Waiting for display V-Sync not supported

RSchubert · Dec 10th 2012

When using double buffering and switching between the buffers it's nice to wait for the VSync.

Code

int ret=ioctl(fbfd,FBIO_WAITFORVSYNC,0);

returns 0xFFFFFFFF - EPERM (operation not permitted).

In the kernel sc3_fb.c driver there's a function to handle the VSync but it never gets called:

Code

static int s3c_fb_ioctl(struct fb_info *info, unsigned int cmd, unsigned long arg) {
struct s3c_fb_win *win = info->par;
struct s3c_fb *sfb = win->parent;
int ret;
u32 crtc;
switch (cmd) {
case FBIO_WAITFORVSYNC:
if (get_user(crtc, (u32 __user *)arg)) {
ret = -EFAULT;
break;
}
ret = s3c_fb_wait_for_vsync(sfb, crtc);
break;
default:
ret = -ENOTTY;
}
return ret;
}

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In assembly debugging ioctl is calling "svc 0x00000000" what is returning 0xFFFFFFF2 (EFAULT - bad address).

Thanks for help.

fs-support_HK · Dec 10th 2012

Quote from "RSchubert"

When using double buffering and switching between the buffers it's nice to wait for the VSync.

Code

int ret=ioctl(fbfd,FBIO_WAITFORVSYNC,0);

returns 0xFFFFFFFF - EPERM (operation not permitted).

Hmm, ioctl() returns -1 on error. So your 0xFFFFFFFF is not the error code, it is just the condition "error". The error number is in errno and should be -EFAULT. Because when looking at the source code that you kindly added, shouldn't the call look something like this?

Code

u32 arg = 0;
int ret=ioctl(fbfd, FBIO_WAITFORVSYNC, &arg);

RSchubert · Dec 10th 2012

Yes, you're right. My mistake. I did not check this, because this was working on PicoMOD7 with Kernel 2.6.29 in my version too.

Thanks a lot.

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