Re: ioremap problem on highmem

[Jon Smirl <jonsmirl@yahoo.com>]

--- Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> 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.
> 

How about removing all ioremaps from the fb drivers. Then fbcon can check if
they are accelerated or not and do the ioremap in fbcon if they are not.

=====
Jon Smirl
jonsmirl@yahoo.com

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