mmap /dev/fb0 fails with "Invalid argument" - linux

Question

I have an embedded system and want to use /dev/fb0 directly. As a first test, I use some code based on example-code found everywhere in the net and SO. Opening succeeds, also fstat and similar. But mmap fails with EINVAL.
Source:
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
int main() {
int fbfd = 0;
struct fb_var_screeninfo vinfo;
struct fb_fix_screeninfo finfo;
long int screensize = 0;
char *fbp = 0;
int x = 0, y = 0;
long int location = 0;
// Open the file for reading and writing
fbfd = open("/dev/fb0", O_RDWR);
if (fbfd == -1) {
perror("Error: cannot open framebuffer device");
exit(1);
}
printf("The framebuffer device was opened successfully.\n");
struct stat stat;
fstat(fbfd, &stat);
printf("/dev/mem -> size: %u blksize: %u blkcnt: %u\n",
stat.st_size, stat.st_blksize, stat.st_blocks);
// Get fixed screen information
if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo) == -1) {
perror("Error reading fixed information");
exit(2);
}
// Get variable screen information
if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo) == -1) {
perror("Error reading variable information");
exit(3);
}
printf("%dx%d, %dbpp\n", vinfo.xres, vinfo.yres, vinfo.bits_per_pixel);
// Figure out the size of the screen in bytes
screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
const int PADDING = 4096;
int mmapsize = (screensize + PADDING - 1) & ~(PADDING-1);
// Map the device to memory
fbp = (char *)mmap(0, mmapsize, PROT_READ | PROT_WRITE, MAP_SHARED, fbfd, 0);
if ((int)fbp == -1) {
perror("Error: failed to map framebuffer device to memory");
exit(4);
}
printf("The framebuffer device was mapped to memory successfully.\n");
munmap(fbp, screensize);
close(fbfd);
return 0;
}
Output:
The framebuffer device was opened successfully.
/dev/mem -> size: 0 blksize: 4096 blkcnt: 0
640x480, 4bpp
Error: failed to map framebuffer device to memory: Invalid argument
strace:
...
open("/dev/fb0", O_RDWR) = 3
write(1, "The framebuffer device was opene"..., 48The framebuffer device was opened successfully.
) = 48
fstat64(3, {st_mode=S_IFCHR|0640, st_rdev=makedev(29, 0), ...}) = 0
write(1, "/dev/mem -> size: 0 blksize: 409"..., 44/dev/mem -> size: 0 blksize: 4096 blkcnt: 0
) = 44
ioctl(3, FBIOGET_FSCREENINFO or FBIOPUT_CONTRAST, 0xbfca6564) = 0
ioctl(3, FBIOGET_VSCREENINFO, 0xbfca6600) = 0
write(1, "640x480, 4bpp\n", 14640x480, 4bpp
) = 14
old_mmap(NULL, 155648, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = -1 EINVAL (Invalid argument)
write(2, "Error: failed to map framebuffer"..., 49Error: failed to map framebuffer device to memory) = 49
write(2, ": ", 2: ) = 2
write(2, "Invalid argument", 16Invalid argument) = 16
write(2, "\n", 1
) = 1
The boot-screen with console and tux is visible. And cat /dev/urandom > /dev/fb0 fills the screen with noise. The pagesize is 4096 on the system (`getconf PAGESIZE). So, 155648 (0x26000) is a multiple. Offset and pointer are both zero. Mapping and filemode are both RW .. what am I missing?
This is for an embedded device build with uClibc and busybox running a single application and I have to port it from an ancient kernel. There is code for linedrawing and such and no need for multiprocessing/ windowing .. please no hints to directfb ;).

Answer 1

The kernel driver that presents the framebuffer doesn't support the legacy direct mmap() of the framebuffer device; you need to use the newer DRM and KMS interface.