devres: device resource management

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

initilaization failure doesn'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 acquision 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_something);

	return 0;

  }

  void my_midlayer_destroy_something()

  {

	devres_release_group(dev, my_midlayer_create_soemthing);

  }

  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/iomap.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

  -----------------------------

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()

  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

	  If you are unsure about this, say N here.

config DEBUG_DEVRES

	bool "Managed device resources verbose debug messages"

	depends on DEBUG_KERNEL

	help

	  This option enables kernel parameter devres.log. If set to

	  non-zero, devres debug messages are printed. Select this if

	  you are having a problem with devres or want to debug

	  resource management for a managed device. devres.log can be

	  switched on and off from sysfs node.

	  If you are unsure about this, Say N here.

config SYS_HYPERVISOR

	bool

	default n

obj-y			:= core.o sys.o bus.o dd.o \

			   driver.o class.o platform.o \

			   cpu.o firmware.o init.o map.o dmapool.o \

			   dma-mapping.o devres.o \

			   attribute_container.o transport_class.o

obj-y			+= power/

obj-$(CONFIG_ISA)	+= isa.o

extern char *make_class_name(const char *name, struct kobject *kobj);

extern void devres_release_all(struct device *dev);

	INIT_LIST_HEAD(&dev->dma_pools);

	INIT_LIST_HEAD(&dev->node);

	init_MUTEX(&dev->sem);

	spin_lock_init(&dev->devres_lock);

	INIT_LIST_HEAD(&dev->devres_head);

	device_init_wakeup(dev, 0);

	set_dev_node(dev, -1);

}