如何為幀緩衝區可載入核心模組分配記憶體（linux）

Question

我正在編寫一個“假”幀緩衝區設備，這樣我就可以使用我的電子閱讀器作為我的樹莓派的外部螢幕。電子閱讀器部分正常運作（https://github.com/lllandsmeer/inkvt），但現在我想讓它顯示為 Linux 的外部螢幕。

我雖然虛擬 Linux 幀緩衝區是“最簡單的”，但也許還有其他選項可以為 Linux 添加假螢幕（顯示在 xrandr 中）？

無論如何，這是當前的核心模組。它創建了一個/dev/fbX設備，在查詢後fbset似乎配置正確。但是，當我嘗試使用它時，它失敗了並且我從fbcat: Corrupted page table at address 7fd7d5c90000dmesg.

我認為這是因為我錯誤配置了記憶體區域（char mem[]）。我嘗試過使用kallocinstread，但這也不起作用。我該如何做事？

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fb.h>

#define WIDTH 256
#define HEIGHT 256

static char mem[WIDTH*HEIGHT*3];

static void op_fillrect(struct fb_info * info, const struct fb_fillrect * rect) {
    printk(KERN_ALERT "fillrect\n");
}

static void op_copyarea(struct fb_info * info, const struct fb_copyarea * area) {
    printk(KERN_ALERT "copyarea\n");
}

static void op_imageblit(struct fb_info * info, const struct fb_image * image) {
    printk(KERN_ALERT "imageblit\n");
}

static struct fb_info * info;

static struct fb_ops ops = {
    .owner = THIS_MODULE,
    .fb_fillrect = op_fillrect,
    .fb_copyarea = op_copyarea,
    .fb_imageblit = op_imageblit,
};

static const struct fb_fix_screeninfo fix = {
    .id = "LennartFB",
    .smem_start = (unsigned long)mem,
    .smem_len = sizeof(mem),
    .type = FB_TYPE_PACKED_PIXELS,
    .visual = FB_VISUAL_TRUECOLOR,
    .line_length = 256,
};

static const struct fb_var_screeninfo var = {
    .xres = WIDTH,
    .yres = HEIGHT,
    .xres_virtual = WIDTH,
    .yres_virtual = HEIGHT,
    .bits_per_pixel = 24,
    .height = 256,
    .width = 256,
    .vmode = FB_VMODE_NONINTERLACED,
    .red = { .length = 8, .offset = 16, },
    .green = { .length = 8, .offset = 8, },
    .blue = { .length = 8, .offset = 0, }
};

static int mod_init(void) {
    int err;
    info = framebuffer_alloc(0, 0);
    if (!info) return -1;
    info->fbops = &ops;
    info->fix = fix;
    info->var = var;
    info->screen_base = mem;
    err = fb_alloc_cmap(&info->cmap, 2, 0);
    if (err < 0) return -2;
    err = register_framebuffer(info);
    if (err < 0) return -3;
    return 0;
}

static void mod_exit(void) {
    return;
}

module_init(mod_init);
module_exit(mod_exit);
MODULE_LICENSE("GPL v3");

訊息：

[ 4984.929380] fbcat: Corrupted page table at address 7fd7d5c90000
[ 4984.929385] PGD 8000000277709067 P4D 8000000277709067 PUD 3eb108067 PMD 2c732e067 PTE 800fffffc0f5d235
[ 4984.929391] Bad pagetable: 000d [#1] SMP PTI
[ 4984.929396] CPU: 11 PID: 8076 Comm: fbcat Tainted: P           OE     5.3.0-46-generic #38~18.04.1-Ubuntu
[ 4984.929398] Hardware name: Acer Aspire A715-72G/Charmeleon_CFS, BIOS V1.19 07/13/2018
[ 4984.929403] RIP: 0033:0x55fa6cb22e98