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

[Tom Rini]

On Tue, Jul 13, 2021 at 10:35:03PM +0200, Marek Vasut wrote:
> On 7/13/21 8:11 PM, Tom Rini wrote:
> > On Tue, Jul 13, 2021 at 07:50:49PM +0200, Marek Vasut wrote:
> > > On 7/13/21 6:47 PM, Simon Glass wrote:
> > > > Hi Marek,
> > > > 
> > > > On Tue, 13 Jul 2021 at 08:53, Marek Vasut <marex at denx.de> wrote:
> > > > > 
> > > > > On 7/13/21 4:41 PM, Tom Rini wrote:
> > > > > > On Tue, Jul 13, 2021 at 04:35:38PM +0200, Marek Vasut wrote:
> > > > > > > On 7/13/21 3:47 PM, Tom Rini wrote:
> > > > > > > > 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 at 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?
> > > > > > > 
> > > > > > > Remember, there is also falcon boot. And we definitely have to be able to
> > > > > > > have old u-boot (SPL) boot new fitImage and vice versa.
> > > > > > 
> > > > > > I don't follow you, sorry.  But since you seem to have the best
> > > > > > understanding of where all of the cases something could go wrong here,
> > > > > > can you perhaps post an RFC patch?  That is likely to be clearer than
> > > > > > another long thread here.
> > > > > 
> > > > > I don't follow you, sorry. I believe the revert did the right thing and
> > > > > new systems should use mkimage -E when generating fitImages, to avoid
> > > > > the string alignment problem. That is all.
> > > > 
> > > > Using -E should be optional and things really should work without it.
> > > 
> > > See the DTSpec, I don't think that is possible unless you relocate fitImage
> > > components, and if you want fast boot time esp. in SPL, that is not good.
> > 
> > This is why I've asked you to make up some patch to perhaps highlight
> > the problem.  Ensuring that the device tree, which is small, is also
> > 8-byte aligned, shouldn't be a big problem nor performance hit.  I'm not
> > sure where the problem case is that isn't "user put things they control
> > in a bad spot, fail and tell them why" but I could just be missing a
> > case.
> 
> The fail case is this:
> - you update SPL with this 8 byte alignment change
> - you have older kernel fitImage with embedded DT for falcon mode
> - system no longer boots because there is off-by-4 error in the DT
>   address passed to the kernel

OK.  Then I think the answer is what I said recently in another part of
this thread, we need to split "find the fdt" from "align the fdt".  The
fdt can come to us with any alignment it happens to have, but we can't
use that fdt in-place unless it's correctly aligned.  In the case of
falcon mode, it needs to end up at CONFIG_SYS_SPL_ARGS_ADDR.  The case
of passing it on to U-Boot proper is where we have at best a hack right
now (as noted by fdt_hack in common/spl/spl.c).  That would be a place
to, as has been also suggested in this thread, pass along more correctly
where the device tree in memory is.

-- 
Tom
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