Re: [Linux-fbdev-devel] Query:Regarding Virtual frame buffer

[Geert U. <ge...@li...>]

On Fri, 6 Apr 2001, Bipin T S wrote:
> >  We are using a Walnut board with a PPC processor. We don't have a
> graphics chip/card in this board and so we planned to use the virtual
> > frame buffer concept for debugging and testing the frame buffer
> system. We build the kernel with the virtual frame buffer enabled in
> Console
> > drivers and we gave video = vfb:enable (Thanks to Simmons)  in the
> command line options.
> 
> 1) We tried giving echo "Hello World" > /dev/fb0. It works fine.

So fbmem.c:fb_write() works, and info->disp->screen_base is the correct virtual
address of the frame buffer.

> 2) We wrote an application that mmaps the /dev/fb0 into user memory.
> This works fine and mmap() returns a valid address
> 
>         fb = open("/dev/fb0", O_RDWR);
>         addr =(unsigned char*)  mmap(NULL, size, PROT_READ|PROT_WRITE,
> MAP_SHARED, fb, 0);
> 
> 3) But when we try to write something into the MAPPED address, we get a
> segmentation fault!!
>             *( addr + 10) = 20;
> 
> We would like to know the reason for the above behaviour. Pointers and
> references will be appreciated

So fbmem.c:fb_mmap() doesn't work. This means either fix.smem_start is not the
correct physical address of the frame buffer, or io_remap_page_range() contains
a bug. The Walnut has a PPC 8xx CPU, IIRC?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@li...

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds