linux驱动移植LCD驱动分析

 一、LCD基础知识

1.1 LCD硬件原理

在Mini2440裸机开发之LCD基础我们介绍了LCD的硬件原理，有兴趣的可以去看看，这里我们仅仅简述一下LCD的原理。

下图是LCD示意图，里面的每个点就是一个像素点。它里面有一个电子枪，一边移动，一边发出各种颜色的光。用动态图表示如下：

电子枪是如何移动的？

• 有一条CLK时钟线与LCD相连，每发出一次CLK(高低电平)，电子枪就移动一个像素。

颜色如何确定？

• 由连接LCD的三组线RGB三原色混合而成：R(Red)、G(Green)、B(Blue)确定。

电子枪如何得知应跳到下一行？

• 有一条HSYNC信号线与LCD相连，每发出一次脉冲(高低电平)，电子枪就跳到下一行，该信号叫做行同步信号。

电子枪如何得知应跳到原点？

• 有一条VSYNC信号线与LCD相连，每发出一次脉冲(高低电平)，电子枪就跳到原点，该信号叫做帧同步信号。

RGB线上的数据从何而来？

• 内存里面划分一块显存(framebuffer)，里面存放了要显示的数据，LCD控制器从里面将数据读出来，通过RGB三组线传给电子枪，电子枪再依次打到显示屏上。

前面的信号由谁发给LCD？

• 有S3C2440里面的LCD控制器来控制发出信号。

工作原理：

• LCD屏可以看作是由许多象素构成的，比如320*240就是由320*240个象素构成的，每个象素由RGB三色调和，每种颜色又由多个位组成。比如我们的开发板上的LCD，有320*240个象素，每个象素由RGB三色调和，RGB三色位数分别为：565。


• S3C2440内集成了LCD控制器，LCD控制器外接LCD，每来一个VLCK，就会从左到右在LCD屏幕上显示一个象素的颜色，而这一个个象素的颜色就存放在显存里，在嵌入式领域，一般不会佩戴专门的显存，而是从内存SDRAM中划分出一部分充当显存；


• HSYNC引脚每发出一个脉冲，表示一行的数据开始发送；


• VSYNC引脚每发出一个冒充，表示一帧的数据开始发送。

1.2  frambuffer设备

我们在Mini2440裸机开发之LCD编程（GB2312、ASCII字库制作） 中介绍了如何在LCD显示屏中显示一张图片，其核心步骤就是向framebuffer中写入图片数据。

在linux中，如果我们的系统想使用GUI(图形界面接口)，这时LCD设备驱动程序就应该编写成frambuffer接口，而不是像裸机中那样只编写操作底层的LCD控制器接口。

framebuffer是linux系统为显示设备提供的一个用户接口，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，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。

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。

对用户程序而言，framebuffer设备它和/dev下面的其它设备没有什么区别，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framebuffer设备驱动来完成。

二、framebuffer设备驱动框架图

framebuffer设备驱动在linux系统框架如下图：

三、基础数据结构

3.1 fb_info结构体

struct fb_info定义在include/linux/fb.h文件中，用于保存我们framebuffer设备信息，其内部提供了对framebuffer设备操作的函数指针：

struct fb_info {
atomic_t count;
int node;
int flags;
/*
* -1 by default, set to a FB_ROTATE_* value by the driver, if it knows
* a lcd is not mounted upright and fbcon should rotate to compensate.
*/
int fbcon_rotate_hint;
struct mutex lock; /* Lock for open/release/ioctl funcs */
struct mutex mm_lock; /* Lock for fb_mmap and smem_* fields */
struct fb_var_screeninfo var; /* Current var */
struct fb_fix_screeninfo fix; /* Current fix */
struct fb_monspecs monspecs; /* Current Monitor specs */
struct work_struct queue; /* Framebuffer event queue */
struct fb_pixmap pixmap; /* Image hardware mapper */
struct fb_pixmap sprite; /* Cursor hardware mapper */
struct fb_cmap cmap; /* Current cmap */
struct list_head modelist; /* mode list */
struct fb_videomode *mode; /* current mode */
#if IS_ENABLED(CONFIG_FB_BACKLIGHT)
/* assigned backlight device */
/* set before framebuffer registration,
remove after unregister */
struct backlight_device *bl_dev;
/* Backlight level curve */
struct mutex bl_curve_mutex;
u8 bl_curve[FB_BACKLIGHT_LEVELS];
#endif
#ifdef CONFIG_FB_DEFERRED_IO
struct delayed_work deferred_work;
struct fb_deferred_io *fbdefio;
#endif
struct fb_ops *fbops;
struct device *device; /* This is the parent */
struct device *dev; /* This is this fb device */
int class_flag; /* private sysfs flags */
#ifdef CONFIG_FB_TILEBLITTING
struct fb_tile_ops *tileops; /* Tile Blitting */
#endif
union {
char __iomem *screen_base; /* Virtual address */
char *screen_buffer;
};
unsigned long screen_size; /* Amount of ioremapped VRAM or 0 */
void *pseudo_palette; /* Fake palette of 16 colors */
#define FBINFO_STATE_RUNNING 0
#define FBINFO_STATE_SUSPENDED 1
u32 state; /* Hardware state i.e suspend */
void *fbcon_par; /* fbcon use-only private area */
/* From here on everything is device dependent */
void *par;
/* we need the PCI or similar aperture base/size not
smem_start/size as smem_start may just be an object
allocated inside the aperture so may not actually overlap */
struct apertures_struct {
unsigned int count;
struct aperture {
resource_size_t base;
resource_size_t size;
} ranges[0];
} *apertures;
bool skip_vt_switch; /* no VT switch on suspend/resume required */
};

