LCD notes (4) analyze the LCD driver of the kernel

1. Driver framework

Linux The driver = Driver framework + Hardware programming . Based on QEMU Write the LCD The driver , Yes LCD The framework of the driver has been analyzed clearly . The core is ：

• Distribute fb_info

• Set up fb_info

• register fb_info

• Hardware related settings

1.1 Entry function registration platform_driver

1.2 The device tree has corresponding nodes

2. Write hardware related code

We just need to focus on IMX6ULL Writing hardware related code , involve 3 part ：

• GPIO Set up

  • LCD Pin

  • Backlight pin

• The clock is set

  • determine LCD The clock of the controller

  • according to LCD Of DCLK Calculate the relevant clock

• LCD The setting of the controller itself

  • Setting up Framebuffer The address of

  • Set up Framebuffer Data format in 、LCD data format

  • Set the timing

2.1 GPIO Set up

There are two ways ：

• Read and write related registers directly

• Use the device tree , Set... In the device tree pinctrl

  • This course focuses on LCD, So use pinctrl Simplify procedures

 

2.2 The clock is set

IMX6ULL Of LCD Controller involves 2 A clock ：

 

The code of the clock subsystem is directly used in the code .

• Specify the frequency in the device tree ：

  • file ：arch/arm/boot/dts/100ask_imx6ull-14x14.dts

  • Code ：clock-frequency

 

Get... From the device tree dot clock, Deposit in display_timing

• file ：drivers\video\of_display_timing.c

• Code ：

ret |= parse_timing_property(np, "clock-frequency", &dt->pixelclock);

 

 

Use display_timing To set up videomode

• file ：drivers\video\videomode.c

• Code ：

void videomode_from_timing(const struct display_timing *dt,
			  struct videomode *vm)
{
	vm->pixelclock = dt->pixelclock.typ;
	vm->hactive = dt->hactive.typ;
	vm->hfront_porch = dt->hfront_porch.typ;
	vm->hback_porch = dt->hback_porch.typ;
	vm->hsync_len = dt->hsync_len.typ;

	vm->vactive = dt->vactive.typ;
	vm->vfront_porch = dt->vfront_porch.typ;
	vm->vback_porch = dt->vback_porch.typ;
	vm->vsync_len = dt->vsync_len.typ;

	vm->flags = dt->flags;
}

according to videomode Value , Use the functions of the clock subsystem to set the clock ：

file ：drivers\video\fbdev\mxc\ldb.c

2.3 LCD Configuration of the controller

Take setting the resolution as an example .

• Specify the resolution in the device tree ：

  • file ：arch/arm/boot/dts/100ask_imx6ull-14x14.dts

  • Code ：clock-frequency

      display-timings {
            native-mode = <&timing0>;

             timing0: timing0_1024x768 {
				hactive = <1024>;
	            vactive = <600>;

Get the resolution from the device tree , Deposit in display_timing

• file ：drivers\video\of_display_timing.c

• Code ：

	ret |= parse_timing_property(np, "hactive", &dt->hactive);
	ret |= parse_timing_property(np, "vactive", &dt->vactive);

Use display_timing To set up videomode

• file ：drivers\video\videomode.c

• Code ：

void videomode_from_timing(const struct display_timing *dt,
			  struct videomode *vm)
{
	vm->hactive = dt->hactive.typ;

    vm->vactive = dt->vactive.typ;

according to videomode Value , Set up fb_videomode

• file ：drivers\video\fbdev\core\fbmon.c

• Code ：

int fb_videomode_from_videomode(const struct videomode *vm,
				struct fb_videomode *fbmode)
{
	unsigned int htotal, vtotal;

	fbmode->xres = vm->hactive;

    fbmode->yres = vm->vactive;

according to fb_videomode Value , Set up fb_info Medium var：

• file ：drivers\video\fbdev\core\modedb.c

• Code ：

void fb_videomode_to_var(struct fb_var_screeninfo *var,
			 const struct fb_videomode *mode)
{
	var->xres = mode->xres;
	var->yres = mode->yres;

• according to var The resolution of the , Set register

  • file ：drivers\video\fbdev\mxsfb.c

  • Code ：

	writel(TRANSFER_COUNT_SET_VCOUNT(fb_info->var.yres) |
			TRANSFER_COUNT_SET_HCOUNT(fb_info->var.xres),
			host->base + host->devdata->transfer_count);