无法在嵌入式 Linux 中使用 libjpeg 以 RGB888 在 LCD(黑色)上显示 JPEG

I am working on an embedded Linux system (kernel-5.10.24), there is a LCD display in it.
Now I am trying to show a JPEG picture on the LCD by using libjpeg.
I found an example code on the internet and tried it out in my system.
I found the display is showing a black rectangle when I chose to use RGB888 mode.

下面是获取RGB数据并将其放入帧缓冲存储器的主要代码.

typedef unsigned int color_t;
///typedef unsigned short color_t;

#define BITS_PER_PIXEL    32
///#define BITS_PER_PIXEL    16

static struct fb_var_screeninfo __g_vinfo;
static color_t *__gp_frame = NULL;

.....
    ioctl(fd, FBIOGET_VSCREENINFO, &__g_vinfo);
    printf("bits_per_pixel = %d\n", __g_vinfo.bits_per_pixel);
    printf("xres_virtual = %d\n", __g_vinfo.xres_virtual);
    printf("yres_virtual = %d\n", __g_vinfo.yres_virtual);
    printf("xres = %d\n", __g_vinfo.xres);
    printf("yres = %d\n", __g_vinfo.yres);

    __gp_frame = mmap(NULL, __g_vinfo.xres_virtual * __g_vinfo.yres_virtual * 
__g_vinfo.bits_per_pixel / 8, PROT_WRITE | PROT_READ, MAP_SHARED, fd, 0);
......
......
int drawjpg(unsigned int x, unsigned int y, const char *name)
{
    color_t col;

    row_stride = cinfo.output_width * cinfo.output_components;
    printf("width  = %d\n", cinfo.output_width);
    printf("height  = %d\n", cinfo.output_height);
    buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);

    while (cinfo.output_scanline < cinfo.output_height)
    {
        jpeg_read_scanlines(&cinfo, buffer, 1);
        line = cinfo.output_scanline - 1 + y;
        for(i = 0; i < cinfo.output_width; i++) {
#if (BITS_PER_PIXEL == 16)
            char b, g, r;
            r = (buffer[0][i*3] >> 3);
            g = ((buffer[0][i*3+1] >> 2));
            b = (buffer[0][i*3+2] >> 3);

            col = (r << 11) | (g << 5) | b;
#elif (BITS_PER_PIXEL == 32)
            col = (buffer[0][i*3] << 16) | (buffer[0][i*3+1] << 8) | buffer[0][i*3+2];
#endif
            *(__gp_frame + line * __g_vinfo.xres + i + x) = col;
        }
        if (line >= __g_vinfo.yres) {
            break;
        }
    }
.....
}

这是我在系统中运行它时得到的结果.

bits_per_pixel = 32
xres_virtual = 720
yres_virtual = 3840
xres = 720
yres = 1280
cinfo.image_width = 442
cinfo.image_height = 442
cinfo.jpeg_color_space = 3
cinfo.num_components = 3
-------------
width  = 442
height  = 442

LCD显示屏上出现了一个黑色矩形，而不是我要显示的图片.

I tried to use #define BITS_PER_PIXEL 16, there are 2 pictures shown in the display. But with 32 there is nothing shown.
With a limited knowledge on framebuffer programing, I think there might be something wrong in RGB data processing.
Since the display supports ARGB, so I changed to use col = 0x0F000000 | (buffer[0][i*3] << 16) | (buffer[0][i*3+1] << 8) | buffer[0][i*3+2];, and got the same result.

What is wrong with the code when in 32 bit mode?
** I think this question is about programing, and should NOT be closed **