Linux為何引入devres機制

devres簡介

在驅動中經常要在初始化函數(shù)或probe函數(shù)中對設備分配一些資源，比如：irq、regulator、gpio等。在驅動進行初始化的時候如果失敗，那么通常會goto到某個地方釋放資源。有時候編寫驅動時會忘記釋放資源，Linux為了解決這個問題而引入了devres機制。devres 是一種設備資源管理機制(device resource management), 類似于一種垃圾收集處理器。而資源的處理時機是在驅動的 install / remove 時候。這樣我們在為設備分配相關資源之后, 就不必要關心如何釋放它們了。

設備資源

Linux中設備資源包含:

Power

Clock

Memory

GPIO

IRQ

DMA

虛擬地址空間

API函數(shù)

Clock

//drivers/clk/clk-devres.c
devm_clk_get()
devm_clk_put()
devm_clk_hw_register()
devm_of_clk_add_hw_provider()

DMA

//drivers/base/dma-mapping.c
dmaenginem_async_device_register()
dmam_alloc_coherent()
dmam_alloc_attrs()
dmam_declare_coherent_memory()
dmam_free_coherent()
dmam_pool_create()
dmam_pool_destroy()

GPIO

//drivers/gpio/gpiolib-devres.c
devm_gpiod_get()
devm_gpiod_get_index()
devm_gpiod_get_index_optional()
devm_gpiod_get_optional()
devm_gpiod_put()
devm_gpiochip_add_data()
devm_gpiochip_remove()
devm_gpio_request()
devm_gpio_request_one()
devm_gpio_free()

IIO

//drivers/iio/industrialio-core.c
devm_iio_device_alloc()
devm_iio_device_free()
devm_iio_device_register()
devm_iio_device_unregister()
devm_iio_kfifo_allocate()
devm_iio_kfifo_free()
devm_iio_triggered_buffer_setup()
devm_iio_triggered_buffer_cleanup()
devm_iio_trigger_alloc()
devm_iio_trigger_free()
devm_iio_trigger_register()
devm_iio_trigger_unregister()
devm_iio_channel_get()
devm_iio_channel_release()
devm_iio_channel_get_all()
devm_iio_channel_release_all()

Input

//drivers/input/input.c
devm_input_allocate_device()

IO region

//kernel/resource.c
devm_release_mem_region()
devm_release_region()
devm_release_resource()
devm_request_mem_region()
devm_request_region()
devm_request_resource()

IOMAP

//lib/devres.c
devm_ioport_map()
devm_ioport_unmap()
devm_ioremap()
devm_ioremap_nocache()
devm_ioremap_wc()
devm_ioremap_resource()
devm_iounmap()
pcim_iomap()
pcim_iomap_regions()
pcim_iomap_table()
pcim_iounmap()

IRQ

//kernel/irq/devres.c
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_sim_init()

Memory

//drivers/base/devres.c
devm_free_pages()
devm_get_free_pages()
devm_kasprintf()
devm_kcalloc()
devm_kfree()
devm_kmalloc()
devm_kmalloc_array()
devm_kmemdup()
devm_kstrdup()
devm_kvasprintf()
devm_kzalloc()

PCI

//drivers/pci/probe.c
devm_pci_alloc_host_bridge()
devm_pci_remap_cfgspace()
devm_pci_remap_cfg_resource()
pcim_enable_device()
pcim_pin_device()

Pinctrl

//drivres/pinctrl/core.c
devm_pinctrl_get()
devm_pinctrl_put()
devm_pinctrl_register()
devm_pinctrl_unregister()

Regulator

//drivers/regulator/core.c
devm_regulator_bulk_get()
devm_regulator_get()
devm_regulator_put()
devm_regulator_register()

驅動對比

非devres驅動:

static int __init soc_camera_probe(struct platform_device *pdev)
{
    ...


    res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
    irq = platform_get_irq(pdev, 0);
    if (!res || (int)irq <= 0) {
        err = -ENODEV;
       goto exit;
   }


   clk = clk_get(&pdev->dev, "csi_clk");
   if (IS_ERR(clk)) {
       err = PTR_ERR(clk);
       goto exit;
   }


   pcdev = kzalloc(sizeof(*pcdev), GFP_KERNEL);
   if (!pcdev) {
       dev_err(&pdev->dev, "Could not allocate pcdev
");
       err = -ENOMEM;
       goto exit_put_clk;
   }


   ...


