1. Intro¶

devres came up while trying to convert libata to use iomap. Eachiomapped address should be kept and unmapped on driver detach. Forexample, a plain SFF ATA controller (that is, good old PCI IDE) innative mode makes use of 5 PCI BARs and all of them should bemaintained.

As with many other device drivers, libata low level drivers havesufficient bugs in ->remove and ->probe failure path. Well, yes,that’s probably because libata low level driver developers are lazybunch, but aren’t all low level driver developers? After spending aday fiddling with braindamaged hardware with no document orbraindamaged document, if it’s finally working, well, it’s working.

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

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

2. Devres¶

devres is basically linked list of arbitrarily sized memory areasassociated with astructdevice. Each devres entry is associated witha release function. A devres can be released in several ways. Nomatter what, all devres entries are released on driver detach. Onrelease, the associated release function is invoked and then thedevres entry is freed.

Managed interface is created for resources commonly used by devicedrivers using devres. For example, coherent DMA memory is acquiredusingdma_alloc_coherent(). The managed version is calleddmam_alloc_coherent(). It is identical todma_alloc_coherent() exceptfor the DMA memory allocated using it is managed and will beautomatically released on driver detach. Implementation looks likethe 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 usesdmam_alloc_coherent(), the area is guaranteed to befreed whether initialization fails half-way or the device getsdetached. If most resources are acquired using managed interface, adriver can have much simpler init and exit code. Init path basicallylooks 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 usingdevres. Complexity is shifted from less maintained low level driversto better maintained higher layer. Also, as init failure path isshared with exit path, both can get more testing.

Note though that when converting current calls or assignments tomanaged devm_* versions it is up to you to check if internal operationslike allocating memory, have failed. Managed resources pertains to thefreeing of these resourcesonly - all other checks needed are stillon you. In some cases this may mean introducing checks that were notnecessary before moving to the managed devm_* calls.

3. Devres group¶

Devres entries can be grouped using devres group. When a group isreleased, all contained normal devres entries and properly nestedgroups are released. One usage is to rollback series of acquiredresources 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, constructslike above are usually useful in midlayer driver (e.g. libata corelayer) where interface function shouldn’t have side effect on failure.For LLDs, just returning error code suffices in most cases.

Each group is identified byvoid *id. It can either be explicitlyspecified by @id argument todevres_open_group() or automaticallycreated by passing NULL as @id as in the above example. In bothcases,devres_open_group() returns the group’s id. The returned idcan be passed to other devres functions to select the target group.If NULL is given to those functions, the latest open group isselected.

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 finisheswhen it is released or destroyed (removed and freed) - no referencecounting.

devres core guarantees atomicity to all basic devres operations andhas support for single-instance devres types (atomiclookup-and-add-if-not-found). Other than that, synchronizingconcurrent accesses to allocated devres data is caller’sresponsibility. This is usually non-issue because bus ops andresource allocations already do the job.

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

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

5. Overhead¶

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

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

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

6. List of managed interfaces¶

CLOCK

devm_clk_get()devm_clk_get_optional()devm_clk_put()devm_clk_bulk_get()devm_clk_bulk_get_all()devm_clk_bulk_get_optional()devm_get_clk_from_child()devm_clk_hw_register()devm_of_clk_add_hw_provider()devm_clk_hw_register_clkdev()

DMA

dmaenginem_async_device_register()dmam_alloc_coherent()dmam_alloc_attrs()dmam_free_coherent()dmam_pool_create()dmam_pool_destroy()

DRM

devm_drm_dev_alloc()

GPIO

devm_gpiod_get()devm_gpiod_get_array()devm_gpiod_get_array_optional()devm_gpiod_get_index()devm_gpiod_get_index_optional()devm_gpiod_get_optional()devm_gpiod_put()devm_gpiod_unhinge()devm_gpiochip_add_data()devm_gpio_request_one()

I2C

devm_i2c_add_adapter()devm_i2c_new_dummy_device()

IIO

devm_iio_device_alloc()devm_iio_device_register()devm_iio_dmaengine_buffer_setup()devm_iio_kfifo_buffer_setup()devm_iio_kfifo_buffer_setup_ext()devm_iio_map_array_register()devm_iio_triggered_buffer_setup()devm_iio_triggered_buffer_setup_ext()devm_iio_trigger_alloc()devm_iio_trigger_register()devm_iio_channel_get()devm_iio_channel_get_all()devm_iio_hw_consumer_alloc()devm_fwnode_iio_channel_get_by_name()

