Message-ID: <84367fd2-9ff9-bdef-adaf-16a2fc96c11b@xs4all.nl>
Date:   Wed, 27 Mar 2019 14:55:49 +0100
From:   Hans Verkuil <hverkuil@...all.nl>
To:     shuah <shuah@...nel.org>, mchehab@...nel.org, perex@...ex.cz,
        tiwai@...e.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        alsa-devel@...a-project.org
Subject: Re: [PATCH v12 1/5] media: Media Device Allocator API

On 3/27/19 2:45 PM, shuah wrote:
> Hi Hans,
> 
> On 3/27/19 4:37 AM, Hans Verkuil wrote:
>> Hi Shuah,
>>
>> Some comments at the end. I suspect you still have to do more work :-(
> 
> I am okay with doing more work. I am expecting it since, I am not happy
> with the config options that are in play. More on this below.
> 
>>
>> On 3/27/19 3:24 AM, 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.
>>>
>>> Signed-off-by: Shuah Khan <shuah@...nel.org>
>>> ---
>>>   Documentation/media/kapi/mc-core.rst |  41 ++++++++
>>>   drivers/media/Makefile               |   6 ++
>>>   drivers/media/media-dev-allocator.c  | 142 +++++++++++++++++++++++++++
>>>   include/media/media-dev-allocator.h  |  54 ++++++++++
>>>   4 files changed, 243 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 f930725e0d6b..05bba0b61748 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..20b488067ec2 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 ($(CONFIG_USB),y)
>>> +	ifeq ($(CONFIG_MODULES),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..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@...nel.org>
>>> + *
>>> + * Credits: Suggested by Laurent Pinchart <laurent.pinchart@...asonboard.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;
>>> +}
>>> +
>>> +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..69aee9a4b4c1
>>> --- /dev/null
>>> +++ b/include/media/media-dev-allocator.h
>>> @@ -0,0 +1,54 @@
>>> +/* SPDX-License-Identifier: GPL-2.0+ */
>>> +/*
>>> + * media-dev-allocator.h - Media Controller Device Allocator API
>>> + *
>>> + * Copyright (c) 2018 Shuah Khan <shuah@...nel.org>
>>> + *
>>> + * Credits: Suggested by Laurent Pinchart <laurent.pinchart@...asonboard.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) && \
>>> +	defined(CONFIG_MODULES)
>>> +/**
>>> + * 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 && CONFIG_USB && CONFIG_MODULES */
>>
>> I am still getting sparse warnings:
>>
>> drivers/media/media-dev-allocator.c /home/hans/work/build/media-git/drivers/media/media-dev-allocator.c:94:21:  warning: symbol 'media_device_usb_allocate' was not declared. Should it be static?
>> drivers/media/media-dev-allocator.c /home/hans/work/build/media-git/drivers/media/media-dev-allocator.c:120:6:  warning: symbol 'media_device_delete' was not declared. Should it be static?
>> drivers/media/media-dev-allocator.c:94:22: warning: no previous prototype for 'media_device_usb_allocate' [-Wmissing-prototypes]
>> drivers/media/media-dev-allocator.c:120:6: warning: no previous prototype for 'media_device_delete' [-Wmissing-prototypes]
> 
> I didn't see the warns - maybe I have to upgrade to the latest gcc.

Ah, media-dev-allocator.c doesn't include media-dev-allocator.h.

I suspect that that's the reason for the sparse warning.

> 
>>
>> This makes sense since if the #if above evaluates to 0, then the
>> static inlines are used. But in media-dev-allocator.c these functions
>> are still present without a corresponding prototype.
>>
>> You likely need a similar #if in media-dev-allocator.c.
>>
>> Have you tested this if CONFIG_MODULES is not set? Does au0828 still work
>> w.r.t. sharing the shared resource? Has the driver been tested when MEDIA_CONTROLLER
>> is not set? Should au0828 select MEDIA_CONTROLLER explicitly perhaps?
> 
> Unfortunately without MEDIA_CONTROLLER and CONFIG_MODULES, no sharing
> happening. The only way to ensure sharing in all cases, is selecting
> MEDIA_CONTROLLER and MODULES in au0828 and that will make sure
> snd-usb-audio enables SND_USB_AUDIO_USE_MEDIA_CONTROLLER and all of
> the sharing logic works.

You can select MEDIA_CONTROLLER for au0828, but never MODULES. Selecting
MEDIA_CONTROLLER makes sense, I have no objection to that.

What do you mean with 'no sharing happening'? You have to deal with
modules to make sure they are not unloaded, but other than that this
feature shouldn't depend on MODULES. Or am I missing something?

> 
>>
>> My main concern is that it looks like au0828's probe() will now fail if MODULES is
>> not set.
>>
> 
> Agreed. At the moment, we have too many config options in play even
> before this work. Maybe it is time au0828 selected MEDIA_CONTROLLER
> and CONFIG_MODULES - this will make sure all of this works.
> 
> I was hoping to start the config option discussion, so we can decide on
> a set of options to select from these drivers.

Regards,

	Hans

> 
> thanks,
> -- Shuah
> 

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