   /*
    * Request the regions.
    */
   if (!request_mem_region(res->start, resource_size(res), DRIVER_NAME)) {
       err = -EBUSY;
       goto exit_kfree;
   }


   base = ioremap(res->start, resource_size(res));
   if (!base) {
       err = -ENOMEM;
       goto exit_release;
   }
   ...


   /* request dma */
   pcdev->dma_chan = imx_dma_request_by_prio(DRIVER_NAME, DMA_PRIO_HIGH);
   if (pcdev->dma_chan < 0) {
       dev_err(&pdev->dev, "Can't request DMA for MX1 CSI
");
       err = -EBUSY;
       goto exit_iounmap;
   }
   ...


   /* request irq */
   err = claim_fiq(&fh);
   if (err) {
       dev_err(&pdev->dev, "Camera interrupt register failed
");
       goto exit_free_dma;
   }


   ...
   err = soc_camera_host_register(&pcdev->soc_host);
   if (err)
       goto exit_free_irq;


   dev_info(&pdev->dev, "MX1 Camera driver loaded
");


   return 0;


exit_free_irq:
   disable_fiq(irq);
   mxc_set_irq_fiq(irq, 0);
   release_fiq(&fh);
exit_free_dma:
   imx_dma_free(pcdev->dma_chan);
exit_iounmap:
   iounmap(base);
exit_release:
   release_mem_region(res->start, resource_size(res));
exit_kfree:
   kfree(pcdev);
exit_put_clk:
   clk_put(clk);
exit:
   return err;
}

devres驅動：

static int __init soc_camera_probe(struct platform_device *pdev)
{
    ...


    res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
    irq = platform_get_irq(pdev, 0);
    if (!res || (int)irq <= 0) {
        return -ENODEV;
    }


   clk = devm_clk_get(&pdev->dev, "csi_clk");
   if (IS_ERR(clk)) {
       return PTR_ERR(clk);
   }


   pcdev = devm_kzalloc(&pdev->dev, sizeof(*pcdev), GFP_KERNEL);
   if (!pcdev) {
       dev_err(&pdev->dev, "Could not allocate pcdev
");
       return -ENOMEM;
   }


   ...


   /*
    * Request the regions.
    */
   if (!devm_request_mem_region(&pdev->dev, res->start, resource_size(res), DRIVER_NAME)) {
       return -EBUSY;
   }


   base = devm_ioremap(&pdev->dev, res->start, resource_size(res));
   if (!base) {
       return -ENOMEM;
   }
   ...


   /* request dma */
   pcdev->dma_chan = imx_dma_request_by_prio(DRIVER_NAME, DMA_PRIO_HIGH);
   if (pcdev->dma_chan < 0) {
       dev_err(&pdev->dev, "Can't request DMA for MX1 CSI
");
       return -EBUSY;
   }
   ...


   /* request irq */
   err = claim_fiq(&fh);
   if (err) {
       dev_err(&pdev->dev, "Camera interrupt register failed
");
       return err;
   }


   ...
   err = soc_camera_host_register(&pcdev->soc_host);
   if (err)
       return err;


   dev_info(&pdev->dev, "MX1 Camera driver loaded
");


   return 0;
}

通過上面的比對大概能知道這些函數(shù)的差別了。

總結

目前除了一些舊代碼之外，大部分驅動都使用devres相關的接口。也推薦大家在代碼中更多的使用相關接口，這樣代碼會更簡潔，不容易出現(xiàn)內存泄露。