INPUT

devm_input_allocate_device()

IO region

devm_release_mem_region()devm_release_region()devm_release_resource()devm_request_mem_region()devm_request_free_mem_region()devm_request_region()devm_request_resource()

IOMAP

devm_ioport_map()devm_ioport_unmap()devm_ioremap()devm_ioremap_uc()devm_ioremap_wc()devm_ioremap_resource() : checks resource, requests memory region, ioremapsdevm_ioremap_resource_wc()devm_platform_ioremap_resource() : callsdevm_ioremap_resource() for platform devicedevm_platform_ioremap_resource_byname()devm_platform_get_and_ioremap_resource()devm_iounmap()

Note: For the PCI devices the specific pcim_*() functions may be used, see below.

IRQ

devm_free_irq()devm_request_any_context_irq()devm_request_irq()devm_request_threaded_irq()devm_irq_alloc_descs()devm_irq_alloc_desc()devm_irq_alloc_desc_at()devm_irq_alloc_desc_from()devm_irq_alloc_descs_from()devm_irq_alloc_generic_chip()devm_irq_setup_generic_chip()devm_irq_domain_create_sim()

LED

devm_led_classdev_register()devm_led_classdev_register_ext()devm_led_classdev_unregister()devm_led_trigger_register()devm_of_led_get()

MDIO

devm_mdiobus_alloc()devm_mdiobus_alloc_size()devm_mdiobus_register()devm_of_mdiobus_register()

MEM

devm_free_pages()devm_get_free_pages()devm_kasprintf()devm_kcalloc()devm_kfree()devm_kmalloc()devm_kmalloc_array()devm_kmemdup()devm_krealloc()devm_krealloc_array()devm_kstrdup()devm_kstrdup_const()devm_kvasprintf()devm_kzalloc()

MFD

devm_mfd_add_devices()

MUX

devm_mux_chip_alloc()devm_mux_chip_register()devm_mux_control_get()devm_mux_state_get()

NET

devm_alloc_etherdev()devm_alloc_etherdev_mqs()devm_register_netdev()

PER-CPU MEM

devm_alloc_percpu()

PCI

devm_pci_alloc_host_bridge() : managed PCI host bridge allocationdevm_pci_remap_cfgspace() : ioremap PCI configuration spacedevm_pci_remap_cfg_resource() : ioremap PCI configuration space resource

pcim_enable_device() : after success, the PCI device gets disabled automatically on driver detachpcim_iomap() : doiomap() on a single BARpcim_iomap_regions() : dorequest_region() andiomap() on multiple BARspcim_iomap_table() : array of mapped addresses indexed by BARpcim_iounmap() : doiounmap() on a single BARpcim_pin_device() : keep PCI device enabled after releasepcim_set_mwi() : enable Memory-Write-Invalidate PCI transaction

PHY

devm_usb_get_phy()devm_usb_get_phy_by_node()devm_usb_get_phy_by_phandle()

PINCTRL

devm_pinctrl_get()devm_pinctrl_put()devm_pinctrl_get_select()devm_pinctrl_register()devm_pinctrl_register_and_init()devm_pinctrl_unregister()

POWER

devm_reboot_mode_register()devm_reboot_mode_unregister()

PWM

devm_pwmchip_alloc()devm_pwmchip_add()devm_pwm_get()devm_fwnode_pwm_get()

REGULATOR

devm_regulator_bulk_register_supply_alias()devm_regulator_bulk_get()devm_regulator_bulk_get_const()devm_regulator_bulk_get_enable()devm_regulator_bulk_put()devm_regulator_get()devm_regulator_get_enable()devm_regulator_get_enable_read_voltage()devm_regulator_get_enable_optional()devm_regulator_get_exclusive()devm_regulator_get_optional()devm_regulator_irq_helper()devm_regulator_put()devm_regulator_register()devm_regulator_register_notifier()devm_regulator_register_supply_alias()devm_regulator_unregister_notifier()

RESET

devm_reset_control_get()devm_reset_controller_register()

RTC

devm_rtc_device_register()devm_rtc_allocate_device()devm_rtc_register_device()devm_rtc_nvmem_register()

SERDEV

devm_serdev_device_open()

SLAVE DMA ENGINE

devm_acpi_dma_controller_register()

SPI

devm_spi_alloc_host()devm_spi_alloc_target()devm_spi_optimize_message()devm_spi_register_controller()devm_spi_register_host()devm_spi_register_target()

WATCHDOG

devm_watchdog_register_device()