Documentation/remoteproc.txt - kernel/msm-4.9 - Gitiles

 Remote Processor Framework

 1. Introduction

 Modern SoCs typically have heterogeneous remote processor devices in asymmetric
 multiprocessing (AMP) configurations, which may be running different instances
 of operating system, whether it's Linux or any other flavor of real-time OS.

 OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP.
 In a typical configuration, the dual cortex-A9 is running Linux in a SMP
 configuration, and each of the other three cores (two M3 cores and a DSP)
 is running its own instance of RTOS in an AMP configuration.

 The remoteproc framework allows different platforms/architectures to
 control (power on, load firmware, power off) those remote processors while
 abstracting the hardware differences, so the entire driver doesn't need to be
 duplicated. In addition, this framework also adds rpmsg virtio devices
 for remote processors that supports this kind of communication. This way,
 platform-specific remoteproc drivers only need to provide a few low-level
 handlers, and then all rpmsg drivers will then just work
 (for more information about the virtio-based rpmsg bus and its drivers,
 please read Documentation/rpmsg.txt).

 2. User API

   int rproc_boot(struct rproc *rproc)
     - Boot a remote processor (i.e. load its firmware, power it on, ...).
       If the remote processor is already powered on, this function immediately
       returns (successfully).
       Returns 0 on success, and an appropriate error value otherwise.
       Note: to use this function you should already have a valid rproc
       handle. There are several ways to achieve that cleanly (devres, pdata,
       the way remoteproc_rpmsg.c does this, or, if this becomes prevalent, we
       might also consider using dev_archdata for this). See also
       rproc_get_by_name() below.

   void rproc_shutdown(struct rproc *rproc)
     - Power off a remote processor (previously booted with rproc_boot()).
       In case @rproc is still being used by an additional user(s), then
       this function will just decrement the power refcount and exit,
       without really powering off the device.
       Every call to rproc_boot() must (eventually) be accompanied by a call
       to rproc_shutdown(). Calling rproc_shutdown() redundantly is a bug.
       Notes:
       - we're not decrementing the rproc's refcount, only the power refcount.
         which means that the @rproc handle stays valid even after
         rproc_shutdown() returns, and users can still use it with a subsequent
         rproc_boot(), if needed.
       - don't call rproc_shutdown() to unroll rproc_get_by_name(), exactly
         because rproc_shutdown() _does not_ decrement the refcount of @rproc.
         To decrement the refcount of @rproc, use rproc_put() (but _only_ if
         you acquired @rproc using rproc_get_by_name()).

   struct rproc *rproc_get_by_name(const char *name)
     - Find an rproc handle using the remote processor's name, and then
       boot it. If it's already powered on, then just immediately return
       (successfully). Returns the rproc handle on success, and NULL on failure.
       This function increments the remote processor's refcount, so always
       use rproc_put() to decrement it back once rproc isn't needed anymore.
       Note: currently rproc_get_by_name() and rproc_put() are not used anymore
       by the rpmsg bus and its drivers. We need to scrutinize the use cases
       that still need them, and see if we can migrate them to use the non
       name-based boot/shutdown interface.

   void rproc_put(struct rproc *rproc)
     - Decrement @rproc's power refcount and shut it down if it reaches zero
       (essentially by just calling rproc_shutdown), and then decrement @rproc's
       validity refcount too.
       After this function returns, @rproc may _not_ be used anymore, and its
       handle should be considered invalid.
       This function should be called _iff_ the @rproc handle was grabbed by
       calling rproc_get_by_name().

 3. Typical usage

 #include <linux/remoteproc.h>

 /* in case we were given a valid 'rproc' handle */
 int dummy_rproc_example(struct rproc *my_rproc)
 {
 	int ret;

 	/* let's power on and boot our remote processor */
 	ret = rproc_boot(my_rproc);
 	if (ret) {
 		/*
 		 * something went wrong. handle it and leave.
 		 */
 	}

 	/*
 	 * our remote processor is now powered on... give it some work
 	 */

 	/* let's shut it down now */
 	rproc_shutdown(my_rproc);
 }

 4. API for implementors

   struct rproc *rproc_alloc(struct device *dev, const char *name,
 				const struct rproc_ops *ops,
 				const char *firmware, int len)
     - Allocate a new remote processor handle, but don't register
       it yet. Required parameters are the underlying device, the
       name of this remote processor, platform-specific ops handlers,
       the name of the firmware to boot this rproc with, and the
       length of private data needed by the allocating rproc driver (in bytes).

