Linux驱动-LCD驱动框架

2024年5月 · 预计阅读时间： 4 分钟

从应用视角到内核，看LCD驱动程序#

整体流程如图：

查看qemu中百问网编写的fb_test程序。

fb-test.c

1.APP-先看main函数#

static struct fb_info fb_info;

int main(int argc, char **argv)
{
    int opt;
    int req_fb = 0;
    int req_pattern = 0;

    printf("fb-test %d.%d.%d (%s)\n", VERSION, PATCHLEVEL, SUBLEVEL,
        VERSION_NAME);
    
    ...

    fb_open(req_fb, &fb_info);

    do_fill_screen(&fb_info, req_pattern);

    return 0;
}

先调用fb_open打开frame buffer，传入默认0，表示打开fb0，返回一个fb_info结构体。获得LCD的信息后，就可以操作屏幕了

2.APP-`fb_open`，获取`fb_info`#

应用程序open、ioctl、mmap#

定义在common.c中

void fb_open(int fb_num, struct fb_info *fb_info)
{
    char str[64];
    int fd,tty;

    tty = open("/dev/tty1", O_RDWR);

    if(ioctl(tty, KDSETMODE, KD_GRAPHICS) == -1)
        printf("Failed to set graphics mode on tty1\n");

    sprintf(str, "/dev/fb%d", fb_num);
    fd = open(str, O_RDWR);

    ASSERT(fd >= 0);

    fb_info->fd = fd;
    IOCTL1(fd, FBIOGET_VSCREENINFO, &fb_info->var);
    IOCTL1(fd, FBIOGET_FSCREENINFO, &fb_info->fix);

    printf("fb res %dx%d virtual %dx%d, line_len %d, bpp %d\n",
            fb_info->var.xres, fb_info->var.yres,
            fb_info->var.xres_virtual, fb_info->var.yres_virtual,
            fb_info->fix.line_length, fb_info->var.bits_per_pixel);

    void *ptr = mmap(0,
            fb_info->var.yres_virtual * fb_info->fix.line_length,
            PROT_WRITE | PROT_READ,
            MAP_SHARED, fd, 0);

    ASSERT(ptr != MAP_FAILED);

    fb_info->ptr = ptr;
}

主要包括以下三个步骤：

fd = open(str, O_RDWR) -> open("/dev/fb0", O_RDWR)调用系统调用-open打开fb0
IOCTRL1 -> ioctl获取LCD信息
#define IOCTL1(fd, ctl, arg1) if (ioctl(fd, ctl, arg1))\
{ perror("ioctl1(" __FILE__ ":" TOSTRING(__LINE__) "): "); exit(1); }
mmap虚拟内存映射：应用程序中需要映射一块虚拟内存到显存的物理地址
```c
void ptr = mmap(0, fb_info->var.yres_virtual fb_info->fix.line_length, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);
```
接下来，将详细解析以上系统调用的过程。

2.1 驱动框架中`file_operations`的`fb_open`#

APP:

fd = open(str, O_RDWR);
=> fd = open("/dev/fb0", O_RDWR);

在fbmem.c，提供了frame buffer操作的框架，就是完成字符设备驱动程序的基本流程，然后再去调用下层的具体单板驱动程序。

static int
fb_open(struct inode *inode, struct file *file)
__acquires(&info->lock)
__releases(&info->lock)
{
    int fbidx = iminor(inode);
    struct fb_info *info;
    int res = 0;

    info = get_fb_info(fbidx);
    if (!info) {
        request_module("fb%d", fbidx);
        info = get_fb_info(fbidx);
        if (!info)
            return -ENODEV;
    }
    ...
    file->private_data = info;
    if (info->fbops->fb_open) {
        res = info->fbops->fb_open(info,1);
        if (res)
            module_put(info->fbops->owner);
    }
    ...
out:
    mutex_unlock(&info->lock);
    if (res)
        put_fb_info(info);
    return res;
}

info = get_fb_info(fbidx);通过传入的次设备号获取显存信息，实际是通过一个数组registered_fb获得，注册过程看下面第4小节。
static struct fb_info *get_fb_info(unsigned int idx)
{
struct fb_info *fb_info;

...
fb_info = registered_fb[idx];
if (fb_info)
atomic_inc(&fb_info->count);
...

return fb_info;
}
在打开过程中，也会把info放入到file中，file->private_data = info;，后面会用到

2.2 驱动框架中`file_operations`的`fb_ioctl`#

IOCTL1(fd, FBIOGET_VSCREENINFO, &fb_info->var);

应用程序的ioctl会调用到驱动的fb_ioctl

fbmem.c

// 通过file获取fb_info
static struct fb_info *file_fb_info(struct file *file)
{
    struct inode *inode = file_inode(file);
    int fbidx = iminor(inode);
    struct fb_info *info = registered_fb[fbidx];

    if (info != file->private_data)
        info = NULL;
    return info;
}

static long fb_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
    struct fb_info *info = file_fb_info(file);

    if (!info)
        return -ENODEV;
    return do_fb_ioctl(info, cmd, arg);
}

static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
            unsigned long arg)
{
    struct fb_ops *fb;
    struct fb_var_screeninfo var;
    struct fb_fix_screeninfo fix;
    struct fb_con2fbmap con2fb;
    struct fb_cmap cmap_from;
    struct fb_cmap_user cmap;
    struct fb_event event;
    void __user *argp = (void __user *)arg;
    long ret = 0;

    switch (cmd) {
    case FBIOGET_VSCREENINFO:
        if (!lock_fb_info(info))
            return -ENODEV;
        var = info->var;
        unlock_fb_info(info);

        ret = copy_to_user(argp, &var, sizeof(var)) ? -EFAULT : 0;
        break;
    ...
    case FBIOGET_FSCREENINFO:
        if (!lock_fb_info(info))
            return -ENODEV;
        fix = info->fix;
        unlock_fb_info(info);

        ret = copy_to_user(argp, &fix, sizeof(fix)) ? -EFAULT : 0;
        break;
    ...
    return ret;
}

这里就是先从file中获得fb_info，然后传入do_fb_ioctl，根据不同的ioctrl命令cmd，获取信息。如FBIOGET_VSCREENINFO就是将LCD的可变参数信息var，拷贝到用户空间的arg.