部分参数含义如下：

• count：fb_info的引用计数，fb_open时使其+1，release时使其-1，为0时销毁；


• node：全局变量registered_fb中的索引值，注册的时候分配，通过node可以索引fb_info；


• flags：一些标志位，有关于硬件加速的，大小端，fb的内存位置(设备或者内存)，具体硬件加速的方法，表明哪个使用了硬件加速；


• var：描述的是具体屏的一些参数，包括可见的分辨率，Bpp(bits_per_pixel),还有具体的时钟信号，包括bp，fp，vsync，hsync等,可以通过应用层设置也可以驱动层配置，相关设置时序的工具有fbset，还有相关的一些调色板配置；


• fix:描述的是不可变量，驱动中控制参数，不能在用户层更改。包括显存起始位置（一般是显示控制器DMA起始地址-物理地址,smem_start），framebuffer的长度(smem_len)；


• monspecs:描述的是显示器的一些参数，时序，生产日期等，一般这种信息描述在显示器中的EDID中，通过解析EDID来填充此参数；


• queue：事件队列，不过分析多个驱动，并没有发现使用该字段的，等遇到再做更新；


• pixmap，sprite（光标）都是像素图，注册framebuffer的时候会默认申请；


• cmap：设备独立的 colormap 信息，可以通过 ioctl 的 FBIOGETCMAP 和 FBIOPUTCMAP 命令设置 colormap；


• modelist:将var参数转化成video mode，然后存入这个链表；


• mode：一些时序，刷新率扫描方式(vmode)（隔行，逐行），极性(sync)；


• 关于背光CONFIG_FB_BACKLIGHT，有关于背光曲线以及背光设备注册，需要注意的是需要在注册framebuffer之前就对其初始化；


• CONFIG_FB_DEFERRED_IO，延迟IO，使用缺页中断的原理操作，减少FBIOPAN_DISPLAY带来的系统调用开支。


• fbops:提供具体的fb操作函数，主要是通过fbmem.c中提供的文件操作函数，间接调用fb_ops,主要的操作有fb_check_var，fb_pan_display，fb_mmap，等，以下三个函数提供了绘图的操作，可以使用系统中的绘图函数，也可以重写硬件加速的绘图函数；


