Linux的LCD驱动分析及移植
测试平台
宿主机平台：Ubuntu 12.04.4 LTS
目标机：Easy-ARM IMX283
目标机内核：Linux 2.6.35.3
LCD驱动分析
LCD屏的驱动总体上分成两块，一块是GUI显示输出驱动；一块是触摸驱动（该部分单独一节另外描述）。
LCD驱动概念
LCD是Liquid Crystal Display的简称，也就是经常所说的液晶显示器。LCD能够支持彩色图像的显示和视频的播放，是一种非常重要的输出设备。如果我们的系统要用GUI（图形界面接口），比如minigui，MicroWindows。这时LCD设备驱动程序就应该编写成frambuffer接口，而不是编写成仅仅操作底层的LCD控制器接口。
framebuffer是Linux系统为显示设备提供的一个接口，它将显示缓冲区抽象，屏蔽图像硬件的底层差异，允许上层应用程序在图形模式下直接对显示缓冲区进行操作。framebuffer又叫帧缓冲，是Linux为操作显示设备提供的一个用户接口。用户应用程序可以通过framebuffer透明地访问不同类型的显示设备。从这个方面来说，framebuffer是硬件设备显示缓冲区的抽象。Linux抽象出framebuffer这个帧缓冲区可以供用户应用程序直接读写，通过更改framebuffer中的内容，就可以立刻显示在LCD显示屏上。
framebuffer是一个标准的字符设备，主设备号是29，次设备号根据缓冲区的数目而定。framebuffer对应/dev/fb%d设备文件。根据显卡的多少，设备文件可能是/dev/fb0、/dev/fb1等。缓冲区设备也是一种普通的内存设备,可以直接对其进行读写。对用户程序而言，它和/dev下面的其他设备没有什么区别，用户可以把frameBuffer看成一块内存，既可以写，又可以读。显示器将根据内存数据显示对应的图像界面。这一切都由LCD控制器和响应的驱动程序来完成。
LCD驱动框架分析
总体上是一个平台设备驱动与字符驱动的组合；
先从LCD的设备 dev/fb* 是怎么实现的来进行追溯；
1.开发板启动，进行设备注册；
在 arch/arm/mach-mx28/device.c 设备注册文件中，注册LCD的fd平台设备 “mxs-fb”
// mxs-fb平台设备资源定义
static struct resource framebuffer_resource[] = { { .flags = IORESOURCE_MEM, .start = LCDIF_PHYS_ADDR, .end = LCDIF_PHYS_ADDR + 0x2000 - 1, }, { .flags = IORESOURCE_IRQ, .start = IRQ_LCDIF, .end = IRQ_LCDIF, }, };
// mxs-fb平台设备私有数据，包含显示屏名称、分辨率、位宽、时钟、面板操作等，在drivers/video/mxs/lcd_43wvf1g.c 中通过subsys_initcall 接口将设备私有数据添加到链表中
static struct mxs_platform_fb_data mxs_framebuffer_pdata = { .list = LIST_HEAD_INIT(mxs_framebuffer_pdata.list), };
// lcd设备启动初始化，在 m28evk.c 中 mx28_device_init()会调用
static void __init mx28_init_lcdif(void) { struct platform_device *pdev; pdev = mxs_get_device("mxs-fb", 0); //获取匹配的设备结构体，定义在 arch/arm/plat-mxs/device.c 中
if (pdev == NULL || IS_ERR(pdev)) return; pdev->resource = framebuffer_resource; pdev->num_resources = ARRAY_SIZE(framebuffer_resource); pdev->dev.platform_data = &mxs_framebuffer_pdata; mxs_add_device(pdev, 3); // 添加到设备注册列表，设备注册在 arch/arm/plat-mxs/device.c 中实现 通过 device_initcall(mxs_device_init);遍历设备列表并进行平台设备注册
}
2.接下来是 platform_driver mxsfb_driver 的注册，匹配之后触发 mxsfb_probe 函数执行以下操作：
进行相关硬件初始化和 framebuffer 设置；
register_framebuffer() 注册 LCD 屏的 fd 设备;
LCD的驱动包含：
drivers/video/mxs/lcd_43wvf1g.c // LCD设备私有数据，包含名称、分辨率、位宽、时钟、面板操作
drivers/video/mxs/lcdif.c // lcd的一些接口操作
drivers/video/mxs/mxsfb.c //
平台设备驱动 platform_driver 注册 drivers/video/mxs/mxsfb.c
会编译成 mxsfb.ko
static struct platform_driver mxsfb_driver = { .probe = mxsfb_probe, .remove = mxsfb_remove, .suspend = mxsfb_suspend, .resume = mxsfb_resume, .driver = { .name = "mxs-fb", // 与启动时的平台设备注册的 platform_device 名称相同 .owner = THIS_MODULE, }, }; static int __init mxsfb_init(void) { return platform_driver_register(&mxsfb_driver); // 显示屏平台设备驱动注册 }
驱动安装时与平台设备匹配之后触发 mxsfb_probe 函数，这个时核心。
static int __devinit mxsfb_probe(struct platform_device *pdev) { int ret = 0; struct mxs_fb_data *data; struct resource *res; struct fb_info *info; struct mxs_platform_fb_data *pdata = pdev->dev.platform_data; struct mxs_platform_fb_entry *pentry = NULL; mydbg("\n"); if (pdata == NULL) { ret = -ENODEV; goto out; } if (default_panel_name) { mydbg("default_panel_name=%s\n",default_panel_name);
// 通过LCD面板名称匹配获取面板参数及设置句柄（平台设备私有数据传递过来） pentry = (void *)mxs_lcd_iterate_pdata(pdata, get_matching_pentry_by_name, default_panel_name); if (pentry) { mxs_lcd_move_pentry_up(pentry, pdata); pdata->cur = pentry; } } if (!default_panel_name || !pentry) { mydbg("\n"); pentry = pdata->cur; } if (!pentry || !pentry->init_panel || !pentry->run_panel || !pentry->release_panel) { mydbg("\n"); ret = -EINVAL; goto out; } data = (struct mxs_fb_data *)framebuffer_alloc(sizeof(struct mxs_fb_data) + sizeof(u32) * 256 - sizeof(struct fb_info), &pdev->dev); if (data == NULL) { ret = -ENOMEM; goto out; } cdata = data; data->dev = &pdev->dev; data->pdata = pdata; platform_set_drvdata(pdev, data); info = &data->info; dev_dbg(&pdev->dev, "resolution %dx%d, bpp %d\n", pentry->x_res, pentry->y_res, pentry->bpp); mxs_lcd_iterate_pdata(pdata, get_max_memsize, data); data->map_size = PAGE_ALIGN(data->mem_size) * NUM_SCREENS; dev_dbg(&pdev->dev, "memory to allocate: %d\n", data->map_size); data->virt_start = dma_alloc_writecombine(&pdev->dev, data->map_size, &data->phys_start, GFP_KERNEL); if (data->virt_start == NULL) { ret = -ENOMEM; goto out_dma; } dev_dbg(&pdev->dev, "allocated at %p:0x%x\n", data->virt_start, data->phys_start); mutex_init(&data->blank_mutex); INIT_WORK(&data->work, mxsfb_task); data->state = F_ENABLE; mxsfb_default.bits_per_pixel = pentry->bpp; /* NB: rotated */ mxsfb_default.xres = pentry->y_res; mxsfb_default.yres = pentry->x_res; mxsfb_default.xres_virtual = pentry->y_res; mxsfb_default.yres_virtual = data->map_size / (pentry->y_res * pentry->bpp / 8); if (mxsfb_default.yres_virtual >= mxsfb_default.yres * 2) mxsfb_default.yres_virtual = mxsfb_default.yres * 2; else mxsfb_default.yres_virtual = mxsfb_default.yres; mxsfb_fix.smem_start = data->phys_start; mxsfb_fix.smem_len = pentry->y_res * pentry->x_res * pentry->bpp / 8; mxsfb_fix.ypanstep = 1; switch (pentry->bpp) { case 32: case 24: mxsfb_default.red.offset = 16; mxsfb_default.red.length = 