[RFC] arm: Defer lookup of machine_type and vet of atags to setup.c

Russell King - ARM Linux linux at arm.linux.org.uk
Wed Jan 12 11:32:21 EST 2011

On Wed, Jan 12, 2011 at 09:24:34AM -0700, Grant Likely wrote:
> Actually it looks like the real problem is that the mmu has been
> turned on, but the virtual mappings for devices have not yet been
> established, and so the debug macros aren't using a valid address.

They should be, as they're valid for use from assembly prior to MMU
turn-on, assembly after MMU turn-on and C code.

If CONFIG_DEBUG_LL is enabled, create_page_tables() should be setting
up the necessary initial mappings for printascii() to work.

Maybe the efforts to unify that stuff ended up breaking the printascii
debugging mechanism, and we now require a new debugging mechanism to
debug the printascii debugging mechanism... :-P

I've used it on Versatile Express during the last week and it did work
right from before setup_arch() was called.

> It looks like I'd need to get past paging_init() in order to get
> ll_debug working between turning on the mmu and paging_init(), but
> paging_init() needs the mdesc pointer, and the whole point of the
> error message is that the mdesc pointer is unknown!  I don't see any
> code that sets up a debug mapping of the uart before paging_init time.

See the #ifdef CONFIG_DEBUG_LL section in create_page_tables in head.S

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