Re: Gives Bus error in memcpy() in coping /dev/fb0 to a file.

["Antonino A. Daplas" <adaplas@gmail.com>]

On Thu, 2007-03-08 at 17:39 +0000, Nuno Lucas wrote:
> On 3/8/07, Antonino A. Daplas <adaplas@gmail.com> wrote:
> > On Wed, 2007-03-07 at 17:20 +0530, Vishal Soni wrote:
> > > hello,
> > >
> > > I am trying to get the snapshot of the screen through a c program by mmaping
> > > the framebuffer device /dev/fb0 .
> > > For this i compiled kernel with virtual framebuffer support and passing
> > > vga=791 as boot line argument.
> > >
> > > Though i am able to get snapshot by
> > > [root@tw pcapp]# cat /dev/fb0 > frame
> > >
> > > and see it on the screen by
> > > [root@tw pcapp]# cat frame > /dev/fb0
> > >
> > > But when i try to capture the frame buffer using the below c code, It gives
> > > bus error @ memcpy()
> > >
> > > Output :::::::::
> > >
> > > [root@tw pcapp]# ./pcapp
> > > 1024x768, 16bpp
> > > framebuffer device mapped to memory @ 0xb7db3000.
> > > FB_FRAME mapped to memory @ 0xb7c33000.
> > > Bus error
> >
> > I don't think vfb's framebuffer can be mmapped.  It's fixable, but
> > nobody seems to have a need for it,  yet.
> 
> Having read about the Hecuba driver, using the deferred IO patch,
> isn't that a better starting point for a virtual fb driver? One could
> just ignore the deferred IO and have an instant virtual frame buffer.
> With just a little bit of effort and one could even make a
> para-virtual driver for Xen and/or KVM.
> 
> I'm just a curious soul lying on this list but would appreciate
> someone to correct my thinking.
> 

Yes, something like that without the mkwrite method.

The classic method was to use get_free_pages to allocate the memory and
mark the range as reserved and io.

Tony


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