8; mxsfb_default.green.offset = 8; mxsfb_default.green.length = 8; mxsfb_default.blue.offset = 0; mxsfb_default.blue.length = 8; break; case 16: #if 0 mxsfb_default.red.offset = 11; mxsfb_default.red.length = 5; mxsfb_default.green.offset = 5; mxsfb_default.green.length = 6; mxsfb_default.blue.offset = 0; mxsfb_default.blue.length = 5; break; #else mxsfb_default.red.offset = 0 ; mxsfb_default.red.length = 5; mxsfb_default.green.offset = 5; mxsfb_default.green.length = 6; mxsfb_default.blue.offset = 11; mxsfb_default.blue.length = 5; break; #endif default: dev_err(&pdev->dev, "unsupported bitwidth %d\n", pentry->bpp); ret = -EINVAL; goto out_dma; } info->screen_base = data->virt_start; info->fbops = &mxsfb_ops; info->var = mxsfb_default; info->fix = mxsfb_fix; info->pseudo_palette = &data->par; data->par = NULL; info->flags = FBINFO_FLAG_DEFAULT; init_waitqueue_head(&data->vsync_wait_q); data->vsync_count = 0; res = platform_get_resource(pdev, IORESOURCE_MEM, 0); if (res == NULL) { dev_err(&pdev->dev, "cannot get IRQ resource\n"); ret = -ENODEV; goto out_dma; } data->regbase = (unsigned long)IO_ADDRESS(res->start); res = platform_get_resource(pdev, IORESOURCE_IRQ, 0); if (res == NULL) { dev_err(&pdev->dev, "cannot get IRQ resource\n"); ret = -ENODEV; goto out_dma; } data->irq = res->start; mxsfb_check_var(&info->var, info); ret = fb_alloc_cmap(&info->cmap, 256, 0); if (ret) goto out_cmap; mxsfb_set_par(info); mxs_init_lcdif(); ret = pentry->init_panel(data->dev, data->phys_start, mxsfb_fix.smem_len, pentry); if (ret) { dev_err(&pdev->dev, "cannot initialize LCD panel\n"); goto out_panel; } dev_dbg(&pdev->dev, "LCD panel initialized\n"); init_timings(data); // not effect dotclk mode ret = request_irq(data->irq, lcd_irq_handler, 0, "fb_irq", data); if (ret) { dev_err(&pdev->dev, "request_irq (%d) failed with error %d\n", data->irq, ret); goto out_panel; } ret = register_framebuffer(info); // 注册fb_info if (ret) goto out_irq; pentry->run_panel(); /* REVISIT: temporary workaround for MX23EVK */ mxsfb_disable_controller(data); mxsfb_enable_controller(data); data->cur_phys = data->phys_start; dev_dbg(&pdev->dev, "LCD running now\n"); #ifdef CONFIG_CPU_FREQ mxsfb_nb.fb_data = data; cpufreq_register_notifier(&mxsfb_nb.nb, CPUFREQ_TRANSITION_NOTIFIER); #endif /* CONFIG_CPU_FREQ */ goto out; out_irq: free_irq(data->irq, data); out_panel: fb_dealloc_cmap(&info->cmap); out_cmap: dma_free_writecombine(&pdev->dev, data->map_size, data->virt_start, data->phys_start); out_dma: kfree(data); out: return ret; }
待续.....
