[PATCH 4/5] [ARM] Auto calculate ZRELADDR and provide option for exceptions

[u.kleine-koenig@pengutronix.de (Uwe Kleine-König)]

Hello Russell,

On Thu, Jun 10, 2010 at 10:16:08AM +0100, Russell King - ARM Linux wrote:
> On Thu, Jun 10, 2010 at 11:00:15AM +0200, Uwe Kleine-K?nig wrote:
> > What do you think about requiring r4 to be set to physoffset as I did in
> > my series?  This way zImage would already know PHYSOFFSET and so didn't
> > need to guess ZRELADDR.  OK, until most bootloaders are fixed we need
> > to guess, too.  But at least this would provide a way to stop guessing
> > wrong for the affected platforms.
> 
> That means it won't get implemented.  Look at the facts.  It's taken
> _years_ (5+ years) for boot loaders to start passing a value in r1.
> We then switched to the ATAG stuff, and it's again taking years for
> boot loaders to start passing right ATAG stuff - and most of them don't
> get it right.  Eg, lots of uboot are happy to print out the memory
> information on the terminal as they start up, but don't pass memory
> information to the kernel.
> 
> So if we want phys offset in r4, we better realise that it'll take
> something like five years of nagging to get it in place, and even then
> people will continue to use boot loaders which don't have support.
> And if it's only necessary for a handful of platforms, I doubt anyone's
> really going to bother - or even validate that r4 is correct.
> 
> You have a whole set of other problems - how do you cope with existing
> boot loaders which happen to call the kernel with a value in r4 which
> _could_ potentially be valid, but should not be used?  At the moment,
> r4 could contain _any_ value what so ever.
> 
> Let's face it - advertising that "the kernel will eventually start using
> r4" doesn't solve the problem either - we did that with the ATAG list
> and we know the results from that.
> 
> Maybe the right experiment to try this time - if we want r4 to contain
> a value - is to say that kernel 2.6.37 will require a value in r4, and
> won't boot without it, and will therefore be incompatible with old
> boot loaders.
I agree with you.  Either we need to completly break bootloaders that
don't pass the right value or it should work without new requirements
for most machines.
For the machines that fail to guess correctly IMHO the better approach
is to let the bootloader pass the information instead of letting the
user guess the right value.  Maybe this is because I work for a company
that usually sells support for both bootloader and kernel.  The people
that don't want/cannot to touch their bootloader might have a different
mileage.

And note we could implement both ways (if provided via .config use this
value otherwise use r4 if it looks valid else guess).  If we choose that
all values in r4 look valid we have the needed breakage :-)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |