[PATCH 1/2] ARM: calculate VMALLOC_END by probing in mdesc->map_io()

[Russell King - ARM Linux]

On Sun, Jan 23, 2011 at 12:59:52PM +0800, Eric Miao wrote:
> On Sun, Jan 23, 2011 at 7:15 AM, Russell King - ARM Linux
> <linux at arm.linux.org.uk> wrote:
> > On Sun, Jan 23, 2011 at 06:56:40AM +0800, Eric Miao wrote:
> >> From: Eric Miao <eric.miao at linaro.org>
> >>
> >> Provided that the only place initializing the fixed IO mapping is
> >> in mdesc->map_io(), the VMALLOC_END can actually be calculated by
> >> first invoking the function without actually doing the map.
> >
> > We really can't do this.  You're assuming that platforms only do IO
> > mapping in their map_io callback - that's way far from the truth.
> >
> > Samsung platforms for instance register clocks and other stuff.
> >
> > OMAP platforms issue cache flushes which won't work on CPUs requiring
> > a mapping to be setup for this to work.  They also check CPU revisions
> > (requiring mappings in place for that), probe the size of sram, etc.
> > None of this will work without mappings setup.
> >
> > And there's a catch-you in OMAP there - it needs accessible mappings
> > to work out what other mappings (eg, sram size) to setup.  It can't map
> > a larger area than is actually present because that's known to cause
> > problems.
> >
> > So, without a lot of effort to clean up platform code first, it's
> > really unsafe to do this double-call of map_io().  Given that it
> > takes some platform support _years_ to change, I really don't see
> > that this is a practical approach.
> 
> Indeed. One other way I was thinking of is to use data representation
> instead of calling iotable_init() in .map_io, i.e. introduce .iotable instead
> of .map_io. All the other hacking stuffs we are currently doing can be
> moved to .init_early(). But this still needs a lot clean-up, thoughts?

No, that still doesn't work.  As I said, some platforms need to create
mappings and access through those mappings to work out what other
mappings to create - iow, they aren't representable as a static set of
mappings.