkuil
Headers show

Comments

Shuah Khan - Jan. 24, 2019, 8:32 p.m.
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               |   4 +
 drivers/media/media-dev-allocator.c  | 144 +++++++++++++++++++++++++++
 include/media/media-dev-allocator.h  |  53 ++++++++++
 4 files changed, 242 insertions(+)
 create mode 100644 drivers/media/media-dev-allocator.c
 create mode 100644 include/media/media-dev-allocator.h
Sakari Ailus - Jan. 25, 2019, 3:38 p.m.
Hi Shuah,

On Thu, Jan 24, 2019 at 01:32:38PM -0700, Shuah Khan wrote:
> 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.

Are there real, non-USB devices that could use the same API?

> 
> Signed-off-by: Shuah Khan <shuah@kernel.org>
> ---
>  Documentation/media/kapi/mc-core.rst |  41 ++++++++
>  drivers/media/Makefile               |   4 +
>  drivers/media/media-dev-allocator.c  | 144 +++++++++++++++++++++++++++
>  include/media/media-dev-allocator.h  |  53 ++++++++++
>  4 files changed, 242 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..4606456c1e86
> --- /dev/null
> +++ b/drivers/media/media-dev-allocator.c
> @@ -0,0 +1,144 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later

GPL-2.0+

> +/*
> + * media-dev-allocator.c - Media Controller Device Allocator API
> + *
> + * Copyright (c) 2018 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>
> +
> +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);

This is a problem, albeit not really more of a problem than it is in a
driver. The refcounting changes can be made here instead. I'll take this
into account in the media device refcounting series I'm planning to start
working on again; would you be perhaps able to help testing with this
device once I have patches in that shape? I have no access to the hardware.

> +
> +	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;
> +}
> +
> +#if IS_ENABLED(CONFIG_USB)

You already compile the file only if CONFIG_USB is enabled. I think you
could remove this.

> +struct media_device *media_device_usb_allocate(struct usb_device *udev,
> +					       const char *module_name)

I'd like to suggest working based on usb_interface instead of usb_device
here: that object already exists and you can find out the device based on
it. It seems all callers of this function already have the usb_interface
around.

> +{
> +	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);
> +#endif
> +
> +void media_device_delete(struct media_device *mdev, const char *module_name)

Same here. The use of the module name seems a bit hackish to me, albeit I
suppose it'd work, too.

> +{
> +	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@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_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
Shuah Khan - Jan. 26, 2019, 12:27 a.m.
Hi Sakari,

On 1/25/19 8:38 AM, Sakari Ailus wrote:
> Hi Shuah,
> 
> On Thu, Jan 24, 2019 at 01:32:38PM -0700, Shuah Khan wrote:
>> 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.
> 
> Are there real, non-USB devices that could use the same API?
> 

There might be. I don't have any to test. This patch is restricted
to USB at the moment.

>>
>> Signed-off-by: Shuah Khan <shuah@kernel.org>
>> ---
>>   Documentation/media/kapi/mc-core.rst |  41 ++++++++
>>   drivers/media/Makefile               |   4 +
>>   drivers/media/media-dev-allocator.c  | 144 +++++++++++++++++++++++++++
>>   include/media/media-dev-allocator.h  |  53 ++++++++++
>>   4 files changed, 242 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..4606456c1e86
>> --- /dev/null
>> +++ b/drivers/media/media-dev-allocator.c
>> @@ -0,0 +1,144 @@
>> +// SPDX-License-Identifier: GPL-2.0-or-later
> 
> GPL-2.0+
> 

I would like to address this in a follow-on patch instead of
re-doing the series again. Hope that is okay.

>> +/*
>> + * media-dev-allocator.c - Media Controller Device Allocator API
>> + *
>> + * Copyright (c) 2018 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>
>> +
>> +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);
> 
> This is a problem, albeit not really more of a problem than it is in a
> driver.

Okay good to know that it can be addressed in your media device
refcounting series.

The refcounting changes can be made here instead. I'll take this
> into account in the media device refcounting series I'm planning to start
> working on again; would you be perhaps able to help testing with this
> device once I have patches in that shape? I have no access to the hardware.
> 

Absolutely. I will be happy to help you with testing on the same
hardware, I used for this series.

>> +
>> +	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;
>> +}
>> +
>> +#if IS_ENABLED(CONFIG_USB)
> 
> You already compile the file only if CONFIG_USB is enabled. I think you
> could remove this.
> 
>> +struct media_device *media_device_usb_allocate(struct usb_device *udev,
>> +					       const char *module_name)
> 
> I'd like to suggest working based on usb_interface instead of usb_device
> here: that object already exists and you can find out the device based on
> it. It seems all callers of this function already have the usb_interface
> around.
> 

Is there an advantage to using interface instead of the parent device?
Is there a problem doing it this way?


>> +{
>> +	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);
>> +#endif
>> +
>> +void media_device_delete(struct media_device *mdev, const char *module_name)
> 
> Same here. The use of the module name seems a bit hackish to me, albeit I
> suppose it'd work, too.
> 

What is hackish about it? I found it very useful to use it in debug and
error messages that are user-friendly.

thanks,
-- Shuah

Patch

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..4606456c1e86
--- /dev/null
+++ b/drivers/media/media-dev-allocator.c
@@ -0,0 +1,144 @@ 
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * media-dev-allocator.c - Media Controller Device Allocator API
+ *
+ * Copyright (c) 2018 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>
+
+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;
+}
+
+#if IS_ENABLED(CONFIG_USB)
+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);
+#endif
+
+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@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_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

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