Erkundung von LCD-Bildschirmtreibern unter Linux für Einsteiger

LCD is a commonly used peripheral, and semiconductor manufacturers typically provide pre-written LCD interface driver programs for their chips. Developers don’t need to modify the LCD driver section; they only need to adjust the device tree according to the specific LCD device they are using. While no modifications are necessary in the driver, it’s still essential to comprehend the LCD driver process.

1. Framebuffer Device

In the Linux operating system, applications ultimately interact with the video memory of RGB LCD to display characters, images, and other information on the LCD screen. Memory management in Linux is rigorous, and video memory needs to be allocated. Moreover, because of the presence of virtual memory, the video memory set by the driver and the video memory accessed by the application must correspond to the same physical memory. To address this issue, the Framebuffer (referred to as “fb”) was introduced. The Framebuffer represents a memory area that stores a frame of an image. Writing data to this memory is equivalent to writing data to the screen. In essence, the Framebuffer maps each point on the screen to a linear memory space, enabling programs to change the value of this memory segment to alter the color of a specific point on the screen.

The Linux kernel abstracts all Framebuffers as fb_info structures. The fb_info structure contains the complete attributes and operation sets of the Framebuffer device. As a result, each Framebuffer device is associated with an fb_info. The structure is defined in the include/linux/fb.h file.

2. Analysis of the LCD Driver

For a given chip, the driver code for the eLCDIF controller for LCD screens with different resolutions remains the same. As an example, let’s consider the LCD driver for the NXP IMX6ULL chip in Linux and briefly outline the LCD driver’s process.

• Opening the Device Tree: Open the imx6ull.dtsi file and locate the content of the lcdif node, which contains configuration details for the LCD interface controller.

lcdif: lcdif@021c8000 {
compatible = “fsl,imx6ul-lcdif”, “fsl,imx28-lcdif”;
reg = <0x021c8000 0x4000>;
interrupts = <GIC_SPI 5 IRQ_TYPE_LEVEL_HIGH>;
clocks = <&clks IMX6UL_CLK_LCDIF_PIX>,
<&clks IMX6UL_CLK_LCDIF_APB>,
<&clks IMX6UL_CLK_DUMMY>;
clock-names = “pix”, “axi”, “disp_axi”;
status = “disabled”;
};

• Device Driver Matching: The compatible property value is used to match the device driver in the Linux source code. The mxsfb.c file contains the platform driver for the MXS Framebuffer.

static const struct of_device_id mxsfb_dt_ids[] = {
{ .compatible = “fsl,imx23-lcdif”, .data = &mxsfb_devtype[0], },
{ .compatible = “fsl,imx28-lcdif”, .data = &mxsfb_devtype[1], },
{ /* sentinel */ }
};
……
……
static struct platform_driver mxsfb_driver = {
.probe = mxsfb_probe,
.remove = mxsfb_remove,
.shutdown = mxsfb_shutdown,
.id_table = mxsfb_devtype,
.driver = {
.name = DRIVER_NAME,
.of_match_table = mxsfb_dt_ids,
.pm = &mxsfb_pm_ops,
},
};

module_platform_driver(mxsfb_driver);

• Probe Function: When the driver matches the device, the mxsfb_probe function is executed. This function initializes the Framebuffer device, allocates resources, maps memory, and sets up the interrupt handler.

static int mxsfb_probe(struct platform_device *pdev) {
const struct of_device_id *of_id = of_match_device(mxsfb_dt_ids, &pdev->dev);
struct resource *res;
struct mxsfb_info *host;
struct fb_info *fb_info;
struct pinctrl *pinctrl;
int irq = platform_get_irq(pdev, 0);
int gpio, ret;
……
res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
if (!res) {
dev_err(&pdev->dev, “Cannot get memory IO resource\n”);
return -ENODEV;
}

host = devm_kzalloc(&pdev->dev, sizeof(struct mxsfb_info), GFP_KERNEL);
if (!host) {
dev_err(&pdev->dev, “Failed to allocate IO resource\n”);
return -ENOMEM;
}

fb_info = framebuffer_alloc(sizeof(struct fb_info), &pdev->dev);
if (!fb_info) {
dev_err(&pdev->dev, “Failed to allocate fbdev\n”);
devm_kfree(&pdev->dev, host);
return -ENOMEM;
}
host->fb_info = fb_info;
fb_info->par = host;

ret = devm_request_irq(&pdev->dev, irq, mxsfb_irq_handler, 0, dev_name(&pdev->dev), host);
if (ret) {
dev_err(&pdev->dev, “request_irq (%d) failed with error %d\n”, irq, ret);
ret = -ENODEV;
goto fb_release;
}

host->base = devm_ioremap_resource(&pdev->dev, res);
if (IS_ERR(host->base)) {
dev_err(&pdev->dev, “ioremap failed\n”);
ret = PTR_ERR(host->base);
goto fb_release;
}
……

fb_info->pseudo_palette = devm_kzalloc(&pdev->dev, sizeof(u32) * 16, GFP_KERNEL);
if (!fb_info->pseudo_palette) {
ret = -ENOMEM;
goto fb_release;
}

INIT_LIST_HEAD(&fb_info->modelist);
pm_runtime_enable(&host->pdev->dev);

ret = mxsfb_init_fbinfo(host);
if (ret != 0)
goto fb_pm_runtime_disable;

mxsfb_dispdrv_init(pdev, fb_info);

if (!host->dispdrv) {
pinctrl = devm_pinctrl_get_select_default(&pdev->dev);
if (IS_ERR(pinctrl)) {
ret = PTR_ERR(pinctrl);
goto fb_pm_runtime_disable;
}
}

if (!host->enabled) {
writel(0, host->base + LCDC_CTRL);
mxsfb_set_par(fb_info);
mxsfb_enable_controller(fb_info);
pm_runtime_get_sync(&host->pdev->dev);
}

ret = register_framebuffer(fb_info);
if (ret != 0) {
dev_err(&pdev->dev, "Fehler bei der Registrierung des Framebuffers\n");
goto fb_destroy;
}
……
return ret;
}

• Framebuffer-Initialisierung: Der Framebuffer wird initialisiert und der Treiber registriert ihn beim Linux-Kernel.

Die Hauptaufgaben der mxsfb_probe Funktion umfassen die Speicherzuweisung für fb_info, die Speicherzuweisung für pseudo_palette, die Initialisierung des eLCDIF-Controllers und die Registrierung des fb_info beim Linux-Kernel.

Tatsächlich stellt die Verwendung des Linux-Systems für viele LCD-Modulhersteller stets eine technische Herausforderung dar. Die Auswahl des richtigen LCD-Modulherstellers ist nicht einfach für diejenigen, die Produkte mit dem Linux-System entwickeln. RJOYTEK ist ein Technologieunternehmen, das sich auf LCD-Module und Board-Entwicklung spezialisiert hat. Wenn Sie weitere technische Unterstützung benötigen, zögern Sie bitte nicht uns zu kontaktieren!

• Produktliste bitte einsehen auf TFT LCD Display Modul
• Überprüfen Sie das LCD-Panel-Video unter LCD SOLUTION RJY
• Wenn Sie technische Unterstützung benötigen, bitte uns zu kontaktieren