linux-kernel - Re: [RFC] arm: Defer lookup of machine

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Date:	Wed, 12 Jan 2011 11:53:31 -0500 (EST)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Catalin Marinas <catalin.marinas@....com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>
Subject: Re: [RFC] arm: Defer lookup of machine_type and vet of atags to setup.c

On Wed, 12 Jan 2011, 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.

A temporary virtual mapping should be there -- look for addruart in 
head.S.

> I'm using printk to get output into the ring buffer in my patch to
> rework lookup_machine_type() in C, and that does indeed output to the
> ring buffer, but the kernel cannot spit stuff out the serial port.
> 
> 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 above.

>  I could try to implement something like that, but it is looking to be
> more complicated than it is worth when the current code works just
> fine.

My bet is that there is a bug with the current code that you are 
exposing.

> Let me know if I've missed something, but I think I should drop the
> removal of __lookup_machine_type from head.S from my patch.

It's not the location of that code which is a problem.  Even if you 
leave that code in place, you want to call it later and I bet that the 
display would be broken even if __lookup_machine_type doesn't move.


Nicolas
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