前言

距上一次写文章有点时间了，今天调通了一块MIPI DSI屏幕, 特写一篇笔记置于此，希望能帮到也想研究这个MIPI DSI的朋友。

正题

博主使用的开发板为Raspbery Pi 4B,系统为Raspberry Pi OS 64Bit (full版本)。

关于系统版本

博主测试了一下各个版本的Raspberry Pi OS对于DRM驱动的兼容性，发现驱动只能在Raspberry Pi OS 11(bullseye)版本以上运行，之前的旧版本系统博主测试过了均不能使用。推测原因是因为旧版本系统使用的legacy GL driver不兼容drm，即使在raspi-config中启用了fakekms驱动也不能运行，kernel报出一长串错误。博主使用的系统版本为2022年1月28日发布的Raspberry Pi OS 64Bit正式版，经测试该系统能正常驱动屏幕。
*32位版也可以正常使用该驱动。

屏幕信息

博主使用的屏幕是一块2.8寸的IPS屏幕，MIPI接口，驱动IC是ST7701S,分辨率是480*640。
为避免广告嫌疑，这里就不放链接了。

提取屏幕信息

博主拿到屏幕后，也拿到了商家提供的初始化代码和屏幕信息，我们主要关注这两个部分:

屏幕接口定义
驱动IC datasheet(如果没有IC的datasheet，屏幕的也行)

博主这里放几张图:

我们要从这两张图中提取到关键的信息。
首先第一张图中我们可以得出来引脚定义，后面需要根据它来lay板子。
还能从第一张图中得知，我们的屏幕是1 Lane的。
第二张图中则包含了关键的初始化序列，我们需要用它来初始化我们的屏幕。
好了，基本信息都齐了，开干。

开发环境准备

打开终端，安装一下raspberrypi-kernel-headers:

sudo apt install raspberrypi-kernel-headers

P.S.如果安装的内核头文件不是您现在使用的内核的版本，那您需要自行下载符合您目前内核版本的内核头文件，或者从(内核)源码编译。如果您无法找到需要的内核头文件(换句话说，必须得从[内核]源码编译了)，请您参考博主的上一篇文章嵌入式Linux使用TFT屏幕:使用TinyDRM点亮ST7789V屏幕中的从内核源码编译章节。
然后我们建个文件夹，就取名叫w280bf036i:

mkdir w280bf036i

然后在那个文件夹中编写我们的驱动源码(panel-wlk-w280bf036i.c):