2.3 驱动框架中`file_operations`的`fb_mmap`#

void *ptr = mmap(0,
            fb_info->var.yres_virtual * fb_info->fix.line_length,
            PROT_WRITE | PROT_READ,
            MAP_SHARED, fd, 0);

同样的，应用程序的mmap也会调用到驱动程序中的fb_mmap

static int
fb_mmap(struct file *file, struct vm_area_struct * vma)
{
    struct fb_info *info = file_fb_info(file);
    struct fb_ops *fb;
    unsigned long mmio_pgoff;
    unsigned long start;
    u32 len;

    fb = info->fbops;
    
    mutex_lock(&info->mm_lock);
    ... //这里会调用具体单板的fb_ops，我们没有提供

    /*
     * Ugh. This can be either the frame buffer mapping, or
     * if pgoff points past it, the mmio mapping.
     */
    start = info->fix.smem_start;
    len = info->fix.smem_len;
    ...
    mutex_unlock(&info->mm_lock);

    vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
    fb_pgprotect(file, vma, start);

    return vm_iomap_memory(vma, start, len);
}

可以看到最后就是调用内核函数vm_iomap_memory

vm_iomap_memory是Linux内核中用于映射设备内存到用户空间的函数。它允许用户空间程序直接访问设备内存，而不需要通过内核态的中间层。

3.具体LCD驱动中，注册`fb_info`过程#

既然上面fb_open是通过获取一个数组的某一项获得fb_info，那么一开始LCD初始化就会注册这个fb_info到数组中。比如，我们编写的imx6ull的LCD驱动，在装载module的时候，就会分配/配置/注册fb_info。

在下层实现的具体单板的LCD驱动中，会注册到上层框架fbmem.c

int __init (void)
{
    dma_addr_t phy_addr;

    /* 1.分配fb_info */
    myfb_info = framebuffer_alloc(0, NULL);

    /* 2.设置fb_info */
    // a.var : LCD分辨率(虚拟分辨率、物理分辨率)、颜色格式
    ...

    // b.fix
    ...

    // c.fbops
    ...

    /* 3.注册fb_info */
    register_framebuffer(myfb_info);

    /* 4.硬件操作 */
    ...

    return 0;
}

此函数上一节编程已经看过了。这里主要看register_framebuffer

int
register_framebuffer(struct fb_info *fb_info)
{
    int ret;

    mutex_lock(&registration_lock);
    ret = do_register_framebuffer(fb_info);
    mutex_unlock(&registration_lock);

    return ret;
}

调用do_register_framebuffer

static int do_register_framebuffer(struct fb_info *fb_info)
{
    int i, ret;
    struct fb_event event;
    struct fb_videomode mode;

    ...

    num_registered_fb++;
    //找到数组的空位置
    for (i = 0 ; i < FB_MAX; i++)
        if (!registered_fb[i])
            break;
    fb_info->node = i;
    atomic_set(&fb_info->count, 1);
    mutex_init(&fb_info->lock);
    mutex_init(&fb_info->mm_lock);
    // 创建一个对应的具体设备节点，供应用程序访问
    fb_info->dev = device_create(fb_class, fb_info->device,
                     MKDEV(FB_MAJOR, i), NULL, "fb%d", i);
    ...

    if (fb_info->pixmap.addr == NULL) {
        fb_info->pixmap.addr = kmalloc(FBPIXMAPSIZE, GFP_KERNEL);
        if (fb_info->pixmap.addr) {
            fb_info->pixmap.size = FBPIXMAPSIZE;
            fb_info->pixmap.buf_align = 1;
            fb_info->pixmap.scan_align = 1;
            fb_info->pixmap.access_align = 32;
            fb_info->pixmap.flags = FB_PIXMAP_DEFAULT;
        }
    }   
    fb_info->pixmap.offset = 0;

    ... // 添加各种信息
    registered_fb[i] = fb_info;     //把当前的fb_info放入数组第i项

    ...

    return 0;
}

近期文章

从应用视角到内核，看LCD驱动程序#

1.APP-先看main函数#

2.APP-fb_open，获取fb_info#

应用程序open、ioctl、mmap#

2.1 驱动框架中file_operations的fb_open#

2.2 驱动框架中file_operations的fb_ioctl#

2.3 驱动框架中file_operations的fb_mmap#

3.具体LCD驱动中，注册fb_info过程#

2.APP-`fb_open`，获取`fb_info`#

2.1 驱动框架中`file_operations`的`fb_open`#

2.2 驱动框架中`file_operations`的`fb_ioctl`#

2.3 驱动框架中`file_operations`的`fb_mmap`#

3.具体LCD驱动中，注册`fb_info`过程#