[RFC] Prohibit ioremap() on kernel managed RAM

[Shilimkar, Santosh]

> -----Original Message-----
> From: Catalin Marinas [mailto:catalin.marinas at arm.com]
> Sent: Friday, April 30, 2010 10:09 PM
> To: George G. Davis
> Cc: Russell King; Shilimkar, Santosh; linux-arm-kernel at lists.infradead.org
> Subject: Re: [RFC] Prohibit ioremap() on kernel managed RAM
> 
> On Fri, 2010-04-30 at 17:33 +0100, George G. Davis wrote:
> > Hi,
> >
> > On Fri, Apr 23, 2010 at 03:40:58PM +0100, Russell King wrote:
> > > > Above change is necessary but what an alternative approach is for this.
> > > > There are many use case where ioremap* is needed.
> > >
> > > This is a very difficult issue to answer; the only way we can safely
> > > remap RAM with different attributes is if we disable the existing
> > > mappings - but since we create those with 1MB sections, that's far
> > > from easy to achieve.
> > >
> > > I think a viable safe solution is to set aside some RAM at boot (which
> > > the kernel doesn't manage at all) and then use ioremap on that; that
> > > approach will still work with this patch in place.
> >
> > So cases such as the omapfb driver which use reserve_bootmem() (in
> > arch/arm/plat-omap/fb.c) and then later use ioremap_wc() to remap
> > reserved memory (in drivers/video/omap2/omapfb/omapfb-main.c)
> > will no longer work with this change.
> 
> Another solution would be to allow the unmapping of sections from the
> kernel linear mapping. I think x86 does this already for the AGP
> aperture.
This seems to be good solution if it's doable. Reserving memory in the boot
is not so flexible and might end up in waste of memory.

Regards,
Santosh