• device:fb_info的设备父节点，对应即sys/device/xxx/fb_info；


• dev：设备指针，注册framebuffer时创建；


• pseudo_palette：伪调色板；


• state：硬件状态，在fbmem中会设置成suspend以及resume；


• skip_vt_switch:关于VT switch，是与console切换以及PM相关的；

3.2 fb_info标志位

fb_info标志位定义如下：

/* FBINFO_* = fb_info.flags bit flags */
#define FBINFO_DEFAULT 0
#define FBINFO_HWACCEL_DISABLED 0x0002
/* When FBINFO_HWACCEL_DISABLED is set:
* Hardware acceleration is turned off. Software implementations
* of required functions (copyarea(), fillrect(), and imageblit())
* takes over; acceleration engine should be in a quiescent state */
/* hints */
#define FBINFO_VIRTFB 0x0004 /* FB is System RAM, not device. */
#define FBINFO_PARTIAL_PAN_OK 0x0040 /* otw use pan only for double-buffering */
#define FBINFO_READS_FAST 0x0080 /* soft-copy faster than rendering */
/* hardware supported ops */
/* semantics: when a bit is set, it indicates that the operation is
* accelerated by hardware.
* required functions will still work even if the bit is not set.
* optional functions may not even exist if the flag bit is not set.
*/
#define FBINFO_HWACCEL_NONE 0x0000
#define FBINFO_HWACCEL_COPYAREA 0x0100 /* required */
#define FBINFO_HWACCEL_FILLRECT 0x0200 /* required */
#define FBINFO_HWACCEL_IMAGEBLIT 0x0400 /* required */
#define FBINFO_HWACCEL_ROTATE 0x0800 /* optional */
#define FBINFO_HWACCEL_XPAN 0x1000 /* optional */
#define FBINFO_HWACCEL_YPAN 0x2000 /* optional */
#define FBINFO_HWACCEL_YWRAP 0x4000 /* optional */
#define FBINFO_MISC_USEREVENT 0x10000 /* event request
from userspace */
#define FBINFO_MISC_TILEBLITTING 0x20000 /* use tile blitting */
/* A driver may set this flag to indicate that it does want a set_par to be
* called every time when fbcon_switch is executed. The advantage is that with
* this flag set you can really be sure that set_par is always called before
* any of the functions dependent on the correct hardware state or altering
* that state, even if you are using some broken X releases. The disadvantage
* is that it introduces unwanted delays to every console switch if set_par
* is slow. It is a good idea to try this flag in the drivers initialization
* code whenever there is a bug report related to switching between X and the
* framebuffer console.
*/
#define FBINFO_MISC_ALWAYS_SETPAR 0x40000
/* where the fb is a firmware driver, and can be replaced with a proper one */
#define FBINFO_MISC_FIRMWARE 0x80000
/*
* Host and GPU endianness differ.
*/
#define FBINFO_FOREIGN_ENDIAN 0x100000
/*
* Big endian math. This is the same flags as above, but with different
* meaning, it is set by the fb subsystem depending FOREIGN_ENDIAN flag
* and host endianness. Drivers should not use this flag.
*/
#define FBINFO_BE_MATH 0x100000
/*
* Hide smem_start in the FBIOGET_FSCREENINFO IOCTL. This is used by modern DRM
* drivers to stop userspace from trying to share buffers behind the kernel's
* back. Instead dma-buf based buffer sharing should be used.
*/
#define FBINFO_HIDE_SMEM_START 0x200000

3.3 fb_ops 

fb_ops里存放时的framebuffer设备操作函数：

/*
* Frame buffer operations
*
* LOCKING NOTE: those functions must _ALL_ be called with the console
* semaphore held, this is the only suitable locking mechanism we have
* in 2.6. Some may be called at interrupt time at this point though.
*
* The exception to this is the debug related hooks. Putting the fb
* into a debug state (e.g. flipping to the kernel console) and restoring
* it must be done in a lock-free manner, so low level drivers should
* keep track of the initial console (if applicable) and may need to
* perform direct, unlocked hardware writes in these hooks.
*/
struct fb_ops {
/* open/release and usage marking */
struct module *owner;
int (*fb_open)(struct fb_info *info, int user);
int (*fb_release)(struct fb_info *info, int user);
/* For framebuffers with strange non linear layouts or that do not
* work with normal memory mapped access
*/
ssize_t (*fb_read)(struct fb_info *info, char __user *buf,
size_t count, loff_t *ppos);
ssize_t (*fb_write)(struct fb_info *info, const char __user *buf,
size_t count, loff_t *ppos);
/* checks var and eventually tweaks it to something supported,
* DO NOT MODIFY PAR */
int (*fb_check_var)(struct fb_var_screeninfo *var, struct fb_info *info);
/* set the video mode according to info->var */
int (*fb_set_par)(struct fb_info *info);
/* set color register */
int (*fb_setcolreg)(unsigned regno, unsigned red, unsigned green,
unsigned blue, unsigned transp, struct fb_info *info);
/* set color registers in batch */
int (*fb_setcmap)(struct fb_cmap *cmap, struct fb_info *info);
/* blank display */
int (*fb_blank)(int blank, struct fb_info *info);
/* pan display */
int (*fb_pan_display)(struct fb_var_screeninfo *var, struct fb_info *info);
/* Draws a rectangle */
void (*fb_fillrect) (struct fb_info *info, const struct fb_fillrect *rect);
/* Copy data from area to another */
void (*fb_copyarea) (struct fb_info *info, const struct fb_copyarea *region);
/* Draws a image to the display */
void (*fb_imageblit) (struct fb_info *info, const struct fb_image *image);
/* Draws cursor */
int (*fb_cursor) (struct fb_info *info, struct fb_cursor *cursor);
/* wait for blit idle, optional */
int (*fb_sync)(struct fb_info *info);
/* perform fb specific ioctl (optional) */
int (*fb_ioctl)(struct fb_info *info, unsigned int cmd,
unsigned long arg);
/* Handle 32bit compat ioctl (optional) */
int (*fb_compat_ioctl)(struct fb_info *info, unsigned cmd,
unsigned long arg);
/* perform fb specific mmap */
int (*fb_mmap)(struct fb_info *info, struct vm_area_struct *vma);
/* get capability given var */
void (*fb_get_caps)(struct fb_info *info, struct fb_blit_caps *caps,
struct fb_var_screeninfo *var);
/* teardown any resources to do with this framebuffer */
void (*fb_destroy)(struct fb_info *info);
/* called at KDB enter and leave time to prepare the console */
int (*fb_debug_enter)(struct fb_info *info);
int (*fb_debug_leave)(struct fb_info *info);
};