       This function should be used by rproc implementations during
       initialization of the remote processor.
       After creating an rproc handle using this function, and when ready,
       implementations should then call rproc_register() to complete
       the registration of the remote processor.
       On success, the new rproc is returned, and on failure, NULL.

       Note: _never_ directly deallocate @rproc, even if it was not registered
       yet. Instead, if you just need to unroll rproc_alloc(), use rproc_free().

   void rproc_free(struct rproc *rproc)
     - Free an rproc handle that was allocated by rproc_alloc.
       This function should _only_ be used if @rproc was only allocated,
       but not registered yet.
       If @rproc was already successfully registered (by calling
       rproc_register()), then use rproc_unregister() instead.

   int rproc_register(struct rproc *rproc)
     - Register @rproc with the remoteproc framework, after it has been
       allocated with rproc_alloc().
       This is called by the platform-specific rproc implementation, whenever
       a new remote processor device is probed.
       Returns 0 on success and an appropriate error code otherwise.
       Note: this function initiates an asynchronous firmware loading
       context, which will look for virtio devices supported by the rproc's
       firmware.
       If found, those virtio devices will be created and added, so as a result
       of registering this remote processor, additional virtio drivers might get
       probed.
       Currently, though, we only support a single RPMSG virtio vdev per remote
       processor.

   int rproc_unregister(struct rproc *rproc)
     - Unregister a remote processor, and decrement its refcount.
       If its refcount drops to zero, then @rproc will be freed. If not,
       it will be freed later once the last reference is dropped.

       This function should be called when the platform specific rproc
       implementation decides to remove the rproc device. it should
       _only_ be called if a previous invocation of rproc_register()
       has completed successfully.

       After rproc_unregister() returns, @rproc is _not_ valid anymore and
       it shouldn't be used. More specifically, don't call rproc_free()
       or try to directly free @rproc after rproc_unregister() returns;
       none of these are needed, and calling them is a bug.

       Returns 0 on success and -EINVAL if @rproc isn't valid.

 5. Implementation callbacks

 These callbacks should be provided by platform-specific remoteproc
 drivers:

 /**
  * struct rproc_ops - platform-specific device handlers
  * @start:	power on the device and boot it
  * @stop:	power off the device
  * @kick:	kick a virtqueue (virtqueue id given as a parameter)
  */
 struct rproc_ops {
 	int (*start)(struct rproc *rproc);
 	int (*stop)(struct rproc *rproc);
 	void (*kick)(struct rproc *rproc, int vqid);
 };

 Every remoteproc implementation should at least provide the ->start and ->stop
 handlers. If rpmsg functionality is also desired, then the ->kick handler
 should be provided as well.

 The ->start() handler takes an rproc handle and should then power on the
 device and boot it (use rproc->priv to access platform-specific private data).
 The boot address, in case needed, can be found in rproc->bootaddr (remoteproc
 core puts there the ELF entry point).
 On success, 0 should be returned, and on failure, an appropriate error code.

 The ->stop() handler takes an rproc handle and powers the device down.
 On success, 0 is returned, and on failure, an appropriate error code.

 The ->kick() handler takes an rproc handle, and an index of a virtqueue
 where new message was placed in. Implementations should interrupt the remote
 processor and let it know it has pending messages. Notifying remote processors
 the exact virtqueue index to look in is optional: it is easy (and not
 too expensive) to go through the existing virtqueues and look for new buffers
 in the used rings.

 6. Binary Firmware Structure

 At this point remoteproc only supports ELF32 firmware binaries. However,
 it is quite expected that other platforms/devices which we'd want to
 support with this framework will be based on different binary formats.

 When those use cases show up, we will have to decouple the binary format
 from the framework core, so we can support several binary formats without
 duplicating common code.

 When the firmware is parsed, its various segments are loaded to memory
 according to the specified device address (might be a physical address
 if the remote processor is accessing memory directly).

 In addition to the standard ELF segments, most remote processors would
 also include a special section which we call "the resource table".

 The resource table contains system resources that the remote processor
 requires before it should be powered on, such as allocation of physically
 contiguous memory, or iommu mapping of certain on-chip peripherals.
 Remotecore will only power up the device after all the resource table's
 requirement are met.

