技术分享 | i.MX8M Mini适配MIPI转eDP芯片

1.方案概述

此方案使用HD-8MMN-CORE的核心板搭配TI公司的芯片SN65DSI86转换芯片实现。

SN65DSI86作为一款MIPI DSI转eDP的芯片，支持双通道DSI输入，最大四通道显示输出，最大支持4K@60fps输出，WUXGA 1080P。本方案中将采用单通道DSI输入，双通道DP输出到1080p的屏幕。

HD8MMN-CORE系列工业级核心板基于NXP(Freescale) i.MX8MM系列Cortex-A53高性能处理器设计，支持硬件加密，支持摄像头接口、USB3.0接口、HDMI/MIPI、PCIe、千兆以太网接口、多路串口等，适用于快速开发一系列最具创新性的应用，如多媒体应用、人机界面、工业4.0、车载终端以及边缘计算设备等。

2.硬件原理图

注：硬件修改REFCLK上需要贴上27M的晶振，TEST2引脚需要通过4.7K电阻上拉到1.8V.

3软件实现

3.1软件介绍

内核版本：Linux5.10；

SN65DSI86驱动：drivers/gpu/drm/bridge/ti-sn65dsi86.c

Panel驱动：drivers/gpu/drm/panel/panel-simple.c

3.2驱动移植

内核配置，需要打开如下两项

1 2	`CONFIG_DRM_TI_SN65DSI86=y` `CONFIG_DRM_PANEL_SIMPLE=y`

1)设备树配置

/ {
        osc_27m: clock-osc-27m {
                compatible = "fixed-clock";
                #clock-cells = <0>;
                clock-frequency = <27000000>;
                clock-output-names = "osc_27m";
        };
 
        lcd_backlight: lcd_backlight {
                compatible = "pwm-backlight";
                pwms = <&pwm1 0 100000>;
                status = "okay";
 
                brightness-levels = < 0  1  2  3  4  5  6  7  8  9
                                     10 11 12 13 14 15 16 17 18 19
                                     20 21 22 23 24 25 26 27 28 29
                                     30 31 32 33 34 35 36 37 38 39
                                     40 41 42 43 44 45 46 47 48 49
                                     50 51 52 53 54 55 56 57 58 59
                                     60 61 62 63 64 65 66 67 68 69
                                     70 71 72 73 74 75 76 77 78 79
                                     80 81 82 83 84 85 86 87 88 89
                                     90 91 92 93 94 95 96 97 98 99
                                    100>;
                enable-gpios = <&gpio1 1 GPIO_ACTIVE_HIGH>;
                default-brightness-level = <80>;
        };
        panel {
                compatible = "test,test-edp-1080p";        //设置自己的屏幕匹配参数组
                backlight = <&lcd_backlight>;
                no-hpd;
 
                port {
                        panel1_in: endpoint {
                                remote-endpoint = <&sn65_out>;
                        };
                };
        };
};
&pwm1 {
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_pwm1>;
        status = "okay";
};
 
&i2c4 {
        clock-frequency = <400000>;
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_i2c4>;
        status = "okay";
        sn65dsi86@2d {
               …….
                clock-names = "refclk";          //默认名字
                //SN65DSI86仅支持12 MHz, 19.2 MHz, 26 MHz, 27 MHz or 38.4 MHz.
                clocks = <&osc_27m>;
 
               ……
                        port@1 {
                                reg = <1>;
                                sn65_out: endpoint {
                                        //data-lanes = <0 1 2 3>;
                                  //根据eDP屏的通道数设置，这里为2通道
                                        data-lanes = <0 1>; 
 ……

2)驱动修改

添加显示屏的显示参数到驱动中，修改drivers/gpu/drm/panel/panel-simple.c：

tatic const struct display_timing test_edp_1080p_timing = {
        .pixelclock = { 153000000, 153000000, 153000000 },
        .hactive = { 1920, 1920, 1920 },
        .hfront_porch = { 100, 100, 100 },
        …..
        .bus_format = MEDIA_BUS_FMT_RGB666_1X18,
        .connector_type = DRM_MODE_CONNECTOR_eDP,
};
 
static const struct of_device_id platform_of_match[] = {
        {
                .compatible = "test,test-edp-1080p",
                .data = &test_edp_1080p,
        },

修改sn65dsi86驱动以适应我们的板卡，修改drivers/gpu/drm/bridge/ti-sn65dsi86.c，修改DSI正确的模式，防止找不到注册的panel出现报错“couldnotfindanypanelnode”，开机过程中打印1次是正常的，SN65DSI86的驱动找不到Panel会被多次调用，直到找到panel为止。

static int ti_sn_bridge_attach(struct drm_bridge *bridge,
                               enum drm_bridge_attach_flags flags)
{
...
        //dsi->mode_flags = MIPI_DSI_MODE_VIDEO;
        dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_SYNC_PULSE |
                          MIPI_DSI_MODE_EOT_PACKET | MIPI_DSI_MODE_VIDEO_HSE;

3)调试报错

在调试过程中碰到“Linktrainingfailed,linkisoff”的报错，是因为SN65DSI86默认只支持ASSR模式的eDP屏幕，对于不支持ASSR模式的eDP屏，硬件上则需要将TEST2引脚拉高到1.8V，且修改相关寄存器将芯片从ASSR模式变为支持标准的DP模式，修改如下：

#define SN_ENH_FRAME_REG                        0x5A
#define  ASSR_CONTROL                           BIT(0)
...
 
static int ti_sn_link_training(struct ti_sn_bridge *pdata, int dp_rate_idx,
                               const char **last_err_str)
{
        unsigned int val;
        int ret;
        int i;
 
        /* set dp clk frequency value */
        regmap_update_bits(pdata->regmap, SN_DATARATE_CONFIG_REG,
                           DP_DATARATE_MASK, DP_DATARATE(dp_rate_idx));
 
        regmap_write(pdata->regmap, 0xff, 0x07);
        regmap_write(pdata->regmap, 0x16, 0x01);
        regmap_write(pdata->regmap, 0xff, 0x00);
       /* For DisplayPort, use the standard DP scrambler seed. */
        regmap_update_bits(pdata->regmap, SN_ENH_FRAME_REG,ASSR_CONTROL, 0);
        /* enable DP PLL */
        regmap_write(pdata->regmap, SN_PLL_ENABLE_REG, 1);
...

如果未接eDP屏幕会出现如下报错：

1 2	`[ 2.299284] ti_sn65dsi86 3-002d: [drm:ti_sn_bridge_enable] ERROR Can't read lane count (-6); assuming 4` `[ 2.765851] ti_sn65dsi86 3-002d: [drm:ti_sn_bridge_enable] ERROR Can't read eDP rev (-6), assuming 1.1`