三、fbmem.c源码分析

fbmem.c是framebuffer设备驱动的核心，它向上给应用程序提供了系统调用的接口，向下对特定的硬件提供了底层的驱动接口，底层驱动可以通过接口向内核注册自己。

fbmem.c位于drivers/video/fbdev/core路径下。我们可以在该文件定位到驱动模块的入口和出口：

module_init(fbmem_init);
module_exit(fbmem_exit);

3.1 入口函数

我们定位到fbmem.c的入口函数，也就是fbmem_init：

/**
* fbmem_init - init frame buffer subsystem
*
* Initialize the frame buffer subsystem.
*
* NOTE: This function is _only_ to be called by drivers/char/mem.c.
*
*/
static int __init
fbmem_init(void)
{
int ret;
if (!proc_create_seq("fb", 0, NULL, &proc_fb_seq_ops))
return -ENOMEM;
ret = register_chrdev(FB_MAJOR, "fb", &fb_fops);
if (ret) {
printk("unable to get major %d for fb devs\n", FB_MAJOR);
goto err_chrdev;
}
fb_class = class_create(THIS_MODULE, "graphics");
if (IS_ERR(fb_class)) {
ret = PTR_ERR(fb_class);
pr_warn("Unable to create fb class; errno = %d\n", ret);
fb_class = NULL;
goto err_class;
}
fb_console_init();
return 0;
err_class:
unregister_chrdev(FB_MAJOR, "fb");
err_chrdev:
remove_proc_entry("fb", NULL);
return ret;
}

简要分析一下该函数执行流程：

• 创建/proc/fb文件；

  
  



• 创建字符设备fb，主设备编号为FB_MAJOR(29)，注册file_operations结构体fb_fops；

  
  


• 调用class_create在/sys/class目录下创建graphics这个类，但是此时并没有调用device_create在/dev下创建设备节点 ；

可以通过如下命令查看字符设备：

root@zhengyang:/work/sambashare/linux-5.2.8# cat /proc/devices
Character devices:
1 mem
4 /dev/vc/0
4 tty
4 ttyS
5 /dev/tty
5 /dev/console
5 /dev/ptmx
5 ttyprintk
6 lp
7 vcs
10 misc
13 input
14 sound/midi
14 sound/dmmidi
21 sg
29 fb // 这个名字来自register_chrdev函数第二个参数
89 i2c

可以看到，确实是创建了主设备号为29的"fb"设备，而这里还没有创建设备节点，后面会提到，内核将该工作放到注册lcd驱动的接口函数里了。

3.2 fb_fops

我们再来看看file_operations结构体fb_fops：

static const struct file_operations fb_fops = {
.owner = THIS_MODULE,
.read = fb_read,
.write = fb_write,
.unlocked_ioctl = fb_ioctl,
#ifdef CONFIG_COMPAT
.compat_ioctl = fb_compat_ioctl,
#endif
.mmap = fb_mmap,
.open = fb_open,
.release = fb_release,
#if defined(HAVE_ARCH_FB_UNMAPPED_AREA) || \
(defined(CONFIG_FB_PROVIDE_GET_FB_UNMAPPED_AREA) && \
!defined(CONFIG_MMU))
.get_unmapped_area = get_fb_unmapped_area,
#endif
#ifdef CONFIG_FB_DEFERRED_IO
.fsync = fb_deferred_io_fsync,
#endif
.llseek = default_llseek,
};

