On other Linux machines using the FBDEV drivers ( Raspberry Pi.. etc. ), I could mmap the /dev/fb0 device and directly create a BMP file that saved what was on the screen.
Now, I am trying to do the same thing with DRM on a TI Sitara AM57XX ( Beagleboard X-15 ). The code that used to work with FBDEV is shown below.
This mmap no longer seems to work the DRM. I'm using a very simple Qt5 Application with the Qt platform linuxfb plugin. It draws just fine into /dev/fb0 and shows on the screen properly, however I cannot read back from /dev/fb0 with a memory mapped pointer and have an image of the screen saved to file. It looks garbled like this:
Code:
#ifdef FRAMEBUFFER_CAPTURE
repaint();
QCoreApplication::processEvents();
// Setup framebuffer to desired format
struct fb_var_screeninfo var;
struct fb_fix_screeninfo finfo;
memset(&finfo, 0, sizeof(finfo));
memset(&var, 0, sizeof(var));
/* Get variable screen information. Variable screen information
* gives information like size of the image, bites per pixel,
* virtual size of the image etc. */
int fbFd = open("/dev/fb0", O_RDWR);
int fdRet = ioctl(fbFd, FBIOGET_VSCREENINFO, &var);
if (fdRet < 0) {
qDebug() << "Error opening /dev/fb0!";
close(fbFd);
return -1;
}
if (ioctl(fbFd, FBIOPUT_VSCREENINFO, &var)<0) {
qDebug() << "Error setting up framebuffer!";
close(fbFd);
return -1;
} else {
qDebug() << "Success setting up framebuffer!";
}
//Get fixed screen information
if (ioctl(fbFd, FBIOGET_FSCREENINFO, &finfo) < 0) {
qDebug() << "Error getting fixed screen information!";
close(fbFd);
return -1;
} else {
qDebug() << "Success getting fixed screen information!";
}
//int screensize = var.xres * var.yres * var.bits_per_pixel / 8;
//int screensize = var.yres_virtual * finfo.line_length;
//int screensize = finfo.smem_len;
int screensize = finfo.line_length * var.yres_virtual;
qDebug() << "Framebuffer size is: " << var.xres << var.yres << var.bits_per_pixel << screensize;
int linuxFbWidth = var.xres;
int linuxFbHeight = var.yres;
int location = (var.xoffset) * (var.bits_per_pixel/8) +
(var.yoffset) * finfo.line_length;
// Perform memory mapping of linux framebuffer
char* frameBufferMmapPixels = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED, fbFd, 0);
assert(frameBufferMmapPixels != MAP_FAILED);
QImage toSave((uchar*)frameBufferMmapPixels,linuxFbWidth,linuxFbHeight,QImage::Format_ARGB32);
toSave.save("/usr/bin/test.bmp");
sync();
#endif
Here is the output of the code when it runs:
Success setting up framebuffer! Success getting fixed screen information! Framebuffer size is: 800 480 32 1966080
Here is the output of fbset showing the pixel format:
mode "800x480" geometry 800 480 800 480 32 timings 0 0 0 0 0 0 0 accel true rgba 8/16,8/8,8/0,8/24 endmode root@am57xx-evm:~#