 In addition to system resources, the resource table may also contain
 resource entries that publish the existence of supported features
 or configurations by the remote processor, such as trace buffers and
 supported virtio devices (and their configurations).

 Currently the resource table is just an array of:

 /**
  * struct fw_resource - describes an entry from the resource section
  * @type: resource type
  * @id: index number of the resource
  * @da: device address of the resource
  * @pa: physical address of the resource
  * @len: size, in bytes, of the resource
  * @flags: properties of the resource, e.g. iommu protection required
  * @reserved: must be 0 atm
  * @name: name of resource
  */
 struct fw_resource {
 	u32 type;
 	u32 id;
 	u64 da;
 	u64 pa;
 	u32 len;
 	u32 flags;
 	u8 reserved[16];
 	u8 name[48];
 } __packed;

 Some resources entries are mere announcements, where the host is informed
 of specific remoteproc configuration. Other entries require the host to
 do something (e.g. reserve a requested resource) and possibly also reply
 by overwriting a member inside 'struct fw_resource' with info about the
 allocated resource.

 Different resource entries use different members of this struct,
 with different meanings. This is pretty limiting and error-prone,
 so the plan is to move to variable-length TLV-based resource entries,
 where each resource will begin with a type and length fields, followed by
 its own specific structure.

 Here are the resource types that are currently being used:

 /**
  * enum fw_resource_type - types of resource entries
  *
  * @RSC_CARVEOUT:   request for allocation of a physically contiguous
  *		    memory region.
  * @RSC_DEVMEM:     request to iommu_map a memory-based peripheral.
  * @RSC_TRACE:	    announces the availability of a trace buffer into which
  *		    the remote processor will be writing logs. In this case,
  *		    'da' indicates the device address where logs are written to,
  *		    and 'len' is the size of the trace buffer.
  * @RSC_VRING:	    request for allocation of a virtio vring (address should
  *		    be indicated in 'da', and 'len' should contain the number
  *		    of buffers supported by the vring).
  * @RSC_VIRTIO_DEV: announces support for a virtio device, and serves as
  *		    the virtio header. 'da' contains the virtio device
  *		    features, 'pa' holds the virtio guest features (host
  *		    will write them here after they're negotiated), 'len'
  *		    holds the virtio status, and 'flags' holds the virtio
  *		    device id (currently only VIRTIO_ID_RPMSG is supported).
  */
 enum fw_resource_type {
 	RSC_CARVEOUT	= 0,
 	RSC_DEVMEM	= 1,
 	RSC_TRACE	= 2,
 	RSC_VRING	= 3,
 	RSC_VIRTIO_DEV	= 4,
 	RSC_VIRTIO_CFG	= 5,
 };

 Most of the resource entries share the basic idea of address/length
 negotiation with the host: the firmware usually asks for memory
 of size 'len' bytes, and the host needs to allocate it and provide
 the device/physical address (when relevant) in 'da'/'pa' respectively.

 If the firmware is compiled with hard coded device addresses, and
 can't handle dynamically allocated 'da' values, then the 'da' field
 will contain the expected device addresses (today we actually only support
 this scheme, as there aren't yet any use cases for dynamically allocated
 device addresses).

 We also expect that platform-specific resource entries will show up
 at some point. When that happens, we could easily add a new RSC_PLAFORM
 type, and hand those resources to the platform-specific rproc driver to handle.

 7. Virtio and remoteproc

 The firmware should provide remoteproc information about virtio devices
 that it supports, and their configurations: a RSC_VIRTIO_DEV resource entry
 should specify the virtio device id, and subsequent RSC_VRING resource entries
 should indicate the vring size (i.e. how many buffers do they support) and
 where should they be mapped (i.e. which device address). Note: the alignment
 between the consumer and producer parts of the vring is assumed to be 4096.

 At this point we only support a single virtio rpmsg device per remote
 processor, but the plan is to remove this limitation. In addition, once we
 move to TLV-based resource table, the plan is to have a single RSC_VIRTIO
 entry per supported virtio device, which will include the virtio header,
 the vrings information and the virtio config space.

