Documentation/driver-model/devres.txt - arm/linux - Git at Google

 Devres - Managed Device Resource
 ================================

 Tejun Heo	<teheo@suse.de>

 First draft	10 January 2007


 1. Intro			: Huh? Devres?
 2. Devres			: Devres in a nutshell
 3. Devres Group			: Group devres'es and release them together
 4. Details			: Life time rules, calling context, ...
 5. Overhead			: How much do we have to pay for this?
 6. List of managed interfaces	: Currently implemented managed interfaces


   1. Intro
   --------

 devres came up while trying to convert libata to use iomap.  Each
 iomapped address should be kept and unmapped on driver detach.  For
 example, a plain SFF ATA controller (that is, good old PCI IDE) in
 native mode makes use of 5 PCI BARs and all of them should be
 maintained.

 As with many other device drivers, libata low level drivers have
 sufficient bugs in ->remove and ->probe failure path.  Well, yes,
 that's probably because libata low level driver developers are lazy
 bunch, but aren't all low level driver developers?  After spending a
 day fiddling with braindamaged hardware with no document or
 braindamaged document, if it's finally working, well, it's working.

 For one reason or another, low level drivers don't receive as much
 attention or testing as core code, and bugs on driver detach or
 initialization failure don't happen often enough to be noticeable.
 Init failure path is worse because it's much less travelled while
 needs to handle multiple entry points.

 So, many low level drivers end up leaking resources on driver detach
 and having half broken failure path implementation in ->probe() which
 would leak resources or even cause oops when failure occurs.  iomap
 adds more to this mix.  So do msi and msix.


   2. Devres
   ---------

 devres is basically linked list of arbitrarily sized memory areas
 associated with a struct device.  Each devres entry is associated with
 a release function.  A devres can be released in several ways.  No
 matter what, all devres entries are released on driver detach.  On
 release, the associated release function is invoked and then the
 devres entry is freed.

 Managed interface is created for resources commonly used by device
 drivers using devres.  For example, coherent DMA memory is acquired
 using dma_alloc_coherent().  The managed version is called
 dmam_alloc_coherent().  It is identical to dma_alloc_coherent() except
 for the DMA memory allocated using it is managed and will be
 automatically released on driver detach.  Implementation looks like
 the following.

   struct dma_devres {
 	size_t		size;
 	void		*vaddr;
 	dma_addr_t	dma_handle;
   };

   static void dmam_coherent_release(struct device *dev, void *res)
   {
 	struct dma_devres *this = res;

 	dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle);
   }

   dmam_alloc_coherent(dev, size, dma_handle, gfp)
   {
 	struct dma_devres *dr;
 	void *vaddr;

 	dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp);
 	...

 	/* alloc DMA memory as usual */
 	vaddr = dma_alloc_coherent(...);
 	...

 	/* record size, vaddr, dma_handle in dr */
 	dr->vaddr = vaddr;
 	...

 	devres_add(dev, dr);

 	return vaddr;
   }

 If a driver uses dmam_alloc_coherent(), the area is guaranteed to be
 freed whether initialization fails half-way or the device gets
 detached.  If most resources are acquired using managed interface, a
 driver can have much simpler init and exit code.  Init path basically
 looks like the following.

   my_init_one()
   {
 	struct mydev *d;

 	d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL);
 	if (!d)
 		return -ENOMEM;

 	d->ring = dmam_alloc_coherent(...);
 	if (!d->ring)
 		return -ENOMEM;

 	if (check something)
 		return -EINVAL;
 	...

 	return register_to_upper_layer(d);
   }

 And exit path,

   my_remove_one()
   {
 	unregister_from_upper_layer(d);
 	shutdown_my_hardware();
   }

 As shown above, low level drivers can be simplified a lot by using
 devres.  Complexity is shifted from less maintained low level drivers
 to better maintained higher layer.  Also, as init failure path is
 shared with exit path, both can get more testing.


   3. Devres group
   ---------------

 Devres entries can be grouped using devres group.  When a group is
 released, all contained normal devres entries and properly nested
 groups are released.  One usage is to rollback series of acquired
 resources on failure.  For example,

   if (!devres_open_group(dev, NULL, GFP_KERNEL))
 	return -ENOMEM;

   acquire A;
   if (failed)
 	goto err;

   acquire B;
   if (failed)
 	goto err;
   ...

   devres_remove_group(dev, NULL);
   return 0;

  err:
   devres_release_group(dev, NULL);
   return err_code;

 As resource acquisition failure usually means probe failure, constructs
 like above are usually useful in midlayer driver (e.g. libata core
 layer) where interface function shouldn't have side effect on failure.
 For LLDs, just returning error code suffices in most cases.

