linux display子系统流程分析

linux display软件总体框图

对于linux diplay软件框架的分析，我认为，应该带着如下几个疑问去阅读代码：

1. /dev/fb, /dev/dri/card, dev/dri/controlD, dev/dri/renderD这些设备是如何生成的？
2. DTS中定义的有关ARM HDLCD与HDMI的信息，在内核代码中是如何被使用到的？
3. /dev/fb设备与ARM HDLCD控制器硬件是如何关联的？
4. ARM HDLCD控制器是如何与HDMI芯片TD998X关联起来？
5. HDMI芯片是如何通过I2C初始化的？
6. 一帧YUV转换后得到的RGB数据，是如何通过fb到达crtc，再到encoder, connector，最后，通过HDMI线到达显示器，并且在显示器上面显示出来的？
7. 为什么DRM框架会取代以往内核中非常普遍使用的Frame Buffer这种框架的？DRM到底有什么好处？
8. DRM框架中用到了component机制，这个机制是干什么用的？
9. DRM框架中KMS、GEM这些概念又是干什么用的？
10. DRM框架是如何对GPU硬件进行支持的？

需要重点理解的核心数据结构

struct drm_device

struct drm_driver

struct drm_master

struct drm_framebuffer

struct drm_plane

struct drm_crtc

​​​​​​​struct drm_encoder

const struct drm_encoder_funcs *funcs;

const struct drm_encoder_helper_funcs *helper_private;

 xxx_helper_funcs里面通常是一些比较common的函数，而xxx_funs通常是一些与特定硬件相关的函数，因此，xxx_heler_funcs可能会调用xxx_funcs中的函数

​​​​​​​struct drm_connector

static const struct drm_connector_helper_funcs  tda998x_connector_helper_funcs = {

               .get_modes = tda998x_connector_get_modes,

               .best_encoder = tda998x_connector_best_encoder,

};

static const struct drm_connector_funcs tda998x_connector_funcs = {

               .reset = drm_atomic_helper_connector_reset,

               .fill_modes = drm_helper_probe_single_connector_modes,

               .detect = tda998x_connector_detect,  //检测是否HDMI插入，读取寄存器cec_read(priv, REG_CEC_RXSHPDLEV) 的bit1是否为1

               .destroy = tda998x_connector_destroy,

               .atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,

               .atomic_destroy_state = drm_atomic_helper_connector_destroy_state,

};

​​​​​​​struct drm_client_funcs

static const struct drm_client_funcs drm_fbdev_client_funcs = {

               .owner                 = THIS_MODULE,

               .unregister          = drm_fbdev_client_unregister,

               .restore = drm_fbdev_client_restore,

               .hotplug               = drm_fbdev_client_hotplug,

};

hdlcd_drm_bind

               -> drm_fbdev_generic_setup(drm, 32)

                              -> drm_client_init(dev, &fb_helper->client, "fbdev", &drm_fbdev_client_funcs)

                              -> drm_fbdev_client_hotplug(&fb_helper->client)

                                             -> drm_fb_helper_init(dev, fb_helper, 0)

                                                            -> dev->fb_helper = helper

                                             -> drm_fb_helper_initial_config(fb_helper, fb_helper->preferred_bpp)

                                                            -> __drm_fb_helper_initial_config_and_unlock(fb_helper, bpp_sel)  //启动时，第一次初始化

                                                                           ->drm_client_modeset_probe(&fb_helper->client, width, height)  //这里读取EDID

                                                                           -> drm_fb_helper_single_fb_probe(fb_helper, bpp_sel)

​​​​​​​​​​​​​​struct drm_fb_helper_funcs

static const struct drm_fb_helper_funcs drm_fb_cma_helper_funcs = {

               .fb_probe = drm_fbdev_cma_create,

};

display相关的dts定义

hdlcd_0

hdlcd@7ff50000 {

                              compatible = "arm,hdlcd";

                              reg = <0 0x7ff50000 0 0x1000>;

                              interrupts = <GIC_SPI 93 IRQ_TYPE_LEVEL_HIGH>;

                              clocks = <&scmi_clk 3>;

                              clock-names = "pxlclk";

                              port {

                                             hdlcd1_output: hdlcd1-endpoint {

                                                            remote-endpoint = <&tda998x_1_input>;

                                             };

                              };

               };

​​​​​​​hdlcd_1

hdlcd@7ff60000 {

                              compatible = "arm,hdlcd";