 Of course, RSC_VIRTIO resource entries are only good enough for static
 allocation of virtio devices. Dynamic allocations will also be made possible
 using the rpmsg bus (similar to how we already do dynamic allocations of
 rpmsg channels; read more about it in rpmsg.txt).
	Remote Processor Framework

	1. Introduction

	Modern SoCs typically have heterogeneous remote processor devices in asymmetric
	multiprocessing (AMP) configurations, which may be running different instances
	of operating system, whether it's Linux or any other flavor of real-time OS.

	OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP.
	In a typical configuration, the dual cortex-A9 is running Linux in a SMP
	configuration, and each of the other three cores (two M3 cores and a DSP)
	is running its own instance of RTOS in an AMP configuration.

	The remoteproc framework allows different platforms/architectures to
	control (power on, load firmware, power off) those remote processors while
	abstracting the hardware differences, so the entire driver doesn't need to be
	duplicated. In addition, this framework also adds rpmsg virtio devices
	for remote processors that supports this kind of communication. This way,
	platform-specific remoteproc drivers only need to provide a few low-level
	handlers, and then all rpmsg drivers will then just work
	(for more information about the virtio-based rpmsg bus and its drivers,
	please read Documentation/rpmsg.txt).

	2. User API

	int rproc_boot(struct rproc *rproc)
	- Boot a remote processor (i.e. load its firmware, power it on, ...).
	If the remote processor is already powered on, this function immediately
	returns (successfully).
	Returns 0 on success, and an appropriate error value otherwise.
	Note: to use this function you should already have a valid rproc
	handle. There are several ways to achieve that cleanly (devres, pdata,
	the way remoteproc_rpmsg.c does this, or, if this becomes prevalent, we
	might also consider using dev_archdata for this). See also
	rproc_get_by_name() below.

	void rproc_shutdown(struct rproc *rproc)
	- Power off a remote processor (previously booted with rproc_boot()).
	In case @rproc is still being used by an additional user(s), then
	this function will just decrement the power refcount and exit,
	without really powering off the device.
	Every call to rproc_boot() must (eventually) be accompanied by a call
	to rproc_shutdown(). Calling rproc_shutdown() redundantly is a bug.
	Notes:
	- we're not decrementing the rproc's refcount, only the power refcount.
	which means that the @rproc handle stays valid even after
	rproc_shutdown() returns, and users can still use it with a subsequent
	rproc_boot(), if needed.
	- don't call rproc_shutdown() to unroll rproc_get_by_name(), exactly
	because rproc_shutdown() _does not_ decrement the refcount of @rproc.
	To decrement the refcount of @rproc, use rproc_put() (but _only_ if
	you acquired @rproc using rproc_get_by_name()).

	struct rproc rproc_get_by_name(const char name)
	- Find an rproc handle using the remote processor's name, and then
	boot it. If it's already powered on, then just immediately return
	(successfully). Returns the rproc handle on success, and NULL on failure.
	This function increments the remote processor's refcount, so always
	use rproc_put() to decrement it back once rproc isn't needed anymore.
	Note: currently rproc_get_by_name() and rproc_put() are not used anymore
	by the rpmsg bus and its drivers. We need to scrutinize the use cases
	that still need them, and see if we can migrate them to use the non
	name-based boot/shutdown interface.

	void rproc_put(struct rproc *rproc)
	- Decrement @rproc's power refcount and shut it down if it reaches zero
	(essentially by just calling rproc_shutdown), and then decrement @rproc's
	validity refcount too.
	After this function returns, @rproc may _not_ be used anymore, and its
	handle should be considered invalid.
	This function should be called _iff_ the @rproc handle was grabbed by
	calling rproc_get_by_name().

	3. Typical usage

	#include <linux/remoteproc.h>

	/* in case we were given a valid 'rproc' handle */
	int dummy_rproc_example(struct rproc *my_rproc)
	{
	int ret;

	/* let's power on and boot our remote processor */
	ret = rproc_boot(my_rproc);
	if (ret) {
	/*
	* something went wrong. handle it and leave.
	*/
	}

	/*
	* our remote processor is now powered on... give it some work
	*/

	/* let's shut it down now */
	rproc_shutdown(my_rproc);
	}

	4. API for implementors

	struct rproc rproc_alloc(struct device dev, const char *name,
	const struct rproc_ops *ops,
	const char *firmware, int len)
	- Allocate a new remote processor handle, but don't register
	it yet. Required parameters are the underlying device, the
	name of this remote processor, platform-specific ops handlers,
	the name of the firmware to boot this rproc with, and the
	length of private data needed by the allocating rproc driver (in bytes).

