Re: [PATCH 1/3] media: Media Device Allocator API

[Hans Verkuil]

Hi Shuah,

Some comments below:

On 05/13/2016 07:09 PM, Shuah Khan wrote:
> Media Device Allocator API to allows multiple drivers share a media device.
> 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 <shuahkh@xxxxxxxxxxxxxxx>
> ---
>  drivers/media/Makefile              |   3 +-
>  drivers/media/media-dev-allocator.c | 139 ++++++++++++++++++++++++++++++++++++
>  include/media/media-dev-allocator.h | 118 ++++++++++++++++++++++++++++++
>  3 files changed, 259 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/media/media-dev-allocator.c
>  create mode 100644 include/media/media-dev-allocator.h
> 
> diff --git a/drivers/media/Makefile b/drivers/media/Makefile
> index e608bbc..b08f091 100644
> --- a/drivers/media/Makefile
> +++ b/drivers/media/Makefile
> @@ -2,7 +2,8 @@
>  # Makefile for the kernel multimedia device drivers.
>  #
>  
> -media-objs	:= media-device.o media-devnode.o media-entity.o
> +media-objs	:= media-device.o media-devnode.o media-entity.o \
> +		   media-dev-allocator.o
>  
>  #
>  # I2C drivers should come before other drivers, otherwise they'll fail
> diff --git a/drivers/media/media-dev-allocator.c b/drivers/media/media-dev-allocator.c
> new file mode 100644
> index 0000000..b49ab55
> --- /dev/null
> +++ b/drivers/media/media-dev-allocator.c
> @@ -0,0 +1,139 @@
> +/*
> + * media-dev-allocator.c - Media Controller Device Allocator API
> + *
> + * Copyright (c) 2016 Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
> + * Copyright (c) 2016 Samsung Electronics Co., Ltd.
> + *
> + * This file is released under the GPLv2.
> + * 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/slab.h>
> +#include <linux/kref.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 list_head list;
> +	struct device *dev;
> +	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);
> +}
> +
> +static struct media_device *__media_device_get(struct device *dev,
> +					       bool allocate)
> +{
> +	struct media_device_instance *mdi;
> +
> +	mutex_lock(&media_device_lock);
> +
> +	list_for_each_entry(mdi, &media_device_list, list) {
> +		if (mdi->dev == dev) {
> +			kref_get(&mdi->refcount);
> +			dev_dbg(dev, "%s: get mdev=%p\n",
> +				 __func__, &mdi->mdev);
> +			goto done;
> +		}
> +	}
> +
> +	if (!allocate) {
> +		mdi = NULL;
> +		goto done;
> +	}
> +
> +	mdi = kzalloc(sizeof(*mdi), GFP_KERNEL);
> +	if (!mdi)
> +		goto done;
> +
> +	mdi->dev = dev;
> +	kref_init(&mdi->refcount);
> +	list_add_tail(&mdi->list, &media_device_list);
> +
> +	dev_dbg(dev, "%s: alloc mdev=%p\n", __func__, &mdi->mdev);
> +
> +done:
> +	mutex_unlock(&media_device_lock);
> +
> +	return mdi ? &mdi->mdev : NULL;
> +}
> +
> +struct media_device *media_device_allocate(struct device *dev)
> +{
> +	dev_dbg(dev, "%s\n", __func__);
> +	return __media_device_get(dev, true);
> +}
> +EXPORT_SYMBOL_GPL(media_device_allocate);

Do we need this function? Whenever a media device is allocated, you also need
to initialize the media device with the media_device_lock held (as happens
below in media_device_usb_allocate). You can't really use this function
standalone as far as I can see.

An alternative might be to pass a callback function with media_device_allocate
that initialized the media device. That would ensure that there are no race
conditions since media_device_allocate can lock media_device_lock before
calling the callback.

> +
> +struct media_device *media_device_usb_allocate(struct usb_device *udev,
> +					       char *driver_name)
> +{
> +	struct media_device *mdev;
> +
> +	mdev = media_device_allocate(&udev->dev);
> +	if (!mdev)
> +		return ERR_PTR(-ENOMEM);
> +
> +	/* check if media device is already initialized */
> +	mutex_lock(&media_device_lock);
> +	if (!mdev->dev)
> +		__media_device_usb_init(mdev, udev, udev->product,
> +					driver_name);
> +	mutex_unlock(&media_device_lock);
> +
> +	return mdev;
> +}
> +EXPORT_SYMBOL_GPL(media_device_usb_allocate);
> +
> +/* don't allocate - get reference if one is found */
> +struct media_device *media_device_get(struct media_device *mdev)
> +{
> +	struct media_device_instance *mdi = to_media_device_instance(mdev);
> +
> +	dev_dbg(mdi->mdev.dev, "%s: mdev=%p\n", __func__, &mdi->mdev);
> +	return __media_device_get(mdi->dev, false);
> +}
> +EXPORT_SYMBOL_GPL(media_device_get);

Do we need this anywhere? It makes no sense to call this standalone, at least, not
that I can see. You just have different drivers that all call media_device_usb_allocate,
and they also call media_device_put when they are done with it. See also my comments
for the next patch.