/*
** Copyright (C) 2022 CNflysky. All rights reserved.
** Kernel DRM driver for W280BF036I LCD Panel in DSI interface.
** Driver IC: ST7701
*/#include <drm/drm_mipi_dsi.h>
#include <drm/drm_modes.h>
#include <drm/drm_panel.h>
#include <drm/drm_print.h>
#include <linux/delay.h>
#include <linux/gpio/consumer.h>
#include <linux/module.h>
#include <linux/of_device.h>
#include <linux/regulator/consumer.h>
#include <video/mipi_display.h>struct w280bf036i_panel_desc {const struct drm_display_mode *mode;unsigned int lanes;unsigned long flags;enum mipi_dsi_pixel_format format;
};struct w280bf036i {struct drm_panel panel;struct mipi_dsi_device *dsi;const struct w280bf036i_panel_desc *desc;struct gpio_desc *reset;
};static inline struct w280bf036i *panel_to_w280bf036i(struct drm_panel *panel) {return container_of(panel, struct w280bf036i, panel);
}static inline int w280bf036i_dsi_write(struct w280bf036i *w280bf036i,const void *seq, size_t len) {return mipi_dsi_dcs_write_buffer(w280bf036i->dsi, seq, len);
}#define w280bf036i_command(w280bf036i, seq...)          \{                                                     \const uint8_t d[] = {seq};                          \w280bf036i_dsi_write(w280bf036i, d, ARRAY_SIZE(d)); \}static void w280bf036i_init_sequence(struct w280bf036i *w280bf036i) {// Command2 BK3 Selection: Enable the BK function of Command2w280bf036i_command(w280bf036i, 0xFF, 0x77, 0x01, 0x00, 0x00, 0x13);// Unknownw280bf036i_command(w280bf036i, 0xEF, 0x08);// Command2 BK0 Selection: Disable the BK function of Command2w280bf036i_command(w280bf036i, 0xFF, 0x77, 0x01, 0x00, 0x00, 0x10);// Display Line Settingw280bf036i_command(w280bf036i, 0xC0, 0x4f, 0x00);// Porch Controlw280bf036i_command(w280bf036i, 0xC1, 0x10, 0x0c);// Inversion selection & Frame Rate Controlw280bf036i_command(w280bf036i, 0xC2, 0x07, 0x14);// Unknownw280bf036i_command(w280bf036i, 0xCC, 0x10);// Positive Voltage Gamma Controlw280bf036i_command(w280bf036i, 0xB0, 0x0a, 0x18, 0x1e, 0x12, 0x16, 0x0c, 0x0e,0x0d, 0x0c, 0x29, 0x06, 0x14, 0x13, 0x29, 0x33, 0x1c);// Negative Voltage Gamma Controlw280bf036i_command(w280bf036i, 0xB1, 0x0a, 0x19, 0x21, 0x0a, 0x0c, 0x00, 0x0c,0x03, 0x03, 0x23, 0x01, 0x0e, 0x0c, 0x27, 0x2b, 0x1c);// Command2 BK1 Selection: Enable the BK function of Command2w280bf036i_command(w280bf036i, 0xFF, 0x77, 0x01, 0x00, 0x00, 0x11);// Vop Amplitude settingw280bf036i_command(w280bf036i, 0xB0, 0x5d);// VCOM amplitude settingw280bf036i_command(w280bf036i, 0xB1, 0x61);// VGH Voltage settingw280bf036i_command(w280bf036i, 0xB2, 0x84);// TEST Command Settingw280bf036i_command(w280bf036i, 0xB3, 0x80);// VGL Voltage settingw280bf036i_command(w280bf036i, 0xB5, 0x4d);// Power Control 1w280bf036i_command(w280bf036i, 0xB7, 0x85);// Power Control 2w280bf036i_command(w280bf036i, 0xB8, 0x20);// Source pre_drive timing set1w280bf036i_command(w280bf036i, 0xC1, 0x78);// Source EQ2 Settingw280bf036i_command(w280bf036i, 0xC2, 0x78);// MIPI Setting 1w280bf036i_command(w280bf036i, 0xD0, 0x88);// Sunlight Readable Enhancementw280bf036i_command(w280bf036i, 0xE0, 0x00, 0x00, 0x02);// Noise Reduce Controlw280bf036i_command(w280bf036i, 0xE1, 0x06, 0xa0, 0x08, 0xa0, 0x05, 0xa0, 0x07,0xa0, 0x00, 0x44, 0x44);// Sharpness Controlw280bf036i_command(w280bf036i, 0xE2, 0x20, 0x20, 0x44, 0x44, 0x96, 0xa0, 0x00,0x00, 0x96, 0xa0, 0x00, 0x00);// Color Calibration Controlw280bf036i_command(w280bf036i, 0xE3, 0x00, 0x00, 0x22, 0x22);// Skin Tone Preservation Controlw280bf036i_command(w280bf036i, 0xE4, 0x44, 0x44);w280bf036i_command(w280bf036i, 0xE5, 0x0d, 0x91, 0xa0, 0xa0, 0x0f, 0x93, 0xa0,0xa0, 0x09, 0x8d, 0xa0, 0xa0, 0x0b, 0x8f, 0xa0, 0xa0);w280bf036i_command(w280bf036i, 0xE6, 0x00, 0x00, 0x22, 0x22);w280bf036i_command(w280bf036i, 0xE7, 0x44, 0x44);w280bf036i_command(w280bf036i, 0xE8, 0x0c, 0x90, 0xa0, 0xa0, 0x0e, 0x92, 0xa0,0xa0, 0x08, 0x8c, 0xa0, 0xa0, 0x0a, 0x8e, 0xa0, 0xa0);w280bf036i_command(w280bf036i, 0xE9, 0x36, 0x00);w280bf036i_command(w280bf036i, 0xEB, 0x00, 0x01, 0xe4, 0xe4, 0x44, 0x88,0x40);w280bf036i_command(w280bf036i, 0xED, 0xff, 0x45, 0x67, 0xfa, 0x01, 0x2b, 0xcf,0xff, 0xff, 0xfc, 0xb2, 0x10, 0xaf, 0x76, 0x54, 0xff);w280bf036i_command(w280bf036i, 0xEF, 0x10, 0x0d, 0x04, 0x08, 0x3f, 0x1f);/* disable Command2 */// w280bf036i_command(w280bf036i, 0xFF, 0x77, 0x01, 0x00, 0x00, 0x00);
