devm_xxx机制

devm是内核提供的基础机制，用于方便驱动开发者所分配资源的自动回收。参考内核文档devres.txt。总的来说，就是驱动开发者只需要调用这类接口分配期望的资源，不用关心释放问题。这些资源的释放会在device对象销毁时自动释放。

devres使用
　　不同的内核模块提供了对应的devm_xxx接口，如下(不仅仅这些)：
MEM
devm_kmalloc()
devm_kzalloc()
devm_kcalloc()
devm_kmalloc_array()
devm_kstrdup()
devm_kfree()

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()