i.MX consolidation patches

[s.hauer@pengutronix.de (Sascha Hauer)]

On Wed, Jun 01, 2011 at 06:04:02PM -0500, Matt Sealey wrote:
> On Wed, Jun 1, 2011 at 4:08 PM, Wolfgang Denk <wd@denx.de> wrote:
> > Dear Sascha Hauer,
> >
> > In message <20110601141847.GG23771@pengutronix.de> you wrote:
> >>
> >> > We probably should disable the uImage target when p2v patching is
> >> > enabled to prevent people getting nasty surprises.
> >> >
> >>
> >> Agreed. Here is a patch. I added Wolfgang Denk to Cc, maybe
> >> he can prove me wrong.
> >>
> >> 8<----------------------------------------------------------
> >> ARM: do not allow to build uImages with ARM_PATCH_PHYS_VIRT
> >>
> >> U-Boot uImages expect a load address and a entry point in
> >> the image header. With CONFIG_ARM_PATCH_PHYS_VIRT these
> >> become variable and thus can not be compiled into the uImage.
> >
> > Would it help if we interpret, for example, the values for load
> > address and entry point not as physical addresses, but instead as
> > offsets relative to the start of physical RAM?
> >
> > This would still require that all systems supported by a single image
> > use the same offsets. ?Is this possible?
> 
> Been having this discussion on and off with various parties and we
> determined that if U-Boot had a little
> extra logic when parsing a uImage header it could perfectly validly
> boot a zImage contained in a uImage
> header.
> 
> In this case, just a load address in the uImage header of 0 (or some
> other totally-impossible value like 0xffffffff in case 0 is perfectly
> valid somewhere)

AFAIK 0x0 is the standard entry point on powerpc, but 0xffffffff should
be fine.

> and then just jump to the entry point by deriving the
> value from the header size - based on the fact that ARM images are PIC
> and the Linux kernel always puts the entry point at 0 offset - to the
> address of the uImage header + size of the uImage header (which U-Boot
> knows already from parsing the header).
> 
> Example: on the MX51 the entry point is usually set to 0x90008000 -
> that's what Freescale put in their BSP
> U-Boots and everyone has copied it.. There's a variable in our U-Boots
> at least that is used in boot scripts to
> ext/fat/whateverload it to 0x90007FC0. That 0x90007FC0 address to load
> to is a nasty hack meant to match
> the header size of a legacy uImage (therefore the first byte of the
> payload will live at 0x90008000).
> 
> We can't support anything but a legacy image right now because of
> that, and if we needed to support a new
> style uImage with FDT, then this load address and entry point magic
> would be totally wrong anyway requiring
> both userspace script and kernel changes.
> 
> So the solution is
> 
> * Assuming all ARM kernels are PIC (guaranteed, right?)

zImages are. The restriction here is:

config AUTO_ZRELADDR
        bool "Auto calculation of the decompressed kernel image address"
        depends on !ZBOOT_ROM && !ARCH_U300
        help
          ZRELADDR is the physical address where the decompressed kernel
          image will be placed. If AUTO_ZRELADDR is selected, the address
          will be determined at run-time by masking the current IP with
          0xf8000000. This assumes the zImage being placed in the first 128MB
          from start of memory.

So U-Boot could interpret load address being set to 0xffffffff as 'put it
somewhere in the first 128MB it jump to this address'.

> * Assuming all ARM kernels start at entry point 0 (true for Image and zImage)

You mean that the entry point is load address + 0? That should be true.
Even if not, when the load address is 0xffffffff, the entry point field
could still describe an offset into the image.

> * Assuming there is a globally invalid magic value you can set in the
> uImage header as load address (not sure)
> * Assuming you can make sure U-Boot only does this for ARM,
> kernel-type images and not ramdisks or so (true)

ramdisks would need the same mechanism if we want to attach them to
multi SoC kernels.

> 
> Set that magic value, U-Boot magically derives the correct entry point
> as the first address of the payload,
> and jumps to it?
> 
> I tested it.. works great but I don't have a wealth of ARM hardware to
> TRULY confirm all the above.
> 
> Let's not kill off uImage generation just yet if we can just patch our
> firmwares once and for all and let Linux
> decide whether it sets the load address to a real address or a magic
> value? Then all variants of kernel will
> work for the boards, patching phys_to_virt or not.
> 
> -- 
> Matt Sealey <matt@genesi-usa.com>
> Product Development Analyst, Genesi USA, Inc.
> 
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