}static int w280bf036i_prepare(struct drm_panel *panel) {struct w280bf036i *w280bf036i = panel_to_w280bf036i(panel);gpiod_set_value(w280bf036i->reset, 0);msleep(50);gpiod_set_value(w280bf036i->reset, 1);msleep(150);mipi_dsi_dcs_soft_reset(w280bf036i->dsi);msleep(5);w280bf036i_init_sequence(w280bf036i);mipi_dsi_dcs_set_tear_on(w280bf036i->dsi, MIPI_DSI_DCS_TEAR_MODE_VBLANK);mipi_dsi_dcs_exit_sleep_mode(w280bf036i->dsi);return 0;
}static int w280bf036i_enable(struct drm_panel *panel) {return mipi_dsi_dcs_set_display_on(panel_to_w280bf036i(panel)->dsi);
}static int w280bf036i_disable(struct drm_panel *panel) {return mipi_dsi_dcs_set_display_off(panel_to_w280bf036i(panel)->dsi);
}static int w280bf036i_unprepare(struct drm_panel *panel) {struct w280bf036i *w280bf036i = panel_to_w280bf036i(panel);mipi_dsi_dcs_enter_sleep_mode(w280bf036i->dsi);gpiod_set_value(w280bf036i->reset, 0);return 0;
}static int w280bf036i_get_modes(struct drm_panel *panel,struct drm_connector *connector) {struct w280bf036i *w280bf036i = panel_to_w280bf036i(panel);const struct drm_display_mode *desc_mode = w280bf036i->desc->mode;struct drm_display_mode *mode;mode = drm_mode_duplicate(connector->dev, desc_mode);if (!mode) {dev_err(&w280bf036i->dsi->dev, "failed to add mode %ux%u@%u\n",desc_mode->hdisplay, desc_mode->vdisplay,drm_mode_vrefresh(desc_mode));return -ENOMEM;}drm_mode_set_name(mode);drm_mode_probed_add(connector, mode);connector->display_info.width_mm = desc_mode->width_mm;connector->display_info.height_mm = desc_mode->height_mm;return 1;
}static const struct drm_panel_funcs w280bf036i_funcs = {.disable = w280bf036i_disable,.unprepare = w280bf036i_unprepare,.prepare = w280bf036i_prepare,.enable = w280bf036i_enable,.get_modes = w280bf036i_get_modes,
};static const struct drm_display_mode w280bf036i_mode = {.clock = 25000,.hdisplay = 480,.hsync_start = 480 + /* HFP */ 10,.hsync_end = 480 + 10 + /* HSync */ 4,.htotal = 480 + 10 + 4 + /* HBP */ 20,.vdisplay = 640,.vsync_start = 640 + /* VFP */ 8,.vsync_end = 640 + 8 + /* VSync */ 4,.vtotal = 640 + 8 + 4 + /* VBP */ 14,.width_mm = 43,.height_mm = 57,.type = DRM_MODE_TYPE_DRIVER | DRM_MODE_TYPE_PREFERRED,
};static const struct w280bf036i_panel_desc w280bf036i_desc = {.mode = &w280bf036i_mode,.lanes = 1,.flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST,.format = MIPI_DSI_FMT_RGB888,
};static int w280bf036i_dsi_probe(struct mipi_dsi_device *dsi) {struct w280bf036i *w280bf036i =devm_kzalloc(&dsi->dev, sizeof(*w280bf036i), GFP_KERNEL);if (!w280bf036i) return -ENOMEM;const struct w280bf036i_panel_desc *desc =of_device_get_match_data(&dsi->dev);dsi->mode_flags = desc->flags;dsi->format = desc->format;dsi->lanes = desc->lanes;w280bf036i->reset = devm_gpiod_get(&dsi->dev, "reset", GPIOD_OUT_LOW);if (IS_ERR(w280bf036i->reset)) {dev_err(&dsi->dev, "Couldn't get our reset GPIO\n");return PTR_ERR(w280bf036i->reset);}drm_panel_init(&w280bf036i->panel, &dsi->dev, &w280bf036i_funcs,DRM_MODE_CONNECTOR_DSI);int ret = drm_panel_of_backlight(&w280bf036i->panel);if (ret) return ret;drm_panel_add(&w280bf036i->panel);mipi_dsi_set_drvdata(dsi, w280bf036i);w280bf036i->dsi = dsi;w280bf036i->desc = desc;return mipi_dsi_attach(dsi);
}static int w280bf036i_dsi_remove(struct mipi_dsi_device *dsi) {struct w280bf036i *w280bf036i = mipi_dsi_get_drvdata(dsi);mipi_dsi_detach(dsi);drm_panel_remove(&w280bf036i->panel);return 0;
}static const struct of_device_id w280bf036i_of_match[] = {{.compatible = "wlk,w280bf036i", .data = &w280bf036i_desc}, {}};
MODULE_DEVICE_TABLE(of, w280bf036i_of_match);static struct mipi_dsi_driver w280bf036i_dsi_driver = {.probe = w280bf036i_dsi_probe,.remove = w280bf036i_dsi_remove,.driver ={.name = "w280bf036i",.of_match_table = w280bf036i_of_match,},
};
module_mipi_dsi_driver(w280bf036i_dsi_driver);MODULE_AUTHOR("CNflysky@qq.com");
MODULE_DESCRIPTION("WLK w280bf036i LCD Panel Driver");
MODULE_LICENSE("GPL");

