Re: Frame buffer / mmap() weirdness

On Tue, 30 Nov 1999, Stephen Edie wrote:

I have been noticing some strange behavior while working with Linux frame
buffer devices on PPC.  Perhaps I am just doing something stupid.  I open
the frame buffer using:

fbdev_handle = open("/dev/fb0", O_RDWR);

I can draw to the frame buffer using:

lseek(fbdev_handle, 0, SEEK_SET);
write(fbdev_handle, pixmap, size);

This part works fine.  What causes problems is attempting to memory map
the frame buffer using:

address = mmap(0, screen.height * screen.stride * screen.bpp / 8,
               PROT_READ | PROT_WRITE, MAP_SHARED, fbdev_handle, 0);

When doing this, address[0] points outside of the video memory and
strangely enough, address[512] points to the first pixel in memory!  So if
I do the following, everything works as expected:

address = (char *)(mmap(0, screen.height * screen.stride * screen.bpp / 8
                        + 512, PROT_READ | PROT_WRITE, MAP_SHARED,
                        fbdev_handle, 0)) + 512;

I don't think this is right!  Is this a kernel bug in the frame buffer
driver code?  (This is being tested using OFFB by the way)  Is there
something weird going on here with page alignment or something?

Perhaps the frame buffer memory is not aligned on a page boundary? What does
`fbset --info' say about the address?

If it's not aligned to a page boundary, the application has to mmap()
(address & 0xfffff000) and add (address & 0x00000fff) to the start address of
the returned region. Make sure to increase the size you pass to mmap(), so the
whole frame buffer memory is covered and size is a multiple of the page size.

Gr{oetje,eeting}s,
--
Geert Uytterhoeven -- Linux/{m68k~Amiga,PPC~CHRP} -- geert@linux-m68k.org

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


** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/