Re: ioremap problem on highmem

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Mon, 13 Oct 2003, Jon Smirl wrote:
> Fix is not that simple. There is a basic conflict with mapping framebuffers
> into the kernel address space and the kernel's need of that address space for
> other purposes. Drivers need to be rewritten to do one of:
> 
> 1) Map the frame buffer into the kernel address space a few pages at a time.
> This is how kernel high mem support works.
> 2) Map the minimal amount of address space necessary for visible framebuffer.
> This is what the VGA driver does. It works most of the time.
> 3) Use a user space program to map the framebuffer and do the drawing.
> 4) Use the reserve= to reserve the needed address space. This boots performance
> sensitive page tables into highmem while keeping your slow framebuffer in
> lowmem.
> 
> It is only fbconsole that uses the kernel mapped framebuffer. If you aren't
> using fbconsole you could just delete the ioremap code from the framebuffer
> driver. Another quick fix would be to eliminate the framebuffer ioremaps from
> each driver and instead do it in the fbconsole driver. This would let people
> still load fb drivers and not load fbconsole.

Another approach: on fully-accelerated drivers, you don't need the ioremap(),
so there's no problem.

On non-accelerated drivers, you need the ioremap(), but usually unaccelerated
hardware is old and has only a few MiB of graphics memory anyway, so there's no
problem.

On partially-accelerated drivers, we can limit the amount of graphics memory
that's ioremapped(), and thus limit the (virtual) screen size. The remaining
problem here is that user space may want to know about the full graphics memory
size, while fbcon cannot handle that.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- 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



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