好了，驱动代码编写完成，我们来解析一下这个代码:
首先在static void w280bf036i_init_sequence函数里面，我们填入了厂家所给的初始化代码:

static void w280bf036i_init_sequence(struct w280bf036i *w280bf036i) {w280bf036i_command(w280bf036i, MIPI_DCS_SOFT_RESET, 0x00);/* We need to wait 5ms before sending new commands */msleep(5);w280bf036i_command(w280bf036i, MIPI_DCS_EXIT_SLEEP_MODE, 0x00);// Command2 BK3 Selection: Enable the BK function of Command2w280bf036i_command(w280bf036i, 0xFF, 0x77, 0x01, 0x00, 0x00, 0x13);// Your initial code here...
}

然后在w280bf036i_mode这个结构体中，我们填上屏幕的Porch参数:

static const struct drm_display_mode w280bf036i_mode = {.clock = 25000,.hdisplay = 480,.hsync_start = 480 + /* HFP */ 10,.hsync_end = 480 + 10 + /* HSync */ 4,.htotal = 480 + 10 + 4 + /* HBP */ 20,.vdisplay = 640,.vsync_start = 640 + /* VFP */ 8,.vsync_end = 640 + 8 + /* VSync */ 4,.vtotal = 640 + 8 + 4 + /* VBP */ 14,.width_mm = 43,.height_mm = 57,.type = DRM_MODE_TYPE_DRIVER | DRM_MODE_TYPE_PREFERRED,
};

最后，在w280bf036i_desc这个结构体中，我们填上屏幕的lane数量:

static const struct w280bf036i_panel_desc w280bf036i_desc = {.mode = &w280bf036i_mode,.lanes = 1,.flags = MIPI_DSI_MODE_VIDEO,.format = MIPI_DSI_FMT_RGB888,
};

然后在当前目录下，编写Makefile:

obj-m += panel-wlk-w280bf036i.o
all:make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

最后，我们执行make命令，编译模块:

make

编译完成后，目录下会出现多个文件，确保有panel-wlk-w280bf036i.ko即可。

编辑设备树

驱动编写完毕，我们还需要设备树用来探测(Probe)设备,编写设备树代码(vc4-kms-dsi-w280bf036i.dts)如下:

// compile: dtc -@ -I dts -O dtb -o vc4-kms-dsi-w280bf036i.dtbo vc4-kms-dsi-w280bf036i.dts/dts-v1/;
/plugin/;/ {compatible = "brcm,bcm2835";  fragment@0 {target = <&dsi1>;__overlay__{status = "okay";#address-cells = <1>;#size-cells = <0>;port {dsi_out_port:endpoint {remote-endpoint = <&panel_dsi_port>;};};w280bf036i:w280bf036i@0 {compatible    = "wlk,w280bf036i";status        = "okay";reg           = <0>;reset-gpios   = <&gpio 47 1>;   // Dummy GPIO , Unusedport {panel_dsi_port: endpoint {remote-endpoint = <&dsi_out_port>;};};};};};fragment@1 {target = <&gpio>;__overlay__ {w280bf036i_pins: w280bf036i_pins {brcm,pins = <47>;brcm,function = <1>; // outbrcm,pull = <0>; // off};};};fragment@2 {target = <&i2c_csi_dsi>;__overlay__ {#address-cells = <1>;#size-cells = <0>;status = "okay";};};
};

我的转接板设计为DSI的I2C SCL脚用于连接Reset，故使用45号引脚作为Reset PIN.
P.S. 该设计有问题，不应该将I2C引脚作为GPIO使用，应加一颗I2C转GPIO芯片(如PCA9536)。
P.P.S. 因为转接板虽然打好了，但是TMD元件还没到，所以先把I2C接口启用了让44和45都高电平阻止屏幕复位，等元件到了会更新的…

应用

dtc -@ -I dts -O dtb -o vc4-kms-dsi-w280bf036i.dtbo vc4-kms-dsi-w280bf036i.dts
sudo cp vc4-kms-dsi-w280bf036i.dtbo /boot/overlays/
sudo cp panel-wlk-w280bf036i.ko /lib/modules/`uname -r`/kernel/drivers/gpu/drm/panel
sudo depmod
echo "ignore_lcd=1" >> /boot/config.txt
echo "dtoverlay=vc4-kms-dsi-w280bf036i" >> /boot/config.txt
sudo reboot

执行完上面的命令，板子重启之后，屏幕上就能显示树莓派桌面了。

展示

由于初始化代码或者是博主的layout有些问题，屏幕的显示效果不太正常，待再次调整。

不过,这个屏幕跑UFOTest居然能跑到80帧，还是挺令人意外的

链接

本文所用的所有代码都能在这里找到: GitHub链接

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前言

正题