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Re: Converting MIPS to Device Tree

To: David Daney <ddaney@caviumnetworks.com>
Subject: Re: Converting MIPS to Device Tree
From: Ralf Baechle <ralf@linux-mips.org>
Date: Fri, 10 Jun 2011 19:57:45 +0100
Cc: David VomLehn <dvomlehn@cisco.com>, Grant Likely <grant.likely@secretlab.ca>, linux-mips@linux-mips.org, Imre Kaloz <kaloz@openwrt.org>, Gabor Juhos <juhosg@openwrt.org>, John Crispin <blogic@openwrt.org>, "Dezhong Diao (dediao)" <dediao@cisco.com>, Thomas Gleixner <tglx@linutronix.de>
In-reply-to: <4DEEB2A8.8050302@caviumnetworks.com>
References: <20110606010753.GA16202@linux-mips.org> <BANLkTik1mRWTcX8WgO5s6mFrUGYwBRmSow@mail.gmail.com> <20110607230218.GA23552@dvomlehn-lnx2.corp.sa.net> <4DEEB2A8.8050302@caviumnetworks.com>
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On Tue, Jun 07, 2011 at 04:22:16PM -0700, David Daney wrote:

> >use a parameter like "devtree=<virtual-address>" on the command line, passed
> >in any way the bootloader likes.
> 
> Some  u-boots for non-mips platforms pass it in the environment of
> the bootm protocol.
> 
> I would say to pass the pointer to the DTB in the environment, but
> not all platforms (like powertv) have an environment.  So I guess
> the command line has to do.

3 steps:

  1) Use command line argument for DT
  2) Iff 1) fails, use DT specified by environment
  3) Iff 1) and 2) fail, use builtin DTB.

> Also I think we should pass the physical address of the DTB, not the
> virtual address.  It would be the kernel's responsibility to figure
> out what the virtual address is.

I like the basic idea - but ...  Most firmware will only use KSEG0 / XKPHYS
mappings so there should be no aliasing issue but still there could be
conflicting cache modes.  So we should also specify that firmware should
writeback and invalidate the DTB from caches.

  Ralf

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