Re: [PATCH] spl: Align device tree blob address at 8-byte boundary

[Tom Rini <trini@konsulko.com>]

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On Mon, Jul 12, 2021 at 11:01:24AM -0500, Alex G. wrote:
> On 7/12/21 10:15 AM, Tom Rini wrote:
> > On Mon, Jul 12, 2021 at 01:36:14PM +0800, Bin Meng wrote:
> > > On Mon, Jul 12, 2021 at 1:21 PM Reuben Dowle <reuben.dowle@4rf.com> wrote:
> > > > 
> > > > I submitted an almost identical patch. See https://github.com/u-boot/u-boot/commit/eb39d8ba5f0d1468b01b89a2a464d18612d3ea76
> > > > 
> > > > This patch eventually had to be reverted (https://github.com/u-boot/u-boot/commit/5675ed7cb645f5ec13958726992daeeed16fd114), because it was causing issues on some platforms that had FIT on 32 bit boundary. However I continue to use it in production code, as without it the boot on my platform aborts.
> > > > 
> > > > I don't have time to investigate why this was happening, but you need to check this code won't just cause exactly the same faults.
> > > 
> > > Thanks for your information.
> > > 
> > > +Marek who did the revert
> > > 
> > > The revert commit message says:
> > > 
> > >      "The commit breaks booting of fitImage by SPL, the system simply
> > > hangs. This is because on arm32, the fitImage and all of its content
> > > can be aligned to 4 bytes and U-Boot expects just that."
> > > 
> > > I don't understand this. If an address is aligned to 8, it is already
> > > aligned to 4, so how did this commit make the system hang on arm32?
> > 
> > I think this had something to do with embedding contents somewhere in
> > the image?  There is a thread on the ML from then but I don't know how
> > informative it will end up being.
> 
> It's true that the flat devicetree spec requires an 8-byte alignment, even
> on 32-bit. The issues here are specific to u-boot.
> 
> SPL and u-boot have to agree where u-boot's FDT is located. We'll look at
> two cases:
> 	1) u-boot as a FIT (binary and FDT separately loaded)
> 	2) u-boot with embedded FDT
> 
> In case (1) SPL must place the FDT at a location where u-boot will find it.
> The current logic is
> 	SPL:	fdt = ALIGN_4(u_boot + u_boot_size)
> 	u-boot:	fdt = ALIGN_4(u_boot + u_boot_size)
> 
> In case (2), SPL's view of the FDT is not relevant, but instead the build
> system must place the FDT correctly:
> 	build:	fdt >> u-boot.bin
> 	u-boot:	fdt = ALIGN_4(u_boot + u_boot_size)
> 
> We have 3 places that must agree. A correct and complete patch could change
> all three, but one has to consider compatibility issues when crossing u-boot
> and SPL versions.
> 
> I had proposed in the revert discussion that SPL use r2 or similar mechanism
> to pass the location of the FDT to u-boot.

I'm not sure that we need to worry too much about mix-and-match
SPL/U-Boot, but documenting what to go change if you must do it
somewhere under doc/ would be good.  I think we can just switch to
ALIGN(8) not ALIGN(4) and be done with it?

-- 
Tom

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