Message-id: <alpine.LFD.2.02.1110141718160.17040@xanadu.home>
Date:	Fri, 14 Oct 2011 17:28:55 -0400 (EDT)
From:	Nicolas Pitre <nico@...xnic.net>
To:	Stephen Warren <swarren@...dia.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Olof Johansson <olof@...om.net>, Arnd Bergmann <arnd@...db.de>,
	Peter De Schrijver <pdeschrijver@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"Colin Cross (ccross@...roid.com)" <ccross@...roid.com>,
	Erik Gilling <konkers@...roid.com>
Subject: RE: [PATCH] arm/tegra: select AUTO_ZRELADDR by default

On Fri, 14 Oct 2011, Stephen Warren wrote:

> Nicolas Pitre wrote at Friday, October 14, 2011 2:45 PM:
> > On Fri, 14 Oct 2011, Stephen Warren wrote:
> ...
> > > I did originally briefly look into getting U-Boot to boot a zImage, but
> > > that looked like a far more invasive patch. There were rumours of some
> > > chip's custom U-Boot already having such support, but I couldn't find
> > > it, nor any evidence of such support in mainline U-Boot.
> > 
> > FRom my clone of the u-Boot repo:
> > 
> > $ git grep -l zImage
> > README
> > arch/sh/lib/zimageboot.c
> > arch/x86/lib/zimage.c
> > ...
> > 
> > Even the name of some of those files clearly hints zImage support.
> > 
> > In any case, loading zImage should be even simpler than loading uImage.
> > It is the same as loading uImage except that you just have to skip the
> > checking and relocating steps.
> 
> Just by way of background in case anyone is wondering why I wrote the
> patch I did:
> 
> Those files both implement custom commands "zimageboot" and "zboot". I
> was looking for integration with the existing "bootm" command.
> 
> The advantage of re-using "bootm" for this is that it already supports
> all the stuff like setting up kernel command-lines, initrds, knowing how
> to pass the FDT to the kernel, and whatever other OS-specific setup might
> be required.

Absolutely.  And that is a must-have.

> The disadvantage of adding zImage support to bootm is that I'd have to
> teach a bunch of U-Boot image handling code about a new image format; it
> already knows about "legacy uImage", "FIT" images, and I'd have to add a
> third "zImage" format. Doing so would at least require adding a lot of
> "case IMAGE_FORMAT_ZIMAGE" everywhere, but it'd probably be best to add
> some kind of vtable for image formats to move all the image-format
> knowledge into format-specific files, leaving the users of the images
> with much smaller code.
> 
> I didn't feel like making such a large change. Hence, I chose to make a
> small change to the existing uImage support.

And that change is certainly valuable.  Some people do want the extra 
feature from the uImage format.  REmoving its absolute address 
limitation is a good thing.

However, also supporting zImage directly on ARM doesn't have to be that 
hard.  Instead of adding "case IMAGE_FORMAT_ZIMAGE" everywhere, all you 
would have to do is to load zImage in memory according to the load 
address provided as an argument to the command, and then fake a uImage 
load by artificially creating the uImage header at run time with the 
data that u-Boot needs later on.


Nicolas
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