下面我们来一一分析这些成员函数。

3.3 fb_open

static int
fb_open(struct inode *inode, struct file *file)
__acquires(&info->lock)
__releases(&info->lock)
{
int fbidx = iminor(inode); // 获取设备节点的次设备号
struct fb_info *info; // 定义fb_info指针
int res = 0;
info = get_fb_info(fbidx); // 根据次设备编号获取lcd驱动信息
if (!info) {
request_module("fb%d", fbidx);
info = get_fb_info(fbidx);
if (!info)
return -ENODEV;
}
if (IS_ERR(info))
return PTR_ERR(info);
mutex_lock(&info->lock); // 获取互斥锁
if (!try_module_get(info->fbops->owner)) {
res = -ENODEV;
goto out;
}
file->private_data = info;
if (info->fbops->fb_open) {
res = info->fbops->fb_open(info,1);
if (res)
module_put(info->fbops->owner);
}
#ifdef CONFIG_FB_DEFERRED_IO
if (info->fbdefio)
fb_deferred_io_open(info, inode, file);
#endif
out:
mutex_unlock(&info->lock); // 释放互斥锁
if (res)
put_fb_info(info);
return res;
}

这里我们来看一下get_fb_info函数的实现：

static struct fb_info *get_fb_info(unsigned int idx)
{
struct fb_info *fb_info;
if (idx >= FB_MAX)
return ERR_PTR(-ENODEV);
mutex_lock(&registration_lock);
fb_info = registered_fb[idx];
if (fb_info)
atomic_inc(&fb_info->count);
mutex_unlock(&registration_lock);
return fb_info;
}

可以看到get_fb_info函数将registered_fb数组的第idx个元素赋值给了fb_info，registered_fb是一个struct fb_info结构类型的全局数组：

struct fb_info *registered_fb[FB_MAX] __read_mostly;

这和数组会在register_framebuffer函数中赋值。

经过分析，我们最终会发现fb_open执行的是fb的操作函数中的fbopen函数。

3.4 fb_read

static ssize_t
fb_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
struct fb_info *info = file_fb_info(file);
u8 *buffer, *dst;
u8 __iomem *src;
int c, cnt = 0, err = 0;
unsigned long total_size;
if (!info || ! info->screen_base)
return -ENODEV;
if (info->state != FBINFO_STATE_RUNNING)
return -EPERM;
if (info->fbops->fb_read)
return info->fbops->fb_read(info, buf, count, ppos);
total_size = info->screen_size;
if (total_size == 0)
total_size = info->fix.smem_len;
if (p >= total_size)
return 0;
if (count >= total_size)
count = total_size;
if (count + p > total_size)
count = total_size - p;
buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,
GFP_KERNEL);
if (!buffer)
return -ENOMEM;
src = (u8 __iomem *) (info->screen_base + p);
if (info->fbops->fb_sync)
info->fbops->fb_sync(info);
while (count) {
c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
dst = buffer;
fb_memcpy_fromfb(dst, src, c);
dst += c;
src += c;
if (copy_to_user(buf, buffer, c)) {
err = -EFAULT;
break;
}
*ppos += c;
buf += c;
cnt += c;
count -= c;
}
kfree(buffer);
return (err) ? err : cnt;
}

3.5 fb_write

static ssize_t
fb_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
{
unsigned long p = *ppos;
struct fb_info *info = file_fb_info(file);
u8 *buffer, *src;
u8 __iomem *dst;
int c, cnt = 0, err = 0;
unsigned long total_size;
if (!info || !info->screen_base)
return -ENODEV;
if (info->state != FBINFO_STATE_RUNNING)
return -EPERM;
if (info->fbops->fb_write)
return info->fbops->fb_write(info, buf, count, ppos);
total_size = info->screen_size;
if (total_size == 0)
total_size = info->fix.smem_len;
if (p > total_size)
return -EFBIG;
if (count > total_size) {
err = -EFBIG;
count = total_size;
}
if (count + p > total_size) {
if (!err)
err = -ENOSPC;
count = total_size - p;
}
buffer = kmalloc((count > PAGE_SIZE) ? PAGE_SIZE : count,
GFP_KERNEL);
if (!buffer)
return -ENOMEM;
dst = (u8 __iomem *) (info->screen_base + p);
if (info->fbops->fb_sync)
info->fbops->fb_sync(info);
while (count) {
c = (count > PAGE_SIZE) ? PAGE_SIZE : count;
src = buffer;
if (copy_from_user(src, buf, c)) {
err = -EFAULT;
break;
}
fb_memcpy_tofb(dst, src, c);
dst += c;
src += c;
*ppos += c;
buf += c;
cnt += c;
count -= c;
}
kfree(buffer);
return (cnt) ? cnt : err;
}

3.6 fb_ioctl

参考文章

[1]十二、Linux驱动之LCD驱动

[2]15.linux-LCD层次分析(详解)

[3]Linux LCD Frambuffer 基础介绍和使用（1）

[4]Linux驱动开发 （framebuffer驱动）

[5]Linux-FrameBuffer fb_info结构体解析申请以及注册