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    SubjectRe: [PATCH v10 1/4] media: Media Device Allocator API
    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

    --
    Regards,

    Sakari Ailus

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