 Each group is identified by void *id.  It can either be explicitly
 specified by @id argument to devres_open_group() or automatically
 created by passing NULL as @id as in the above example.  In both
 cases, devres_open_group() returns the group's id.  The returned id
 can be passed to other devres functions to select the target group.
 If NULL is given to those functions, the latest open group is
 selected.

 For example, you can do something like the following.

   int my_midlayer_create_something()
   {
 	if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL))
 		return -ENOMEM;

 	...

 	devres_close_group(dev, my_midlayer_create_something);
 	return 0;
   }

   void my_midlayer_destroy_something()
   {
 	devres_release_group(dev, my_midlayer_create_something);
   }


   4. Details
   ----------

 Lifetime of a devres entry begins on devres allocation and finishes
 when it is released or destroyed (removed and freed) - no reference
 counting.

 devres core guarantees atomicity to all basic devres operations and
 has support for single-instance devres types (atomic
 lookup-and-add-if-not-found).  Other than that, synchronizing
 concurrent accesses to allocated devres data is caller's
 responsibility.  This is usually non-issue because bus ops and
 resource allocations already do the job.

 For an example of single-instance devres type, read pcim_iomap_table()
 in lib/devres.c.

 All devres interface functions can be called without context if the
 right gfp mask is given.


   5. Overhead
   -----------

 Each devres bookkeeping info is allocated together with requested data
 area.  With debug option turned off, bookkeeping info occupies 16
 bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded
 up to ull alignment).  If singly linked list is used, it can be
 reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit).

 Each devres group occupies 8 pointers.  It can be reduced to 6 if
 singly linked list is used.

 Memory space overhead on ahci controller with two ports is between 300
 and 400 bytes on 32bit machine after naive conversion (we can
 certainly invest a bit more effort into libata core layer).


   6. List of managed interfaces
   -----------------------------

 MEM
   devm_kzalloc()
   devm_kfree()
   devm_kmemdup()
   devm_get_free_pages()
   devm_free_pages()

 IIO
   devm_iio_device_alloc()
   devm_iio_device_free()
   devm_iio_trigger_alloc()
   devm_iio_trigger_free()
   devm_iio_device_register()
   devm_iio_device_unregister()

 IO region
   devm_request_region()
   devm_request_mem_region()
   devm_release_region()
   devm_release_mem_region()

 IRQ
   devm_request_irq()
   devm_free_irq()

 DMA
   dmam_alloc_coherent()
   dmam_free_coherent()
   dmam_alloc_noncoherent()
   dmam_free_noncoherent()
   dmam_declare_coherent_memory()
   dmam_pool_create()
   dmam_pool_destroy()

 PCI
   pcim_enable_device()	: after success, all PCI ops become managed
   pcim_pin_device()	: keep PCI device enabled after release

 IOMAP
   devm_ioport_map()
   devm_ioport_unmap()
   devm_ioremap()
   devm_ioremap_nocache()
   devm_iounmap()
   devm_ioremap_resource() : checks resource, requests memory region, ioremaps
   devm_request_and_ioremap() : obsoleted by devm_ioremap_resource()
   pcim_iomap()
   pcim_iounmap()
   pcim_iomap_table()	: array of mapped addresses indexed by BAR
   pcim_iomap_regions()	: do request_region() and iomap() on multiple BARs

 REGULATOR
   devm_regulator_get()
   devm_regulator_put()
   devm_regulator_bulk_get()
   devm_regulator_register()

 CLOCK
   devm_clk_get()
   devm_clk_put()

 PINCTRL
   devm_pinctrl_get()
   devm_pinctrl_put()

 PWM
   devm_pwm_get()
   devm_pwm_put()

 PHY
   devm_usb_get_phy()
   devm_usb_put_phy()

 SLAVE DMA ENGINE
   devm_acpi_dma_controller_register()

 SPI
   devm_spi_register_master()

 GPIO
   devm_gpiod_get()
   devm_gpiod_get_index()
   devm_gpiod_get_optional()
   devm_gpiod_get_index_optional()
   devm_gpiod_put()
	Devres - Managed Device Resource
	================================

	Tejun Heo <teheo@suse.de>

	First draft 10 January 2007


	1. Intro : Huh? Devres?
	2. Devres : Devres in a nutshell
	3. Devres Group : Group devres'es and release them together
	4. Details : Life time rules, calling context, ...
	5. Overhead : How much do we have to pay for this?
	6. List of managed interfaces : Currently implemented managed interfaces


	1. Intro
	--------

	devres came up while trying to convert libata to use iomap. Each
	iomapped address should be kept and unmapped on driver detach. For
	example, a plain SFF ATA controller (that is, good old PCI IDE) in
	native mode makes use of 5 PCI BARs and all of them should be
	maintained.