LCD驱动移植总结
1.LCD引脚配置及初始化
上述原理图包含了数据、时钟、背光控制、触摸、复位等引脚的分配，具体有机会在深入理解LCD硬件驱动原理有在进行说明。
在 arch/arm/mach-mx28/mx28evk_pins.c 的 mx28evk_fixed_pins[ ] 引脚列表中添加 LCD 的驱动引脚，相关引脚转义在 mx28_pins.h 结合 arch/arm/mach/pinctrl.h 实现
#if defined(CONFIG_FB_MXS) || defined(CONFIG_FB_MXS_MODULE) { .name = "LCD_D00", .id = PINID_LCD_D00, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D01", .id = PINID_LCD_D01, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D02", .id = PINID_LCD_D02, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D03", .id = PINID_LCD_D03, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D04", .id = PINID_LCD_D04, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D05", .id = PINID_LCD_D05, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D06", .id = PINID_LCD_D06, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D07", .id = PINID_LCD_D07, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D08", .id = PINID_LCD_D08, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D09", .id = PINID_LCD_D09, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D10", .id = PINID_LCD_D10, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D11", .id = PINID_LCD_D11, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D12", .id = PINID_LCD_D12, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D13", .id = PINID_LCD_D13, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D14", .id = PINID_LCD_D14, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D15", .id = PINID_LCD_D15, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, /* { .name = "LCD_D16", .id = PINID_LCD_D16, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D17", .id = PINID_LCD_D17, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D18", .id = PINID_LCD_D18, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D19", .id = PINID_LCD_D19, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D20", .id = PINID_LCD_D20, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D21", .id = PINID_LCD_D21, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D22", .id = PINID_LCD_D22, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_D23", .id = PINID_LCD_D23, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, */ { .name = "LCD_RESET", .id = PINID_LCD_RESET, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_VSYNC", .id = PINID_LCD_RD_E, .fun = PIN_FUN2, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_HSYNC", .id = PINID_LCD_WR_RWN, .fun = PIN_FUN2, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_ENABLE", .id = PINID_LCD_CS, .fun = PIN_FUN2, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_DOTCLK", .id = PINID_LCD_RS, .fun = PIN_FUN2, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, { .name = "LCD_BACKLIGHT", .id = PINID_PWM3, .fun = PIN_FUN1, .strength = PAD_8MA, .voltage = PAD_3_3V, .drive = 1, }, #endif
然后通过类似 LCDIF_PHYS_ADDR + BM_LCDIF_CTRL1_RESET 组合就可以操作相关引脚寄存器了
2. 驱动移植到drivers 目录
1）.将LCD驱动放到 drivers/video/ 目录下，本例为 mxs
2）.修改 drivers/video/Kconfig，添加如下配置，表示会提取 mxs 驱动的 Kconfig 配置
if ARCH_MXS source "drivers/video/mxs/Kconfig" endif
3. 修改板级文件
板级文件有两个
mach-mx28 // mx28系列特有的
plat-mxs // fsl通用共有的功能
1）在设备注册 arch/arm/mach-mx28/device.c 中添加 mxs LCD 平台设备注册
#if defined(CONFIG_FB_MXS) || defined(CONFIG_FB_MXS_MODULE)
// LCD平台设备资源 resource static struct resource framebuffer_resource[] = { { .flags = IORESOURCE_MEM, //寻址地址空间资源
.start = LCDIF_PHYS_ADDR, .end = LCDIF_PHYS_ADDR + 0x2000 - 1, }, { .flags = IORESOURCE_IRQ, //中断资源 .start = IRQ_LCDIF, .end = IRQ_LCDIF, }, }; static struct mxs_platform_fb_data mxs_framebuffer_pdata = { .list = LIST_HEAD_INIT(mxs_framebuffer_pdata.list), }; static void __init mx28_init_lcdif(void) { struct platform_device *pdev; pdev = mxs_get_device("mxs-fb", 0); //获取匹配的设备结构体，定义在 arch/arm/plat-mxs/device.c 中 if (pdev == NULL || IS_ERR(pdev)) return; pdev->resource = framebuffer_resource; pdev->num_resources = ARRAY_SIZE(framebuffer_resource); pdev->dev.platform_data = &mxs_framebuffer_pdata; //设备私有数据链表，包含名称、分辨率、位宽、时钟、面板操作，在drivers/video/mxs/lcd_43wvf1g.c 中通过subsys_initcall 接口将设备私有数据添加到链表中
mxs_add_device(pdev, 3); //添加到设备注册列表 } #else static void __init mx28_init_lcdif(void) { ; } #endif
2）在设备列表注册 arch/arm/plat-mxs/device.c 中添加 mxs LCD 平台设备结构及设备列表匹配信息
#if defined(CONFIG_FB_MXS) || defined(CONFIG_FB_MXS_MODULE) // LCD面板平台设备结构体
static struct platform_device mxs_fb = { .name = "mxs-fb", //平台设备名称，后面的平台设备驱动名称要与这个一致 .id = 0, .dev = { .dma_mask = &common_dmamask, .coherent_dma_mask = DMA_BIT_MASK(32), .release = mxs_nop_release, }, }; #endif static struct mxs_dev_lookup dev_lookup[] = { ......
#if defined(CONFIG_FB_MXS) || defined(CONFIG_FB_MXS_MODULE) { .name = "mxs-fb", .size = 1, .pdev = &mxs_fb, }, #endif
...... }