	This function should be used by rproc implementations during
	initialization of the remote processor.
	After creating an rproc handle using this function, and when ready,
	implementations should then call rproc_register() to complete
	the registration of the remote processor.
	On success, the new rproc is returned, and on failure, NULL.

	Note: _never_ directly deallocate @rproc, even if it was not registered
	yet. Instead, if you just need to unroll rproc_alloc(), use rproc_free().

	void rproc_free(struct rproc *rproc)
	- Free an rproc handle that was allocated by rproc_alloc.
	This function should _only_ be used if @rproc was only allocated,
	but not registered yet.
	If @rproc was already successfully registered (by calling
	rproc_register()), then use rproc_unregister() instead.

	int rproc_register(struct rproc *rproc)
	- Register @rproc with the remoteproc framework, after it has been
	allocated with rproc_alloc().
	This is called by the platform-specific rproc implementation, whenever
	a new remote processor device is probed.
	Returns 0 on success and an appropriate error code otherwise.
	Note: this function initiates an asynchronous firmware loading
	context, which will look for virtio devices supported by the rproc's
	firmware.
	If found, those virtio devices will be created and added, so as a result
	of registering this remote processor, additional virtio drivers might get
	probed.
	Currently, though, we only support a single RPMSG virtio vdev per remote
	processor.

	int rproc_unregister(struct rproc *rproc)
	- Unregister a remote processor, and decrement its refcount.
	If its refcount drops to zero, then @rproc will be freed. If not,
	it will be freed later once the last reference is dropped.

	This function should be called when the platform specific rproc
	implementation decides to remove the rproc device. it should
	_only_ be called if a previous invocation of rproc_register()
	has completed successfully.

	After rproc_unregister() returns, @rproc is _not_ valid anymore and
	it shouldn't be used. More specifically, don't call rproc_free()
	or try to directly free @rproc after rproc_unregister() returns;
	none of these are needed, and calling them is a bug.

	Returns 0 on success and -EINVAL if @rproc isn't valid.

	5. Implementation callbacks

	These callbacks should be provided by platform-specific remoteproc
	drivers:

	/**
	* struct rproc_ops - platform-specific device handlers
	* @start: power on the device and boot it
	* @stop: power off the device
	* @kick: kick a virtqueue (virtqueue id given as a parameter)
	*/
	struct rproc_ops {
	int (start)(struct rproc rproc);
	int (stop)(struct rproc rproc);
	void (kick)(struct rproc rproc, int vqid);
	};

	Every remoteproc implementation should at least provide the ->start and ->stop
	handlers. If rpmsg functionality is also desired, then the ->kick handler
	should be provided as well.

	The ->start() handler takes an rproc handle and should then power on the
	device and boot it (use rproc->priv to access platform-specific private data).
	The boot address, in case needed, can be found in rproc->bootaddr (remoteproc
	core puts there the ELF entry point).
	On success, 0 should be returned, and on failure, an appropriate error code.

	The ->stop() handler takes an rproc handle and powers the device down.
	On success, 0 is returned, and on failure, an appropriate error code.

	The ->kick() handler takes an rproc handle, and an index of a virtqueue
	where new message was placed in. Implementations should interrupt the remote
	processor and let it know it has pending messages. Notifying remote processors
	the exact virtqueue index to look in is optional: it is easy (and not
	too expensive) to go through the existing virtqueues and look for new buffers
	in the used rings.

	6. Binary Firmware Structure

	At this point remoteproc only supports ELF32 firmware binaries. However,
	it is quite expected that other platforms/devices which we'd want to
	support with this framework will be based on different binary formats.

	When those use cases show up, we will have to decouple the binary format
	from the framework core, so we can support several binary formats without
	duplicating common code.

	When the firmware is parsed, its various segments are loaded to memory
	according to the specified device address (might be a physical address
	if the remote processor is accessing memory directly).

	In addition to the standard ELF segments, most remote processors would
	also include a special section which we call "the resource table".

	The resource table contains system resources that the remote processor
	requires before it should be powered on, such as allocation of physically
	contiguous memory, or iommu mapping of certain on-chip peripherals.
	Remotecore will only power up the device after all the resource table's
	requirement are met.