	As with many other device drivers, libata low level drivers have
	sufficient bugs in ->remove and ->probe failure path. Well, yes,
	that's probably because libata low level driver developers are lazy
	bunch, but aren't all low level driver developers? After spending a
	day fiddling with braindamaged hardware with no document or
	braindamaged document, if it's finally working, well, it's working.

	For one reason or another, low level drivers don't receive as much
	attention or testing as core code, and bugs on driver detach or
	initialization failure don't happen often enough to be noticeable.
	Init failure path is worse because it's much less travelled while
	needs to handle multiple entry points.

	So, many low level drivers end up leaking resources on driver detach
	and having half broken failure path implementation in ->probe() which
	would leak resources or even cause oops when failure occurs. iomap
	adds more to this mix. So do msi and msix.


	2. Devres
	---------

	devres is basically linked list of arbitrarily sized memory areas
	associated with a struct device. Each devres entry is associated with
	a release function. A devres can be released in several ways. No
	matter what, all devres entries are released on driver detach. On
	release, the associated release function is invoked and then the
	devres entry is freed.

	Managed interface is created for resources commonly used by device
	drivers using devres. For example, coherent DMA memory is acquired
	using dma_alloc_coherent(). The managed version is called
	dmam_alloc_coherent(). It is identical to dma_alloc_coherent() except
	for the DMA memory allocated using it is managed and will be
	automatically released on driver detach. Implementation looks like
	the following.

	struct dma_devres {
	size_t size;
	void *vaddr;
	dma_addr_t dma_handle;
	};

	static void dmam_coherent_release(struct device dev, void res)
	{
	struct dma_devres *this = res;

	dma_free_coherent(dev, this->size, this->vaddr, this->dma_handle);
	}

	dmam_alloc_coherent(dev, size, dma_handle, gfp)
	{
	struct dma_devres *dr;
	void *vaddr;

	dr = devres_alloc(dmam_coherent_release, sizeof(*dr), gfp);
	...

	/* alloc DMA memory as usual */
	vaddr = dma_alloc_coherent(...);
	...

	/* record size, vaddr, dma_handle in dr */
	dr->vaddr = vaddr;
	...

	devres_add(dev, dr);

	return vaddr;
	}

	If a driver uses dmam_alloc_coherent(), the area is guaranteed to be
	freed whether initialization fails half-way or the device gets
	detached. If most resources are acquired using managed interface, a
	driver can have much simpler init and exit code. Init path basically
	looks like the following.

	my_init_one()
	{
	struct mydev *d;

	d = devm_kzalloc(dev, sizeof(*d), GFP_KERNEL);
	if (!d)
	return -ENOMEM;

	d->ring = dmam_alloc_coherent(...);
	if (!d->ring)
	return -ENOMEM;

	if (check something)
	return -EINVAL;
	...

	return register_to_upper_layer(d);
	}

	And exit path,

	my_remove_one()
	{
	unregister_from_upper_layer(d);
	shutdown_my_hardware();
	}

	As shown above, low level drivers can be simplified a lot by using
	devres. Complexity is shifted from less maintained low level drivers
	to better maintained higher layer. Also, as init failure path is
	shared with exit path, both can get more testing.


	3. Devres group
	---------------

	Devres entries can be grouped using devres group. When a group is
	released, all contained normal devres entries and properly nested
	groups are released. One usage is to rollback series of acquired
	resources on failure. For example,

	if (!devres_open_group(dev, NULL, GFP_KERNEL))
	return -ENOMEM;

	acquire A;
	if (failed)
	goto err;

	acquire B;
	if (failed)
	goto err;
	...

	devres_remove_group(dev, NULL);
	return 0;

	err:
	devres_release_group(dev, NULL);
	return err_code;

	As resource acquisition failure usually means probe failure, constructs
	like above are usually useful in midlayer driver (e.g. libata core
	layer) where interface function shouldn't have side effect on failure.
	For LLDs, just returning error code suffices in most cases.