> +
> +void media_device_put(struct media_device *mdev)
> +{
> +	struct media_device_instance *mdi = to_media_device_instance(mdev);
> +
> +	dev_dbg(mdi->mdev.dev, "%s: mdev=%p\n", __func__, &mdi->mdev);
> +	kref_put(&mdi->refcount, media_device_instance_release);
> +}
> +EXPORT_SYMBOL_GPL(media_device_put);
> diff --git a/include/media/media-dev-allocator.h b/include/media/media-dev-allocator.h
> new file mode 100644
> index 0000000..a63dfc6
> --- /dev/null
> +++ b/include/media/media-dev-allocator.h
> @@ -0,0 +1,118 @@
> +/*
> + * media-dev-allocator.h - Media Controller Device Allocator API
> + *
> + * Copyright (c) 2016 Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
> + * Copyright (c) 2016 Samsung Electronics Co., Ltd.
> + *
> + * This file is released under the GPLv2.
> + * 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;
> +
> +#ifdef CONFIG_MEDIA_CONTROLLER
> +/**
> + * DOC: Media Controller Device Allocator API
> + *
> + * When 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.
> + *
> + * Shared media device should stay in registered state until the last driver
> + * unregisters it. In addition, 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, allocate API bumps up the refcount and return the existing media
> + * device. Driver puts the reference back from its disconnect routine when it
> + * calls media_device_unregister_put().
> + *
> + * 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.
> + *
> + * Media Device can be in any the following states:
> + *
> + * - Allocated

I don't think this is a real state. It's either unregistered or allocated and
registered. Just 'allocated' doesn't really make sense. This ties in to my
comment about media_device_allocate().

> + * - Registered (could be tied to more than one driver)
> + * - Unregistered and released in the kref put handler.

I would probably drop these 5 lines altogether.

> + *
> + * Driver Usage:
> + *
> + * Drivers should use the media-core routines to get register reference and
> + * use the media_device_unregister_put() routine to make sure the shared
> + * media device delete is handled correctly.
> + *
> + * driver probe:
> + *	Call media_device_usb_allocate() to allocate or get a reference
> + *	Call media_device_register(), if media devnode isn't registered
> + *
> + * driver disconnect:
> + *	Call media_device_unregister_put() to put the reference.

There is no media_device_unregister_put() (yet). That should be in the
next patch.

> + *
> + */
> +
> +/**
> + * media_device_usb_allocate() - Allocate and return media device
> + *
> + * @udev - struct usb_device pointer
> + * @driver_name
> + *
> + * 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 and calls media_device_usb_init() to initialize it.
> + *
> + */
> +struct media_device *media_device_usb_allocate(struct usb_device *udev,
> +					       char *driver_name);
> +/**
> + * media_device_allocate() - Allocate and return global media device
> + *
> + * @dev - struct device pointer
> + *
> + * This interface should be called to allocate media device. A new media
> + * device instance is created and added to the system wide media device
> + * instance list. If media device instance exists, media_device_allocate()
> + * increments the reference count and returns the media device. When more
> + * than one driver control the media device, the first driver to probe will
> + * allocate and the second driver when it calls  media_device_allocate(),
> + * it will get a reference.
> + *
> + */
> +struct media_device *media_device_allocate(struct device *dev);
> +/**
> + * media_device_get() -	Get reference to a registered media device.
> + *
> + * @mdev - struct media_device pointer
> + *
> + * This interface should be called to get a reference to an allocated media
> + * device.
> + */
> +struct media_device *media_device_get(struct media_device *mdev);
> +/**
> + * media_device_put() - Release media device. Calls kref_put().
> + *
> + * @mdev - struct media_device pointer
> + *
> + * This interface should be called to put Media Device Instance kref.
> + */
> +void media_device_put(struct media_device *mdev);
> +#else
> +static inline struct media_device *media_device_usb_allocate(
> +			struct usb_device *udev, char *driver_name)
> +			{ return NULL; }
> +static inline struct media_device *media_device_allocate(struct device *dev)
> +			{ return NULL; }
> +static inline struct media_device *media_device_get(struct media_device *mdev)
> +			{ return NULL; }
> +static inline void media_device_put(struct media_device *mdev) { }
> +#endif /* CONFIG_MEDIA_CONTROLLER */
> +#endif
> 

Regards,

	Hans