	In addition to system resources, the resource table may also contain
	resource entries that publish the existence of supported features
	or configurations by the remote processor, such as trace buffers and
	supported virtio devices (and their configurations).

	Currently the resource table is just an array of:

	/**
	* struct fw_resource - describes an entry from the resource section
	* @type: resource type
	* @id: index number of the resource
	* @da: device address of the resource
	* @pa: physical address of the resource
	* @len: size, in bytes, of the resource
	* @flags: properties of the resource, e.g. iommu protection required
	* @reserved: must be 0 atm
	* @name: name of resource
	*/
	struct fw_resource {
	u32 type;
	u32 id;
	u64 da;
	u64 pa;
	u32 len;
	u32 flags;
	u8 reserved[16];
	u8 name[48];
	} __packed;

	Some resources entries are mere announcements, where the host is informed
	of specific remoteproc configuration. Other entries require the host to
	do something (e.g. reserve a requested resource) and possibly also reply
	by overwriting a member inside 'struct fw_resource' with info about the
	allocated resource.

	Different resource entries use different members of this struct,
	with different meanings. This is pretty limiting and error-prone,
	so the plan is to move to variable-length TLV-based resource entries,
	where each resource will begin with a type and length fields, followed by
	its own specific structure.

	Here are the resource types that are currently being used:

	/**
	* enum fw_resource_type - types of resource entries
	*
	* @RSC_CARVEOUT: request for allocation of a physically contiguous
	* memory region.
	* @RSC_DEVMEM: request to iommu_map a memory-based peripheral.
	* @RSC_TRACE: announces the availability of a trace buffer into which
	* the remote processor will be writing logs. In this case,
	* 'da' indicates the device address where logs are written to,
	* and 'len' is the size of the trace buffer.
	* @RSC_VRING: request for allocation of a virtio vring (address should
	* be indicated in 'da', and 'len' should contain the number
	* of buffers supported by the vring).
	* @RSC_VIRTIO_DEV: announces support for a virtio device, and serves as
	* the virtio header. 'da' contains the virtio device
	* features, 'pa' holds the virtio guest features (host
	* will write them here after they're negotiated), 'len'
	* holds the virtio status, and 'flags' holds the virtio
	* device id (currently only VIRTIO_ID_RPMSG is supported).
	*/
	enum fw_resource_type {
	RSC_CARVEOUT = 0,
	RSC_DEVMEM = 1,
	RSC_TRACE = 2,
	RSC_VRING = 3,
	RSC_VIRTIO_DEV = 4,
	RSC_VIRTIO_CFG = 5,
	};

	Most of the resource entries share the basic idea of address/length
	negotiation with the host: the firmware usually asks for memory
	of size 'len' bytes, and the host needs to allocate it and provide
	the device/physical address (when relevant) in 'da'/'pa' respectively.

	If the firmware is compiled with hard coded device addresses, and
	can't handle dynamically allocated 'da' values, then the 'da' field
	will contain the expected device addresses (today we actually only support
	this scheme, as there aren't yet any use cases for dynamically allocated
	device addresses).

	We also expect that platform-specific resource entries will show up
	at some point. When that happens, we could easily add a new RSC_PLAFORM
	type, and hand those resources to the platform-specific rproc driver to handle.

	7. Virtio and remoteproc

	The firmware should provide remoteproc information about virtio devices
	that it supports, and their configurations: a RSC_VIRTIO_DEV resource entry
	should specify the virtio device id, and subsequent RSC_VRING resource entries
	should indicate the vring size (i.e. how many buffers do they support) and
	where should they be mapped (i.e. which device address). Note: the alignment
	between the consumer and producer parts of the vring is assumed to be 4096.

	At this point we only support a single virtio rpmsg device per remote
	processor, but the plan is to remove this limitation. In addition, once we
	move to TLV-based resource table, the plan is to have a single RSC_VIRTIO
	entry per supported virtio device, which will include the virtio header,
	the vrings information and the virtio config space.

	Of course, RSC_VIRTIO resource entries are only good enough for static
	allocation of virtio devices. Dynamic allocations will also be made possible
	using the rpmsg bus (similar to how we already do dynamic allocations of
	rpmsg channels; read more about it in rpmsg.txt).