	Each group is identified by void *id. It can either be explicitly
	specified by @id argument to devres_open_group() or automatically
	created by passing NULL as @id as in the above example. In both
	cases, devres_open_group() returns the group's id. The returned id
	can be passed to other devres functions to select the target group.
	If NULL is given to those functions, the latest open group is
	selected.

	For example, you can do something like the following.

	int my_midlayer_create_something()
	{
	if (!devres_open_group(dev, my_midlayer_create_something, GFP_KERNEL))
	return -ENOMEM;

	...

	devres_close_group(dev, my_midlayer_create_something);
	return 0;
	}

	void my_midlayer_destroy_something()
	{
	devres_release_group(dev, my_midlayer_create_something);
	}


	4. Details
	----------

	Lifetime of a devres entry begins on devres allocation and finishes
	when it is released or destroyed (removed and freed) - no reference
	counting.

	devres core guarantees atomicity to all basic devres operations and
	has support for single-instance devres types (atomic
	lookup-and-add-if-not-found). Other than that, synchronizing
	concurrent accesses to allocated devres data is caller's
	responsibility. This is usually non-issue because bus ops and
	resource allocations already do the job.

	For an example of single-instance devres type, read pcim_iomap_table()
	in lib/devres.c.

	All devres interface functions can be called without context if the
	right gfp mask is given.


	5. Overhead
	-----------

	Each devres bookkeeping info is allocated together with requested data
	area. With debug option turned off, bookkeeping info occupies 16
	bytes on 32bit machines and 24 bytes on 64bit (three pointers rounded
	up to ull alignment). If singly linked list is used, it can be
	reduced to two pointers (8 bytes on 32bit, 16 bytes on 64bit).

	Each devres group occupies 8 pointers. It can be reduced to 6 if
	singly linked list is used.

	Memory space overhead on ahci controller with two ports is between 300
	and 400 bytes on 32bit machine after naive conversion (we can
	certainly invest a bit more effort into libata core layer).


	6. List of managed interfaces
	-----------------------------

	MEM
	devm_kzalloc()
	devm_kfree()
	devm_kmemdup()
	devm_get_free_pages()
	devm_free_pages()

	IIO
	devm_iio_device_alloc()
	devm_iio_device_free()
	devm_iio_trigger_alloc()
	devm_iio_trigger_free()
	devm_iio_device_register()
	devm_iio_device_unregister()

	IO region
	devm_request_region()
	devm_request_mem_region()
	devm_release_region()
	devm_release_mem_region()

	IRQ
	devm_request_irq()
	devm_free_irq()

	DMA
	dmam_alloc_coherent()
	dmam_free_coherent()
	dmam_alloc_noncoherent()
	dmam_free_noncoherent()
	dmam_declare_coherent_memory()
	dmam_pool_create()
	dmam_pool_destroy()

	PCI
	pcim_enable_device() : after success, all PCI ops become managed
	pcim_pin_device() : keep PCI device enabled after release

	IOMAP
	devm_ioport_map()
	devm_ioport_unmap()
	devm_ioremap()
	devm_ioremap_nocache()
	devm_iounmap()
	devm_ioremap_resource() : checks resource, requests memory region, ioremaps
	devm_request_and_ioremap() : obsoleted by devm_ioremap_resource()
	pcim_iomap()
	pcim_iounmap()
	pcim_iomap_table() : array of mapped addresses indexed by BAR
	pcim_iomap_regions() : do request_region() and iomap() on multiple BARs

	REGULATOR
	devm_regulator_get()
	devm_regulator_put()
	devm_regulator_bulk_get()
	devm_regulator_register()

	CLOCK
	devm_clk_get()
	devm_clk_put()

	PINCTRL
	devm_pinctrl_get()
	devm_pinctrl_put()

	PWM
	devm_pwm_get()
	devm_pwm_put()

	PHY
	devm_usb_get_phy()
	devm_usb_put_phy()

	SLAVE DMA ENGINE
	devm_acpi_dma_controller_register()

	SPI
	devm_spi_register_master()

	GPIO
	devm_gpiod_get()
	devm_gpiod_get_index()
	devm_gpiod_get_optional()
	devm_gpiod_get_index_optional()
	devm_gpiod_put()