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Date: Fri, 17 Aug 2012 14:21:24 -0400 (EDT)
From: Nicolas Pitre <nico@fluxnic.net>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2 02/31] arm64: Kernel booting and initialisation
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On Fri, 17 Aug 2012, Arnd Bergmann wrote:

> On Thursday 16 August 2012, Nicolas Pitre wrote:
> > Some people will want to use bzip2 or whatever other decompressor du 
> > jour.  Maybe this shouldn't be gzip specific, or just presented as a 
> > possible option?
> 
> Good point. Whether this should be part of this document depends on
> what assumptions we make about the boot loader getting the image
> in the first place.
> 
> In the strict sense, we are documenting the interface between the boot
> loader and the kernel here, which already specifies that  the kernel
> must be uncompressed by the time we enter it. If the boot loader wants
> to add its own encryption or compression methods, or its own headers
> in front of the binary, the boot protocol isn't really impacted.

Right.  And someone else will insist on wrapping the kernel into a boot 
loader specific image format e.g. u-Boot.  If all those variations could 
be kept out of the kernel build that would be a good thing.

That means the kernel should be wrapped/compressed/scrambled at 
installation time, not at build time.  This way the kernel image remains 
universal and flexibility in its installation is possible.

> That said, I think it's a good idea to also specify what kind of 
> format we want to be used, e.g. a stripped ELF Image compressed with 
> one of gzip/bzip2/lzo/xz and with no other headers added, on a 
> vfat/ext4/btrfs formatted file system. There are probably a lot of 
> other things one might want to specify if we go down this route. Or we 
> could refer to the UEFI spec and mandate that the same format that 
> UEFI uses should be used here independent of what boot loader is used. 
> I think we can still allow other ways to get to the image for deeply 
> embedded systems, e.g. linking the kernel into the boot loader as a 
> blob on tightly constrained systems. For that case, we'd only specify 
> the interface between boot loader and kernel as described above.

I don't think we'll have to concern ourselves with tightly constrained 
systems that much on ARM64.


Nicolas
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