                              reg = <0 0x7ff60000 0 0x1000>;

                              interrupts = <GIC_SPI 85 IRQ_TYPE_LEVEL_HIGH>;

                              clocks = <&scmi_clk 3>;

                              clock-names = "pxlclk";

                              port {

                                             hdlcd0_output: hdlcd0-endpoint {

                                                            remote-endpoint = <&tda998x_0_input>;

                                             };

                              };

               };

​​​​​​​tda998(hdmi0和hdmi1)

i2c@7ffa0000 {

                              compatible = "snps,designware-i2c";

                              reg = <0x0 0x7ffa0000 0x0 0x1000>;  //I2C控制器基地址

                              #address-cells = <1>;

                              #size-cells = <0>;

                              interrupts = <GIC_SPI 104 IRQ_TYPE_LEVEL_HIGH>;

                              clock-frequency = <400000>;  //i2c速率400K

                              i2c-sda-hold-time-ns = <500>;

                              clocks = <&soc_smc50mhz>;

hdmi_transmitter0: hdmi-transmitter@70 {

                                             compatible = "nxp,tda998x";

                                             reg = <0x70>;  //I2C从设备地址

                                             #sound-dai-cells = <1>;

                                             audio-ports = <TDA998x_I2S 0x03

                                                                           TDA998x_SPDIF 0x04>;

                                             port {

                                                            tda998x_0_input: tda998x-0-endpoint {

                                                                           remote-endpoint = <&hdlcd0_output>; 

                                                            };

                                             };

                              };

hdmi_transmitter1: hdmi-transmitter@71 {

                                             compatible = "nxp,tda998x";

                                             reg = <0x71>;  //I2C从设备地址

                                             port {

                                                            tda998x_1_input: tda998x-1-endpoint {

                                                                           remote-endpoint = <&hdlcd1_output>;

                                                            };

                                             };

                              };

               };

arm hdlcd驱动分析

ARM HDLCD是在多个ARM开发平台上用到的显示控制器，由 ARM Ltd 提供。

HDLCD从帧缓冲区读取RGB数据并将其发送到数字编码器（DVI 或 HDMI）。

​​​​​​​hdlcd控制器硬件说明

如下是juno文档中对HDLCD控制器的描述：

​​​​​​​hdlcd控制器寄存器定义

//drivers/gpu/drm/arm/hdlcd_regs.h

/* register offsets */

#define HDLCD_REG_VERSION                  0x0000  /* ro */

#define HDLCD_REG_INT_RAWSTAT                       0x0010  /* rw */

#define HDLCD_REG_INT_CLEAR                              0x0014  /* wo */

#define HDLCD_REG_INT_MASK                0x0018  /* rw */

#define HDLCD_REG_INT_STATUS                           0x001c  /* ro */

#define HDLCD_REG_FB_BASE                  0x0100  /* rw */

#define HDLCD_REG_FB_LINE_LENGTH   0x0104  /* rw */

#define HDLCD_REG_FB_LINE_COUNT                   0x0108  /* rw */

#define HDLCD_REG_FB_LINE_PITCH                      0x010c  /* rw */

#define HDLCD_REG_BUS_OPTIONS                        0x0110  /* rw */

#define HDLCD_REG_V_SYNC                     0x0200  /* rw */

#define HDLCD_REG_V_BACK_PORCH                    0x0204  /* rw */

#define HDLCD_REG_V_DATA                    0x0208  /* rw */

#define HDLCD_REG_V_FRONT_PORCH                 0x020c  /* rw */

#define HDLCD_REG_H_SYNC                    0x0210  /* rw */

#define HDLCD_REG_H_BACK_PORCH                    0x0214  /* rw */

#define HDLCD_REG_H_DATA                    0x0218  /* rw */

#define HDLCD_REG_H_FRONT_PORCH                 0x021c  /* rw */

#define HDLCD_REG_POLARITIES                             0x0220  /* rw */

#define HDLCD_REG_COMMAND                             0x0230  /* rw */

#define HDLCD_REG_PIXEL_FORMAT                      0x0240  /* rw */

#define HDLCD_REG_RED_SELECT                           0x0244  /* rw */

#define HDLCD_REG_GREEN_SELECT                      0x0248  /* rw */

#define HDLCD_REG_BLUE_SELECT                         0x024c  /* rw */

/* version */

#define HDLCD_PRODUCT_ID                    0x1CDC0000

#define HDLCD_PRODUCT_MASK                             0xFFFF0000

#define HDLCD_VERSION_MAJOR_MASK               0x0000FF00

#define HDLCD_VERSION_MINOR_MASK               0x000000FF

​​​​​​​hdlcd_probe

drivers/gpu/drm/arm/hdlcd_drv.c

static struct platform_driver  hdlcd_platform_driver = {

               .probe                  = hdlcd_probe,

               .remove               = hdlcd_remove,

               .driver   = {

                              .name = "hdlcd",

                              .pm = &hdlcd_pm_ops,

                              .of_match_table               = hdlcd_of_match,

               },

};

