From mboxrd@z Thu Jan  1 00:00:00 1970
From: Daniel Thompson <daniel.thompson@linaro.org>
Subject: Re: Change of TEXT_OFFSET for multi_v7_defconfig
Date: Tue, 22 Apr 2014 11:26:53 +0100
Message-ID: <535643ED.8030406@linaro.org>
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To: Nicolas Pitre <nicolas.pitre@linaro.org>, Rob Herring <robherring2@gmail.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>, Peter Maydell <peter.maydell@linaro.org>, Joel Fernandes <joelf@ti.com>, linux-arm-msm <linux-arm-msm@vger.kernel.org>, Stephen Boyd <sboyd@codeaurora.org>, Peter Crosthwaite <peter.crosthwaite@xilinx.com>, QEMU Developers <qemu-devel@nongnu.org>, Christopher Covington <cov@codeaurora.org>, "linux-arm-kernel@lists.infradead.org" <linux-arm-kernel@lists.infradead.org>, Kumar Gala <galak@codeaurora.org>

On 18/04/14 05:34, Nicolas Pitre wrote:
>> I'm not suggesting to break anything or changing existing platforms,
>> > but how do we improve the Image format in a compatible way. If
>> > bootloaders want to support booting Image files or vmlinux directly,
>> > then we should support that including any compatible changes to make
>> > things work better.
> And why would bootloaders want that?  Just to create confusion with 
> the established boot protocol?

I'd say that they don't. My original concern was how the different
architectures negotiate if more than one arch wants a special text
offset, not how to write a correct bootloader.

The existing uImage files already provide sufficient information to load
the kernel regardless of the TEXT_OFFSET chosen by negotiation among the
enabled architectures.

The entry point is PAGE_OFFSET + TEXT_OFFSET and, although only
implicitly defined, the entry point cannot be set to any other value
without making a backward incompatible to arm/Booting:
    "The boot loader is expected to call the kernel image by jumping
    directly to the first instruction of the kernel image."

Therefore providing PAGE_OFFSET remains 1G aligned and the hardware
meets the not-unreasonably-stupid test (i.e. TEXT_OFFSET < 1G) then
deriving the right value for TEXT_OFFSET is a trivial mask operation on
the entry point.


Daniel.