static const struct of_device_id  hdlcd_of_match[] = {

               { .compatible      = "arm,hdlcd" },

               {},

};

hdlcd_probe

               ->of_graph_get_next_endpoint(pdev->dev.of_node, NULL)

                              ->port = of_get_child_by_name(parent, "port")   //查找port结点

                              ->endpoint = of_get_next_child(port, prev) //查找port中的endpoint结点

               ->port = of_graph_get_remote_port_parent(ep)

                              ->np = of_parse_phandle(node, "remote-endpoint", 0)  //解析得到"remote-endpoint"属性值：tda998x_1_input

               ->component_match_add(&pdev->dev, &match, compare_dev, port)

                              ->component_match_add_release(master, matchptr, NULL, compare, compare_data)

                                             -> match = devres_alloc(devm_component_match_release, sizeof(*match), GFP_KERNEL)

                                             -> match->compare[match->num].compare = compare

               ->component_master_add_with_match(&pdev->dev, &hdlcd_master_ops, match)  //hdlcd_master_ops定义在下面，里面有bind()接口

                              -> try_to_bring_up_master(master, NULL)

compare_dev定义如下：

static int compare_dev(struct device *dev, void *data)

{

               return dev->of_node == data;

}

hdlcd_master_ops定义在下面，里面有bind()接口

static const struct component_master_ops hdlcd_master_ops = {

               .bind                     = hdlcd_drm_bind,

               .unbind                 = hdlcd_drm_unbind,

};

​​​​​​​hdlcd_drm_bind

static const struct  component_master_ops    hdlcd_master_ops = {

               .bind                    = hdlcd_drm_bind,

               .unbind                 = hdlcd_drm_unbind,

};

hdlcd_drm_bind

               ->drm_dev_alloc(&hdlcd_driver, dev)  // hdlcd_driver定义如下

               ->hdlcd_setup_mode_config(drm);

                              ->drm->mode_config.funcs = &hdlcd_mode_config_funcs  //定义在下面

               ->hdlcd_load(drm, 0);

                              ->hdlcd->mmio = devm_ioremap_resource(drm->dev, res)

                              ->version = hdlcd_read(hdlcd, HDLCD_REG_VERSION)  //读hdlcd version寄存器

                              ->of_reserved_mem_device_init(drm->dev)

                              ->hdlcd_setup_crtc(drm)

                                             ->hdlcd_plane_init(drm)

                                                            ->drm_plane_helper_add(plane, &hdlcd_plane_helper_funcs) // 里面有一个atomic_update()接口，会配置hdlcd的FB BASE寄存器

                                                                           -> hdlcd_write(hdlcd, HDLCD_REG_FB_BASE, scanout_start)

                                             -> drm_crtc_init_with_planes(drm, &hdlcd->crtc, primary, NULL, &hdlcd_crtc_funcs, NULL)

                                             ->drm_crtc_helper_add(&hdlcd->crtc, &hdlcd_crtc_helper_funcs)

                              ->drm_irq_install(drm, platform_get_irq(pdev, 0))

                                             ->request_irq(irq, dev->driver->irq_handler, sh_flags, dev->driver->name, dev)

->drm_dev_register(drm, 0);  //注册drm字符设备

               ->drm_minor_register(dev, DRM_MINOR_CONTROL);

->drm_minor_register(dev, DRM_MINOR_RENDER);

               ->drm_minor_register(dev, DRM_MINOR_PRIMARY);

               ->dev->driver->load(dev, flags)  // hdlcd_driver中没有定义load()接口，所以这里为空

               ->drm_modeset_register_all(dev)

                               ->drm_plane_register_all(dev); //调用各自的 late_register()接口

                               ->drm_crtc_register_all(dev);

                               ->drm_encoder_register_all(dev);  //遍历dev->mode_config.encoder_list